Monthly Archives: May 2018

Atlas of fan deltas

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The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source as indicated below (it is the polite, and professional thing to do).  I retain copyright of all images presented herein

Brian Ricketts –  www.geological-digressions.com

Fan deltas are like alluvial fans except they dip their toes in lakes and shallow seas. So, in addition to the alluvial component, there is subaqueous deposition down a relatively steep, angle-of-repose slope. Sedimentation along the delta front, or slope, commonly produces large, basinward-dipping foresets, one of the defining characteristics of fan deltas.

Fan delta deposits are generally coarse-grained; there is much sand and gravel. Distributary systems tend to be braided. Sediment is supplied to the delta front from where it avalanches down-slope or transforms to debris flows. Gravitational instability may also influence depositional mechanisms.

Fan deltas tend to accumulate where there is a decent supply of sediment; close to steep uplands, active faults, mountain fronts, thrust fronts, glacial lakes and fiords, and pull-apart basins.    Deposition outboard of active extension faults can produce spectacular fan delta stacks on the hanging-wall block. Fan deltas associated with thrust faults may accumulate as basinward overlapping packages in the footwall, that are subsequently overthrust. In pull-apart basins, the locus of fan delta stacking parallels strike-slip displacement; often likened to a horizontal stack of dominoes – the Devonian Hornelen Basin (Norway) and Late Miocene Ridge Basin (California) are classic examples.

Here’s a paper on Bowser Basin fan deltas: Ricketts, B.D., and Evenchick, C.A. 2007. Evidence of different contractional styles along foredeep margins provided by Gilbert deltas; examples from Bowser Basin, British Columbia, Canada: Bulletin of the Canadian Petroleum Geologists, v. 55, p. 243-261.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

 

The images:

A sizeable fan delta encroaching into Tanquary Fiord, Ellesmere Island. Arrow points to a Geological Survey of Canada base camp in 1988. The gravel delta top and foreslope are derived from Paleozoic rocks.

 

 

The head of Strand Fiord, Axel Heiberg Island, contains a braid-plain fan delta (center), the outwash drainage from Strand Glacier (distant right). A smaller, ‘radial’ fan delta is growing along the south (right) fiord shore.  See image below for a different perspective of this fan delta.

 

 

Looking west along Strand Fiord (Axel Heiberg Island); several small fan deltas drain the bordering ridges. In the foreground is the fan delta shown in the preceding image, fed by a braided river.

 

 

 

Typical Arctic fan deltas: Left Slidre Fiord. Braided stream supply to the delta front is clear, with the active channels regularly moving across the delta top. A gravel beach ridge formed along the inactive delta front, has become detached.  Right: Small, steep sloped fan delta along Emma Fiord.

 

This small, very recent, dissected fan delta accumulated on the beach face at Kariotahi, south Auckland (Tasman Sea coast). Storm drainage through the weakly indurated Pleistocene dune-beach sands behind, deposited sand during high tide. The small delta built across the beach, and as the tide ebbed, the stream eroded into its delta. The overall concave (down) top surface is evident in both images.

Cross-section through the Kariotahi mini fan delta. Mostly Laminated and rippled sand and a few mud stringers, with a layer of disrupted sand-mud at the red arrow.

 

 

Pleistocene Gilbert delta exposed in the Bradner Road pit, Fraser Valler, Vancouver. The dipping foresets have a clear topset sand unit (laminated and small crossbeds).  Foresets show numerous pinchouts and local discordances, probably reflecting changing stream flow and sediment supply, and possibly local slumping down the foreslope. The delta is at least 6m thick.  It accumulated in a glacial outwash lake. The overlying grey deposit is a diamictite.

 

An impressive stack of Upper Jurassic fan deltas in Bowser Basin, northern British Columbia. Each delta package is separated by recessive, interfan turbidites and mudstone. The stack accumulated during active faulting close to the basin margin. Icebox Canyon.

 

 

A different perspective of the Icebox Canyon fan delta stack: fan foresets are dipping towards the viewer (top to the left). Some fan packages coalesce, others are separated by thin turbiditic sandstone and mudstone.

 

 

Closer view of delta packages, shows foresets, and thin bedded interfan deposits. Icebox Canyon, Bowser Basin

 

 

Foreset geometry is clearly expressed in this view of the Icebox Canyon fan delta stack

 

 

 

Interfan turbidites, mostly Tb-d components of Bouma cycles. Top to the right.

 

 

 

Gravel ripples developed along some fan delta foresets, indicating some down-slope bedload movement of sand and gravel. Icebox Canyon, Bowser Basin.

 

 

 

Clear discordances between foreset conglomerate beds, and topset conglomerates in fan deltas at Mt. Cartmel (left), and Tsargoss Lake (right). Topset beds at Mt. Cartmel contain planar and trough crossbedded, clast-supported conglomerate that is interpreted as the briaded, alluvial portion of the fan delta. Bowser Basin.

Some fan Deltas in Bowser Basin, migrated to the shelf-slope break, and were probably instrumental in supplying gravel to the deeper basin submarine gullies, canyons, and submarine fans. Here, foreset toes interfinger with slope shale and thin sandstone. West of Tsatia Mt.

 

Admittedly a bit dark, but look closely and you will see fan delta foreset toes interfingering with slope mudrocks, and overlying the delta, coarsening-upward shelf deposits. West of Tsatia Mt, Bowser Basin.

 

 

Non-cohesive – greater degree of clast-support (left) and cohesive-muddy (right) debris flow conglomerate composing some fan delta foresets at the Mt Cartmel delta.

Reconstruction of fan delta-shelf and shelf-break gullies, outboard of active Late Jurassic thrusting, Bowser Basin, BC.  For details, see: Ricketts, B.D., and Evenchick, C.A. 2007. Evidence of different contractional styles along foredeep margins provided by Gilbert deltas; examples from Bowser Basin, British Columbia, Canada: Bulletin of the Canadian Petroleum Geologists, v. 55, p. 243-261.

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Atlas of stromatolites and cryptalgal laminates

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The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source as indicated below (it is the polite, and professional thing to do).  I retain copyright of all images presented herein

Brian Ricketts –  www.geological-digressions.com

Stromatolites. The Precambrian is replete with them. In many ways they define the Precambrian, that period of earth history, about 90% of it, that set the scene for the world we currently live in – its atmosphere, hydrosphere, lithosphere, and biosphere. It’s the period when life began more than 3.4 billion years ago, taking its time (about 3 billion years) to get over that first rush of DNA replication.

Stromatolites are the sedimentary record of that really prolonged period of geological time. Some of the oldest known, bona fide cryptalgal structures are found in the 3.4 Ga North Pole deposits. They represent fossil slime – mats of photosynthetic, prokaryotic cyanobacteria. They were responsible for producing the oxygen we, and most other life forms breath.

Stromatolites really came into their own by about 2.5Ga, forming extensive buildups, and reef-like structures, by slow, incremental addition, mat-by-mat, in the ancient shallow seas. Growth habits varied from broad flat domes to intricately branched columns. Stromatolite structure, shape and distribution were primarily controlled by environmental conditions such as water depth, wave and current energy, and substrate (muddy, sandy).  Glacially polished rock outcrops on Belcher Islands (where all the following images are from) show these structures in exquisite detail.

Stromatolites in outcrop commonly appear huge, as columns or domes extending vertically several metres. But their sea floor profiles, or synoptic relief during growth was low. We can visualize this when tracing individual laminae or sets of laminae (ie. the original mat surface) from one column to the next. Your average shallow shelf or platform stromatolite extended no more than a few millimeters or centimeters above the sea floor. Some large mounds, or reef-like structures had a few metres relief; but nothing like more recent coral reefs. This also means that the environmental conditions for incremental growth must have been stable for long periods of time. This needs to be kept in mind when looking at cryptalgal structures in outcrop; their apparent size can be misleading.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

A few publications that have a bearing on the set of images below:

Ricketts, B.D.  1983: The evolution of a Middle Precambrian dolostone sequence – a spectrum of dolomitization regimes; Journal of Sedimentary Petrology, v. 53, p. 565-596.

Ricketts, B.D. and Donaldson, J.A. 1981: Sedimentary history of the Belcher Group of Hudson Bay; Geological Survey of Canada, Paper 81-10, p. 235-254. F.A.H. Campbell, Editor

Ricketts, B.D. and Donaldson, J.A.  1989: Stromatolite reef development on a mud-dominated platform in the Middle Precambrian Belcher Group of Hudson Bay; Canadian Society of Petroleum Geologists, Memoir 13, p. 113-119.

Donaldson, J.A. and Ricketts, B.D.  1979:     Beachrock in Proterozoic dolostone of the Belcher Islands, Northwest Territories; Journal of Sedimentary Petrology, v. 49, p. 1287-1294.

 

The images:

 

Bulbous, dolomitized stromatolites in the lower part of the outcrop become progressively more branched towards the top. The view is oblique to bedding; the surface polished by Laurentide glacial ice. McLeary Fm. Right: dashes follow the synoptic surface, which approximates the actual growing mat morphology and relief at the sea floor.  Whereas the stromatolites in outcrop appear large, at the time of growth (2 billion years ago) the sea floor would have looked vaguely dimpled or domed. Bedding-parallel stylolites have thinned the rock sequence by 10-20%.

 

The cartoon refers to the synoptic surface outlined in the image above. Even though columns and dome appear in outcrop to be quite large (10s of cm to metres), their actual growth profiles at the sediment-water interface was measured in only millimetres to centimetres.

 

 

Bulbous stromatolites similar to those shown above.  The original carbonate (calcite-high Mg calcite-aragonite) has been completely replaced by dolomite.  Some of the upstanding, resistant edges are subsequent chert replacement. Image on the right shows excellent preservation of original laminae that in some cases can be traced across 2 and 3 branches. Both are oblique to bedding. McLeary Fm. Intercolumn sediment is dolomitized carbonate mud.

 

Stromatolite form here changes from bulbous to more digitate branching, back to bulbous. McLeary Fm.

 

 

 

Large, laterally extensive stromatolite domes more than 8m thick, but having synoptic relief of only a few centimetres. There were very few interruptions in growth; they probably accumulated on a subtidal shelf-platform lacking strong bottom currents. Kasegalik Fm.

 

 

Large, closely-spaced, low relief stromatolite domes; synoptic relief was 5-8cm.  Look closely at the laminae and in some you will see continuity from one dome to another, and in others discontinuities and overlaps 2-4 laminae thick. Mavor Fm.

 

Large stromatolite domes like those above, can transform to more digitate columns higher in the bedding unit. This probably represents subtle changes in environment, such as local bottom currents, or growth that was interrupted by storms. Mavor Fm.

 

 

Exhumed stromatolite domes on bedding, McLeary Fm. Their internal structure is similar to the domes shown above. The domes are slightly elongated, with long axis parallel to subtidal paleocurrents (determined from other sedimentary structures).  Inter-dome sediment is dolomitized carbonate mud. Hammer, centre-right.

 

Bedding and cross-section views of subtidal platform, domal stromatolites. Synoptic relief here is a bit less than in the image above. McLeary Fm. Stromatolites in the uppermost bed are eroded, overturned, or oversteepened, probably by storm waves.

 

 

This distinctive stromatolite unit can be traced 10s of kilometres. Closely spaced vertical, digitate columns grew on a shallow subtidal platform. Columns are relatively uniform width, usually branched, with tangential laminae forming a sturdy wall. Synoptic relief was only a few millimetres. McLeary Fm.

Polished slabs of the digitate stromatolites shown above. The scale on the right is centimetres. Preserved laminae are mm to sub-mm thick. The rock has been completely dolmitized, and yet delicate structure is preserved. McLeary Fm.

 

Isometric reconstruction of slabbed digitate stromatolites (based on several polished slabs like the one above). McLeary Fm. The (barely visible) scale is in centimetres.

 

 

Closely packed columnar stromatolites – bedding view. Raised rims on each column is due to silicification. McLeary Fm.

 

 

 

Several growth stages from domal stromatolites to narrower, closely-spaced, digitate columns, Mavor Fm. Intercolumn sediment is dolomitized mud. Three stylolites (top, centre, bottom) have reduced section thickness by 15-20%. Although completely dolomitized, mm and sub-mm scale laminae are well preserved.

 

Disruption of stromatolite columns and small domes by erosion. Rip-ups include largish mudstone slabs. McLeary Fm.

 

 

 

Digitate stromatolite columns in cross-section (left) and bedding (right). Dolomitiztion here has produced coarse crystalline textures that have partly obliterated outlines and laminae. Mavor Fm.

Domal, digitate, and coalescing stromatolite columns, growth habits that changed with environmental conditions or interruptions in growth (e.g. storms), McLeary Fm. Image on right has an erosional discordance at the pen tip. branching began during mat regrowth.

 

Radiating, digitate, branching columns. Left: the radiating cluster is a solitary buildup in surrounding flat, laminated mats. Right: The digitate cluster has been disrupted and partly eroded by crossbedded sandstone, indicating a significant change in local environmental conditions (shallow subtidal to intertidal).

 

Domal masses with silicified, subsidiary columns growing from the margins. An erosional discordance (just below the coin) terminated growth. Kasegalik Fm.

 

 

 

Wavy mats give way to columnar stromatolites with cone-shaped laminae. This form has historically been called Conophyton.  McLeary fm.

Irregularly branched columns with significant silica replacement. The white crystals are coarse, late diagenetic dolomite

 

 

 

Ornately branched stromatolite, a possible example of what historically was called Tungussia.  Mavor Fm.

 

 

 

Left: Dolomite pseudomorphs of gypsum in dolarenite.  Right: Fine-grained dolarenites interbedded with carbonate mudstone (dolomite) and simple, laminated crpytalgal mats (partly silicified). Gypsum psuedomorphs (spots) are scattered throughout. A layer of algal mat and mud rip-ups is present at the lens cap. McLeary Fm.

 

Teepee structures in carbonate mudstone and laminated cryptalgal mats; disruption of the mudstone slabs was probably caused by salt-gypsum expansion. McLeary Fm.

 

 

 

Beachrock is common in the McLeary Fm. Here, a block of dislodged beachrock (preferentially cemented dolomitic sandstone) has been overturned, as evidenced by the small, upside-down stromatolite columns.

 

Molar tooth structures in dolomitic mud. Their origin has been described variously as shrinkage cracks caused by changes in salinity, CO2 gas expansion (from decaying mats?), wave loading, clathrates, and seismically-induced changes in pore pressures.  They are not worm burrows!

 

 

 

Subtidal to outer platform stromatolite mounds that have undergone more intense recrystallization during dolomite replacement of the original carbonate, such that original column-bulb outlines are partly obscured. Remnants of small columns are visible in the upper dome layers (right). There is a hint of coloumn or mat detachment, and possibly pisoliths in the centre. The vugs are secondary diagenetic features from dissolution of (?) sparry calcite and dolomite replacement. Tukarak Fm (immediately overlies the McLeary Fm).

 

Recrystallized, dolomitic mounds where the original carbonate has been replaced by one or two generations of dolomite spar. The void is lined with late diagenetic dolomite spar, and even later calcite (white crystals).  Tukarak Fm.

 

Microdigitate mats, here associated with grainstone. Left: mats above the dark cherty layer show at least three stages of growth, each following an episode of erosion. Mats below the chert are more simple wavy forms. The grainstone above contains numerous mud and mat rip-ups. Right: Slightly larger, but no less delicate microdigitate mats and columns, again showing evidence of erosion and regrowth. Both examples formed in intertidal to supratidal flats. McLeary Fm.

 

A coarse grainstone (completely dolomitized) containing abundant mat rip-ups, pisoliths, and a single continuous mat that has regrown over pisoliths. Subtidal to supratidal flat, McLeary Fm.

 

 

 

Wavy and crinkley mats, and faintly preserved microdigitate columns, show the changes in growth habit possible over a scale of millimetres to centimetres. Scale top (bottom left) is 20 mm wide. McLeary Fm.

 

 

Left: Small dome, beginning with flat laminae at the base, and successions of microdigitate columns above.  Right: Small domes capped by microdigitate columns.  Laminated mudstone above are discordant and eroded. The white, silicified masses were probably larger domal structures. McLeary Fm.

 

Partly silicified microdigitate mats overlying a pavement of edgewise lutite slabs, or beach rosettes. Grainstone above contains mat rip-ups and pisoliths. McLeary Fm.

 

 

Dolomitized carbonate mudstone and thin mats, totally disrupted, ripped up, and folded by storm surges into a supratidal flat. McLeary Fm.

 

 

Successive microdigitate columns and laminated dololutite-mat interbeds. The resistant ridges are silicified, cherty mats. McLeary Fm.

 

 

 

Both images show wavy mats and microdigitate columns, disrupted by supratidal desiccation, storm-loading pull-aparts, and fragmentation. The interval in the left image is capped by larger domal masses that in turn have been locally overturned. McLeary Fm.

 

Bulbous to domal masses, partly disrupted and overturned, have stabilized an edgewise conglomerate (beach rosette) pavement. Slabs in the pavement are thin, probably partly lithified-cemented lutite, ripped up during earlier storm events.  McLeary Fm.

 

 

One of the more spectacular stromatolite buildups, or reefs, in the Proterozoic Mavor Formation, Belcher Group. The aerial view shows a transition from shallow subtidal, flat laminates to simple mounds, to large domes with 3-5m synoptic relief at the platform margin – slope deposits (Costello Fm) extend from the margin on the right. Smaller mounds on the left coalesce into larger mounds. Field of view along mound length is about 800m. Stratigraphic thickness is about 150m along this section of Tukarak Island.

 

Slightly oblique view across several large mounds and intervening troughs. The relief here is close to synoptic relief. Keep in mind the entire structure was made up of cryptalgal laminates. There were interruptions in growth, at the scale of individual mounds, evidenced by numerous discontinuity surfaces. There is little evidence for wholesale erosion, and the conclusion is that the larger mound structures accumulated below storm wave-base. Mavor Fm.

Left: View approximately along strike. Second-order mounds are nicely exposed here (by the hammer). Right: View is slightly oblique to depositional dip. Here too we can see smaller mounds superposed on the larger structure. Mavor Fm.

 

View down dip across smaller mounds that are superposed on the larger structures. Mavor Fm.

 

 

 

Cross-section through the smaller mounds (hammer right centre) showing the distinctive geometry and regularity of the laminae. There are numerous stylolites (thin dark bands) that tend to mimic the mound outline.  Synoptic relief is 20-40 cm. Mavor Fm.

 

 

Detail of the wavy and crinkley cryptalgal laminae, through a (2nd order) mound crest (left) and trough (right). The synoptic relief on any lamination is rarely more than a centimetre.

 

Flat to wavy cryptalgal laminae in a 2nd order mound, with prominent stylolites. At least 5 seams here account for about 20% loss of stratigraphic thickness. Below the upper stylolite seam is a thin layer of mat rip-ups, evidence for briefly interrupted growth. Mavor Fm.

 

 

Reconstruction of the progressive changes in mound amplitude and spacing, from shallow subtidal platform at the base (corresponds with the left side of the aerial image above), through coalescing mounds at the platform margin, to the slope deposits beyond (Costello Fm). For completeness, an example of the slope rocks is shown below.

 

Regular bedded (that can be traced laterally for 100s to 1000s of metres) calci-dololutite and red marls in slope deposits, outboard of the Mavor Formation platform-wide buildups-reefs. There are a few slumps and the occasional small channel filled with eroded lutite and shale. There are a few thin, graded beds, likely deposited as calci-turbidites.

 

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Atlas of trace fossils

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Ancient and modern trace fossils,

The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source as indicated below (it is the polite and professional thing to do).  I retain copyright of all images presented herein

 

Trace fossils have the privilege of being two things at once: sedimentary structures, and fossils. They occur in sediment, are made of sediment, but represent the activities of creeping, crawling or burrowing critters, mostly at or immediately below the sediment water interface (marine, lacustrine, estuarine, swamp), or subaerial environments such as dune fields. As such, trace fossils represent the range of activities that critters are normally occupied with – grazing or foraging for food, home construction and house-keeping, predating or escaping predators, wandering aimlessly, or taking a nap after an exhausting day. Some critters like to rough it, preferring the tumble of waves or strong currents, while others like the peace and quiet of deeper realms. Lives are frequently interrupted by storms or violent, turbulent flows of sand and mud; their traces, or lack of them, also reflect these events.

Most animals produce more than one kind of trace depending on what they are doing, which means that in most cases, traces reflect animal activity and biometrics, rather than the specific critter species.  Most traces do not contain any remnants of the animal that made them (there are a few exceptions); finding a trilobite body fossil at the end of its scampering trail is pretty rare.

Trace fossils provide valuable information on benthic communities, environmental conditions such as wave or current energy, redox conditions, rates of sedimentation, or periods of time when sedimentation slowed (e.g. hiatuses, disconformities, omission surfaces).

Intense bioturbation can also obliterate other kinds of sedimentary structures; for a geologist, this may be an annoyance or a happy circumstance.  Most Precambrian successions are free of trace fossils and bioturbation; this changed during the Ediacaran, the period that appears to have been a kind of precursor to the Cambrian invertebrate explosion. Most Phanerozoic sedimentary successions (since 540 million years ago) have enjoyed the munching-burrowing efforts of a myriad nameless critters.

Trace fossils, like regular body fossils, are part of the rock.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

The images:

ant nest

Modern ants nest found beneath a paving stone. The small white patches are piles of eggs.

 

 

 

 

star fish trace fossilResting impression of a brittle starfish (echinoid), Asteriacites,  Jurassic, Ellesmere Island.

 

 

 

zoophycusRadiating spreite of Zoophycus, organized into a 3-dimension cork-screw like structure, probably representing systematic feeding by a marine worm or similar creature. Mid-Miocene Mohakatino Fm. North Taranaki

 

 

zoophycus in 3DA nice representation of the 3-dimensionality of spreite radiating about a central vertical burrow in Zoophycus, Permian, Canon Fiord Fm, Ellesmere Island.

 

 

Miocene zoophycusEarly Miocene Zoophycus from basal Waitemata Basin mudstone (turbidite d,e interval), Matheson’s Bay, NZ.

 

 

 

Scolicia trace fossil                     Eocene Scolicia

Two examples of Scolicia. Left: basin floor fan, Mid-Miocene Mohakatino Fm, N Taranaki. Right: shelf, Iceberg Bay Fm. Axel Heibe4rg Island.

 

Thalassinoides trace                     Paleocene Thalassinoides

Thalassinoides, Left from submarine fan Mohakatino Fm. N Taranaki. Right: shallow shelf, Expedition Fm. Axel Heiberg Island.

 

Rhizocorallium trace fossil                      Cretaceous Rhizocorallium

Rhizocorallium, both images from Grizzly Gorge, Yukon

 

Jurassic RoseliaRosselia, possible sea anemone, from mid-Jurassic Bowser Basin, Tsatia Mt. British Columbia.

 

 

 

Paleocene SkolithosSkolithos, a worm or arthropod feeding or domicile burrow, Paleocene Expedition Fm, Axel Heiberg Island.

 

 

 

trace fossils and ripples                     ripples and trace fossils

Rippled bedding, with numerous small feeding traces, one Paleophycus, and the slightly raised mounds of abundant vertical Skolithis burrows. Left Paleocene Expedition Fm. Right Eocene Iceberg Bay Fm, Axel Heiberg Island.

Eocene PlanolitesRandom Eocene feeding traces, delta plain, Iceberg Bay Fm. Axel Heiberg Island. ?Planolites

 

 

 

Cambrian trace fossilsMultiple crossing trails in Cambrian shelf deposits on rippled sandstone, Cathedral Mountain, Alberta.

 

 

 

Ophiomorpha                     Paleocene Ophiomorpha from NZ

Ophiomorpha, with typical ornamented burrow lining. Both images from the Paleocene Wangaloa Fm, Otago coast, NZ.

 

Ophiomorpha burrowsPaleocene Ophiomorpha, Wangaloa Fm, Otago coast.

 

 

 

Ophiomorpha filled with volcanic ashOphiomorpha filled with volcanic ash, Ngarupuru Fm, North Taranaki

 

 

 

 

Miocene sting ray feeding holes                     Sting ray feeding holes Waitemata Basin

Ray jetting holes (feeding), in basal Waitemata Basin shallow shelf deposits (Lower Miocene).  The holes are filled with poorly sorted grit and shell fragments. Some holes overlap. The right image shows the density of holes over several square metres. Matheson’s Bay, Auckland.

 

Sting ray jetting holesAnother view of ray jetting holes (feeding), in Lower Miocene basal Waitemata Basin shallow shelf deposits, The Outpost, Auckland.

 

 

 

Modern Pholad borings                    Miocene Pholad borings

Left: recent Pholad borings into indurated greywacke.  Fragments of the bivalve shells remain. Diameter of the coin (top left) is about 25mm. Right: Lower Miocene Pholad borings into a greywacke sea-stack (eventually blanketed by turbidites) in basal Waitemata Basin shelf-beach deposits. The borings are mostly filled with mudstone. Matheson’s Bay, Auckland.

 

intense bioturbationIntense bioturbation has all but obliterated primary depositional layering, Paleocene, Expedition Fm. Ellesmere Island.

 

 

 

Chondrites burrowsAbundant Chondrites have completely churned this shelf deposit. Grizzly Gorge, Yukon.

 

 

 

calcite cemented burrowsExhumed, calcite-cemented burrows in  Permian calcarenite, Canon Fiord Fm, Vesle Fiord, Ellesmere Island. These critters enjoyed fairly high energy wave-current activity, indicated by grain-size and cross bedding.

 

 

modern bivalve feeding burrowsSmall sand mounds produced during bivalve (Paphies subtriangulata) feeding and water expulsion.

 

 

 

sand dune tracesAbundant invertebrate, and one hopping vertebrate tracks and trails on Death Valley dune sands.

 

 

 

Pleistocene sand dune burrowsNumerous sand-filled, unlined burrows in Late Pleistocene, vegetated sand-dune flat. Great Exhibition Bay, NZ.

 

 

 

Pseudo burrows in dunes                pseudo burrows in Pleistocene dunes

Pseudo-burrows, formed as reduction spots in Late Pleistocene dune sands. Reduction commonly occurrs around small plant fragments. The host sands are a brownish colour from iron mineral oxidation.  Note the primary dune laminae that pass through the spots. Late Pleistocene, Great Exhibition Bay, NZ.

unknown trace fossilA very pretty trace fossil, but at the time I saw it, of unknown affinity, in Point Lobos submarine canyon strata. If anyone knows more about this, let me know.

 

 

 

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Atlas of syntectonic sediments

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The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source as indicated below (it is the polite, and professional thing to do).  I retain copyright of all images presented herein

Brian Ricketts –  www.geological-digressions.com

This category is a bit different to the other Atlas collections. It does not refer to a specific environmental state, like fluvial or submarine fan, but to erosion, deposition, and deformation associated with active tectonics. This includes uplift, folding, faulting, the erosion of landscapes created by each of these, and subsequent deposition. Syn-tectonic deposits may be constrained in time to specific events (e.g. faulting), or to periods of mountain building, or other modes of deformation along plate boundaries. Classic examples include the Molasse of central Europe, and basins outboard of the Cordilleran fold and thrust belt in western Canada.

Most of the images here are inferred to have been associated with specific tectonic events. Conglomerate facies are common in fluvial and alluvial settings in close proximity to active faults and uplifts (Eurekan Orogeny in the Canadian Arctic, Alberta Foreland Basin, evolving transform faults in Ridge Basin, and active extension – strike slip faulting in Death Valley), to deep marine turbidites that were also influenced by active (Waitemata) basin tectonics. There’s also a few shots of coastal exposure of an active accretionary prism on New Zealand’s east coast.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

 

The images:

Diabase sills intrude Jurassic through Permian successions in the Arctic Sverdrup Basin. Unroofing of these older rocks during the Eurekan Orogeny (climaxing about mid Eocene) provided large volumes of coarse sediment to alluvial fans, braided and high sinuosity rivers. In these two examples the Stolz Thrust is at the base of slope, with tectonic transport to the right (east). Here, the older rocks have been thrust over the syntectonic deposits (Buchanan Lake Fm.). Axel Heiberg Island.

 

Stolz Thrust at Geodetic Hills (the site of the Middle Eocene Fossil Forest). Left: Diabase sills are thrust over syntectonic conglomerate. Right: Upturned and sheared Triassic rocks in the hanging wall; the fault trace is located in the depression (upper left).

 

Detail of shear and boudinage of Triassic sandstone-mudstone in Stolz Thrust zone, Geodetic Hills.  Location is the right image above.

 

 

 

Stolz Thrust, with Permo-Triassic rocks in the hanging wall (including slivers of anhydrite), over middle Eocene syntectonic conglomerate and sandstone (Buchanan Lake Fm.) North of Whitsunday Bay, Axel Heiberg Island.  Coarse-grained sediment was shed from the uplifted older rocks, and subsequently over-ridden by continued thrusting.

 

 

Intensely deformed anhydrite in the hanging wall of Stolz Thrust, Axel Heiberg Island. It is likely anhydrite debris was shed with the coarse sediment, but did not survive the first cycle of transport and deposition.

 

 

Left: Syntectonic conglomerate (Buchanan Lake Fm.) over-thrust by Ordovician limestone (that also contributed debris to the conglomerate), Franklin Pierce Bay, Ellesmere Island. Right: Syntectonic conglomerate-sandstone braided river deposits that accumulated outboard of faulted uplifts. Boulder Hills, Ellesmere Island.

 

Panorama of Jurassic-Triassic rocks above Stolz Thrust over syntectonic conglomerate at Geodetic Hills (Buchanan Lake Fm.), Axel Heiberg Island (left), and a compositional unroofing sequence in conglomerate (right). The lighter coloured deposits near the base of conglomerate were derived from Jurassic sandstones. the progressive change upward to darker brown conglomerate reflects access to deeper, older Triassic sandstone and diabase sills in the eroding hanging wall.

 

Aerial views of Middle Eocene, syntectonic alluvial fan – braidplain conglomerate outboard of thrusted uplands. Left: Emma Fiord, Ellesmere Island. Right: Geodetic Hills, Axel Heiberg Island.

 

Small thrust fault through proximal, bouldery, syntectonic conglomerate, Geodetic Hills, Axel Heiberg Island.  Hammer lower center. Boulders to 50cm wide.

 

 

 

Syntectonic boulder-cobble (mostly diabase) proximal alluvial fan deposits, with scattered sand wedges, Geodetic Hills, Axel Heiberg Island. At the time of deposition, they would have been close to the uplifted source rocks.

 

Thick, crudely bedded debris flows and sheet flood alluvial fan conglomerates, probably close to sediment source. Diabase clasts up to a metre wide. Middle Eocene, Geodetic Hills, Axel Heiberg Island.

 

 

 

Lower Paleozoic carbonates have been thrust over Upper Cretaceous foreland basin strata (approximately east-dipping bedding visible at top right), Kananaskis, Alberta Basin. The U. Cretacous units accumulated during an earlier phase of thrusting, farther west, and then subsquently over-ridden.

 

 

Left: older foreland basin deposits (Kootenay Gp), overlain by conglomerate, shed from a renewed phase of thrusting and folding (resistant units at top) – The Lower Cretaceous Cadomin Fm. interpreted variously as braidplain, alluvial fan, and pediment. Right: Trough crossbedded, pebbly sandstone, Cadomin Fm.

 

Interbedded conglomerate-sandstone, mostly as planar tabular crossbeds. Cadomin Fm. Mt Allan, Kananaskis.

 

 

 

Lower Cretaceous foreland basin strata involved in a later phase of thrusting. View is to the north of Highwood Pass. Lewis Thrust charges down the valley beyond. Front Ranges, Alberta Foreland basin.

 

 

Iconic views of the Front Ranges, Kananaskis. Left: Upturned Lower Paleozoic carbonates and sandstones, and in the valley, recessive Jurassic-Lower Cretaceous foreland basin strata. Right: Probably one of the most photographed fold pairs in Canada – Lewis Thrust terminates at the base of this fold pair. Kananaskis Highway.

 

The northern segment of Lower Miocene Waitemata Basin (Auckland) developed atop a moving slab of obducted lithosphere – the Northland Allochthon. The Allochthon, now fragmented, consists of ophiolite (including possible seamounts), marls, terrigenous clastics and limestones. Allochthon rocks, like those shown here (Algies Bay) commonly are intensely deformed. Movement of the Allochthon is implicated in some of the syn-sedimentary – weak rock deformation in Waitemata Basin itself. This view shows thrusted marls, north of Algies Bay.

Examples of intense shearing in Northland Allochthon marls and mudstones. Left: multiple generations of fracturing. Right: Boudinage and shear of siderite nodules in the mudrocks (above). Algies Bay, Auckland.

 

 

Sedimentary dyke through Northland Allochthon mudrocks. The dyke contains fragments of Lower Miocene Waitemata Basin sandstone and mudstone, attesting to the dynamic relationship between the two.  The dyke in turn is fractured by later deformation. Algies Bay, Auckland.

 

 

Examples of soft and weak-rock deformation – slumping in Waitemata Basin turbidites, possibly dynamically linked to Northland Allochthon deformation. Left: Thrust-folds near Waiwera. Right: Recumbent isoclinal folds, and rotated boudins in sandstone, Army Bay.

 

Intensely folded and faulted turbidites above an undeformed glide plane, south of Orewa Beach, possibly dynamically linked to Northland Allochthon deformation.

 

 

 

Violin Breccia, Ridge Basin, California. fault plane talus, and or debris flows, adjacent San Gabriel Fault, a Late Miocene splay of the evolving San Andreas transform. Breccia clasts are mainly gneiss. The breccia extends many km along the fault strand, but only about 2km down-dip into the basin.

 

Left: Lacustrine shoreface – delta sandstone, and stringers of Violin Breccia. Right: detail of the left image, showing crossbedded sandstone and grit-pebble sized material from the Violin Breccia. Ridge Basin, California.

 

Left: down dip view of dissected Panamint Range alluvial fan, Death Valley. The coarse fan deposits reflect erosion of the uplifted Panamint metamorphic core complex.  The fan canyon-head is shown in the right image.

 

Hole in the Wall, Death Valley. Here, lacustrine sands and muds contain sporadic debris flows (resistant unit). Right image shows debris flow scours. They accumulated during Miocene-Pliocene extension  that resulted in Death Valley basin subsidence. Subsequent deformation took place as the Furnace Creek strike-slip fault created an en echelon stack of fan deltas and associated lacustrine deposits.

 

Hole in the Wall, Death Valley. Discordant packages of lacustrine shoreface and prodelta mudstone-sandstone, and pebble conglomerate. The debris flow in the images above can be traced from the lower right to the central part of the cliff.

 

Hole in the Wall, Death Valley. Lacustrine silt and clay, in prodelta or basin floor. The right image shows small grit-filled scours from periodic influxes down the prodelta slope.

 

Coastal exposure of an active accretionary prism, Waimarama, eastern North Island. The accretionary prism here consists of telescoped slivers of sea-floor sediment, above Hikurangi subduction zone.  Left: Thrusts and associated shearing in bentonitic mudrocks, sandstones, and marls (arrows), looking north. Right: Looking south at similar lithologies, and the modern expression of sedimentation associated with the deformation – a cobble beach.

 

Closer view of thrusts and intensely sheared mudstone-sandstone melange, Waimarama, eastern North Island.

 

 

 

Sheared and stretched sandstone (left), and sheared bentonitic melange (right), within thin, accretionary prism thrust sheets, Waimarama, eastern North Island.

 

A lozenge of resistant cherty mudstone within the softer bentonitic melange, detached during thrusting, Waimarama, eastern North Island.

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Atlas of synsedimentary deformation

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The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source as indicated below (it is the polite, and professional thing to do).  I retain copyright of all images presented herein

Brian Ricketts –  www.geological-digressions.com

Deformation of sediment while it is soft or semi-consolidated, is common. The rock record is replete with folded and slumped strata, strata that slid in coherent packages, strata that lose their coherence during liquefaction or fluidization, displacement by faults where soft or plastic sediment seemingly acts like its brittle rock counterparts, or dyke-like injections where sediment is locally overpressured.

The term syn-sedimentary tends to be used rather loosely, as deformation that takes place during or soon after deposition; the ‘soon’ is the loose part of this broad definition. Sediment begins to compact almost immediately following deposition, where framework grains begin to move closer together.  Interstitial water is expelled, and this process in itself can deform the sediment. Water expulsion in compacting deeper strata can also increase local pore pressures that in turn reduce sediment shear strength. Other common triggers are gravitational instability  and seismic tremors. Coastal storm surges can also produce instability in sea floor sediments caused by rapid fluctuations in pore pressure.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

Five of the images shown in this tranche also appear in the Submarine fans and channels category; there are several more examples of soft- or syn-sedimentary structures on the Submarine fans page.

The images:

Sediment deposited rapidly will have a high water content.  Immediately following deposition and incipient compaction, excess water will be forced towards the sea floor.  Muddy sediment, like a turbidite will present natural permeability barriers to water expulsion, resulting in either deformation of laminae and other structures, or formation of dewatering pipes, or pillars.  Water escaping at the sea floor will deposit fine sediment that it has picked up along its trajectory, and deposit this as small mud-silt pimples or volcanoes.  This example (Omarolluk Fm, Proterozoic, Belcher Islands) shows the mud volcanoes on bedding, and in cross-section, thin whitish pillars that represent water escape routes – they are white because the muddy matrix has been removed.

Dewatering pillars can occur in sheets within single beds, in this case a Proterozoic turbidite. The position of sheets within a bed is probably associated with permeability barriers. Omarolluk Fm. Belcher Islands (1.8-1.9 billion years). The black patches are calcite concretions; pillars pass through the concretions.

 

Calcite concretions in turbidites, Omarolluk Fm, Belcher Islands, formed during very early stages of burial.  This very early stage of diagenesis was shallow enough for the concretions to be reworked in submarine channels.

Dish structures are another product of dewatering. Water that escapes though narrow conduits, will drag sand laminae upwards; laminae between adjacent dewatering pillars will appear concave upwards, or dish-shaped. Left is from Lower Miocene Waitemata Basin, Musick Point; Right is from the Rosario Group, San Diego.

Fluvial trough crossbeds here have been turned on end during early compaction and dewatering, producing what are commonly called ball and pillow structure.  Proterozoic Loaf Fm. Belcher Islands.

 

 

Ripples and bed contacts in this sandstone-mudstone unit have been deformed by liquefaction, and in places pulled apart.  Evidence from nearby strata indicates a possible seismic event has jostled these beds (see image below). Fairweather Fm, Belcher Islands, about 2 billion years old.

 

This sandstone dyke terminates in, and probably breached shallow intertidal deposits.  The surrounding layers have been dragged upwards during sand intrusion.  Structures like this  commonly form during earthquakes when soft sediment is liquefied.  Fairweather Fm, Belcher Islands, about 2 billion years old.

 

Detached load casts in laminated, locally rippled volcaniclastics. Flaherty (volcanic) Fm, Belcher Islands, about 2 billion years old.

 

 

 

A sandstone dyke that originated from deformed – slumped sandy turbidites; the dyke intrudes a slope mudstone-siltstone succession, and extends about 40m up the exposed face.  Both the slumping and sandstone intrusion are thought to have formed during a seismic event. Upper Jurassic, Tsatia Mt, Bowser Basin.

 

 

Dololutite beds deformed while in a plastic state, are interbedded with undeformed crossbedded (dolomitic) grainstone.  These folds probably formed as packages of sediment were moved during karstification. Some layers have detached from one another, forming voids that were eventually filled by aragonite-fibrous calcite – subsequently converted to dolomite. Both images from the Rowatt Fm, Belcher Islands, about 2 billion years old.

Detached folds in ice-contact glacial outwash. Elevated pore pressures and deformation were probably caused by ice loading. Late Pleistocene, Ottawa.

 

 

 

Classic folded, faulted, and detached turbidite beds, caused by sliding, slumping, syn-depositional faulting, and some liquefaction.  Left; Lower Miocene Waitemata Basin, Army Bay, Right: Lower Miocene Waitemata Basin Manly Beach, Auckland.

 

Classic folded, faulted, and detached turbidite beds, caused by sliding, slumping, synsedimentary faulting, and local liquefaction.  Left; Lower Miocene Waitemata Basin, Takapuna Beach , Auckland. Right: highway roadcut, Albany, Auckland.

 

Small, detached slump fold carried along the base of a turbidity current. Lower Miocene Waitemata Basin, Cockle Bay, Auckland

 

 

 

Detached slump folded dololutite-calcilutite. These thin carbonate beds were deposited on a Proterozoic slope (Costello Fm. Belcher Islands), outboard of really large, platform stromatolite reefs.

 

 

 

Folded, and possibly thrust faulted turbidites, above which are undisturbed beds.  Lower Miocene Waitemata Basin, Pukenihinihi Point, north Auckland.

 

 

 

Slump folds in Late Miocene Castaic Fm, Ridge Basin turbidites, probably initiated by a seismic events on the bounding San Gabriel strike-slip fault, that probably was an offshoot of the evolving San Andreas transform system. Left: a fold pair.  Right: folding, pull-apart, and small accommodation faults.

Folding and intrusion of liquefied sand in the Rosario Group, San Diego

 

 

 

Large recumbent slump fold in a Late Miocene, basin floor submarine fan, Mt. Messenger Formation, North Taranaki, New Zealand.

 

 

 

Seriously deformed slump unit in Late Miocene, basin floor submarine fan, Mt. Messenger Formation, North Taranaki, New Zealand. Strata below the detachment are not deformed.

 

Detail of deformation associated with slumping – here, boudinage and tight recumbent folds in dark brown sandstone layers. Late Miocene, basin floor submarine fan, Mt. Messenger Formation, North Taranaki, New Zealand.

 

 

The iconic ‘Jam Roll’ slump in Late Miocene, basin floor submarine fan, Mt. Messenger Formation, North Taranaki, New Zealand. Folded sandstone-mudstone almost completely encloses the structure.

 

 

Detail of liquefaction and dewatering structures in the Jam Roll slump. Left: a modest size mud volcano. Right: highly fluid mud layers that flowed during liquefaction. Late Miocene, basin floor submarine fan, Mt. Messenger Formation, North Taranaki, New Zealand.

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Atlas of sedimentary textures and fabrics

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The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source as indicated below (it is the polite, and professional thing to do).  I retain copyright of all images presented herein

Brian Ricketts –  www.geological-digressions.com

 

Texture in a rock describes the relationship of its components – grains, minerals, other chunks of rock – to one another.  In detrital sediments and sedimentary rocks, a distinction is made between clasts that form the framework (silt, sand, grit, pebble, cobble, boulder), and detrital sediment that is the matrix – matrix resides in the spaces between grains and usually consists of very fine-grained sediment, such as clays and silt. Detailed description of the matrix usually requires a microscope.

We can describe the framework in terms of the size (sand, cobble etc.) and shape of individual clasts (sherical, oblate, angular versus rounded), the proportions of different clast sizes (e.g. sorting), and the proportion of framework to matrix.  These are all useful descriptors of a sediment, but they can also provide valuable information on depositional processes, such as:

  • the degree of sediment reworking during transport (e.g. beach versus glacial diamitite),
  • depositional energy (e.g. river channel versus floodplain, beach versus estuary),
  • the removal, or winnowing of lighter, or hydraulically more buoyant mineral grains (e.g. micas), or
  • removal of mechanically less stable grains or minerals – for example quartz is mechanically more stable than feldspar because the latter usually has good cleavage.

Sedimentary fabric refers to detrital components that impart some kind of directionality to rock and sediment. It can be thought of as a vector, that has both magnitude (size, shape) and  direction (texture only has qualities like size, shape, proportion and so on). Thus, the alignment of clasts or fossils imparts a fabric (e.g. pebble imbrication in a channel, or current-aligned fossils).

Together, texture and fabric are important additions to a geologists toolbox, for description and interpretation.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then the ‘back page’ arrow to return to the Atlas.

 

The images:

Beach gravels (Left, Haumoana, NZ; Right, Fundy Bay, Nova Scotia: both have clast-supported frameworks of well rounded, oblate to platey pebbles and cobbles.  Their texture and lack of fine matrix are in keeping with the high wave energy along these coasts.

Beach storm ridge shell accumulations, clast (shell) supported.  The shells are predominantly gastropods, including abundant Struthiolaria.  There is no obvious preferred orientation of shells and shell fragments. Mangawhai Heads, NZ

 

 

Exhumed surface of a Jurassic, marine, debris flow. Pebbles of radiolarian chert are all well rounded, and most participate in a clast-supported framework.  Rounding of the pebbles must have taken place in a shallower water environment, probably fluvial.  This was probably a highly fluid debris flow. Bowser Basin, northern British Columbia.

 

 

Contrasting debris flow textures. Left; matrix-supported clasts of radiolarian chert and mudstone rip-ups – this was a very muddy, cohesive debris flow. Right; mixed last-supported and matrix-supported clasts in a less cohesive, more fluid debris flow.  Bowser Basin, northern British Columbia.

Bouldery debris flow, with mixed clast- and matrix-supported frameworks. Dana Point, California.

 

 

 

Lower Miocene volcaniclastic debris flow, mostly matrix-supported but pockets of clast-supported frameworks. Waitakere Volcanic arc, west Auckland.

 

 

Scanning electron microsope image of a moderately  well-sorted sandstone, Ellerslie Fm, Alberta.  Sand grains show varying degrees of depositional rounding.  The fuzzy surface of most grains is caused by incipient diagenetic clays (illite-kaolinite), i.e. clays formed by chemical reactions after deposition. This rock has excellent porosity and permeability. the image width spans 3mm.

 

Thin section micrographs of a lithic sandstone (arenite). Left: plane polarized light, showing individual grain shapes and intergrain contacts.  The blue areas are pore spaces filled with blue resin. Right: Crossed nicols, showing quartz in various stages of extinction, lithic grains (speckled), and minor potassium feldspar. Ellerslie Fm, Alberta. Most grains here are about 0.2 – 0.3mm across.

check out this and related posts for an explanation of polarizing microscopy

Thin section micrograph, under crossed nicols of a calcite-cemented lithic sandstone, Ellerslie Fm, Alberta.  The calcite has a yellow-orange colour.  Most grains here are about 0.2 – 0.3mm across.

 

 

The fusulinid foraminifera incorporated into these Permian sediments, have all been aligned approximately parallel to the local paleocurrents (determined from crossbedded calcarenites).  South Bay, Ellesmere Island.

 

 

Modern river gravels containing abundant platy rock fragments that are aligned by the prevailing currents. The imbrication here indicates flow to the right.

 

 

Parting lineation in laminated sandstone forms when the long axes of sand grains, in layers a few grains thick, are aligned parallel to current flow directions.  The streakiness, or lines form when sandstone splits along the laminae. They are thought to form during upper flow-regime plane bed flow (high energy flow). Paleoflow in this example was either to the right or left; a decision as to which direction can only be determined from unidirectional structures.

Stone rosettes form on beaches where there is a plentiful supply of platy rock fragments – in this case much older shale.  The flat clasts are turned edgewise by wave action, and commonly are organised into crude radiating or stacked patterns (also called edgewise conglomerate). Similar structures can form from bivalve shells.  On some beaches they form extensive pavements.  Somewhat similar structures have been reported from periglacial regions.

An example of ancient stone rosettes, or edgewise conglomerate forming extensive pavements on a Proterozoic beach (about 2 billion years old). In this example (Mavor Fm., Belcher Islands, Hudon Bay), the thin slabs consist of dolomitized lutite crusts, formed probably in a supratidal flat, and subsequently ripped up by storm waves.

Here is a paper on these examples: Ricketts, B.D. and Donaldson, J.A.  1979: Stone rosettes as indicators of ancient shorelines: examples from the Precambrian Belcher Group, Northwest Territories; Canadian Journal of Earth Sciences, v. 16, p. 1187-1891

Bedding cross-section views of ancient, Proterozoic stone rosette beach pavements.  The edgewise stacking of platy dololutite crusts is readily apparent.  A bedding view of the same structures is shown above.  Mavor Fm, Belcher Islands, Hudson bay.

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Atlas of submarine fans and channels

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Submarine fans and channels

The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source as indicated below (it is the polite, and professional thing to do).  I retain copyright of all images presented herein

Brian Ricketts –  www.geological-digressions.com

 

Beyond the slope (continental slope or delta slope) is the deep ocean floor, at depths usually measured in 100s to 1000s of metres.  Sediment that has bypassed the shelf is transported through submarine canyons and gullies by turbulent flows of mud and sand (turbidites), or debris flows that are capable of moving a much greater range of clast sizes, from pebbles to chunks of rock or dislodged sediment having dimensions in the 10s to 100s of metres. A lower sea floor gradient at the base of the slope, plus frictional forces along the sea floor and overlying water, causes these flows to decelerate. The sediment accumulates in submarine fans, that have dimensions measured in 10s to 100s of kilometres.

The earliest models of submarine fan construction and architecture in the late 60s early 70s (e.g. Walker, Normark, Mutti and Ricci Luchi), and the plethora of model variations since, are based primarily on reconstructions from the rock record, with a smattering of new, actualistic observations.  All these models have certain commonalities – in terms of their stratigraphic and geomorphic architecture, they contain elements of proximal to distal components of fan lobes, submarine channels, channel levees and overbank, and dislocation of slope, fan or channel sediment packages by slumping and sliding. Sediment dispersal is generally attributed to turbulent flows (turbidity currents),  debris flows (ranging from highly fluid to plastic), and grain flows (less common), against a background of normal oceanic traction currents and pelagic-hemipelagic sedimentation.  I have tried to illustrate as many of these attributes as possible in the images that follow.

Ancient submarine fan deposits illustrated here include: the Lower Miocene Waitemata Basin near Auckland, New Zealand; the Paleocene of Point San Pedro,  Upper Cretaceous Pigeon Point, and Dana Point successions, all in California; and Proterozoic examples from Belcher Islands (about 1800-1900 Ma).

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

 

The images:

                

Typical exposure of Waitemata Basin strata around Auckland coastal cliffs: Left; mid-fan turbidites at Takapuna Beach. Right; thick, proximal submarine fan-channel capped by thinning-upward overbank facies, north end of Goat Island Marine Reserve.

Waitemata Basin turbidites near the base of the succession, folded by compaction over paleotopographic highs on Jurassic-Permian metagreywacke basement. Omana Beach, south Auckland

 

 

 

Thick, proximal to mid-fan turbidites and possible channel overbank, Waitemata Basin, Goat Island Marine Reserve.

 

 

 

The thicker, upper unit is is a laminated Tb Bouma interval with mudstone rip-up clasts, and a partly eroded-disrupted Td interval at the top – traced laterally this unit becomes composite.  The thick mudstone beds are probably a combination of Td,e.  Takapuna Beach, Auckland.

 

 

                  

Turbidite beds, well developed Bouma Tb-d intervals, with oversteepened and convoluted ripple drift (Tc interval), Lower Miocene Waitemata Basin, Cockle Bay, Auckland.

A thin Bouma Tb layer (at the coin) is overlain by a thin, rippled Tc (just above the coin), that subsequently was eroded by a thin, but coarse-grained sandy flow that ripped up local mudstone slabs and wafers. The middle grey mudstone is mostly Te (hemipelagic) with small bottom-current ripples redistributing sand across a thin layer. Waitemata Basin, Cockle Bay, south Auckland.

 

Convoluted siltstone-fine sandstone, truncated by the next flow unit, in which there is a thin, gritty Ta interval. Waitemata Basin, Cockle Bay, south Auckland.

 

 

 

A composite flow unit with well developed Tb laminations (lowest), and near the top a scour surface formed by the succeeding flow.  Waitemata Basin, north end of Goat Island Marine Reserve.

 

 

 

Thick, coarse-grained laminated Tb interval, Musick Point, Auckland.

 

 

 

 

Thick Bouma Ta-b composites; most of the intervening, skinny Td mudstone (center) has been eroded.  Waitemata Basin, Cockle Bay.

 

 

 

Dewatering of this turbidite (during very early burial) is indicated the concave-up dish structures, and small synsedimentary faults that terminate just above the dish structures. Waitemata Basin, Musick Point,

 

 

Coalified wood fragment (outlined), intensely bored by Miocene Toredo-like marine worms, Waitemata Basin, Goat Island Marine Reserve.

 

 

 

                  

Very think, composite debris flows containing abundant pebbles, cobbles and boulders of basalt, and subordinate sedimentary and mafic igneous clasts. Interpreted provenance of the clasts varies between two extremes: an active, early Miocene volcanic arc on the western margin of Waitemata Basin; and more recently as debris from oceanic islands (see Shane et al, 2010, Geochemistry, Geophysics, Geosystems, open access). Left: Motuihe Island, Auckland. Right: an iconic outcrop at Waiwera, north Auckland. Lower flow units have large rafts of locally derived, deformed mudstone. The debris flow is overlain by thick, proximal fan turbidites.

                  

Mixed matrix-supported and some clast-supported textures in Waitemata Basin debris flows. Left: Waiwera (same as the left image above); Right: Karekare, Auckland west coast.

A massive raft of columnar-jointed basalt, a remnant of either a lava flow of dyke from an oceanic island somewhere west of the basin. The weight of the block and compaction have pushed it into the underlying turbidite beds. Waitemata Basin, Army Bay, Auckland.

 

 

Probably the most photographed slump fold in Waitamata Basin, Army Bay. The recumbent structure is detached from strata below along a relatively undisturbed glide plane.  The lower limb is also cut by small faults.

 

 

                 

Left: Classic slump folded turbidites, confined to a specific interval; strata above and below are relatively undeformed.  Fold sandstone limbs are partly detached or pulled apart, and some mudrocks have been fluidized,  Waitemata Basin, Takapuna, Auckland. Right: Broken soft-sediment fold, with partially fluidized mudrock below the central detached limb.  Waitemata Basin, Little Manly Beach.

Isoclinal folding in thin-bedded mudstone-sandstone (left center), and a sandy turbidite bed deformed by rotated boudins (upper right). All these structures formed while the sediment was at a transition from relatively soft to weakly indurated. Waitemata Basin, Army Bay, north Auckland.

 

 

Soft sediment deformation in Waitemata Basin, includes small thrusts (fault plane indicated by arrows), with folded strata in the hanging wall, and small drag folds in the footwall.  Waiwera, north Auckland.

 

 

Intensely folded turbidites on a horizontal, undeformed glide plane, Waitemata Basin, Orewa Beach, Auckland.

 

 

 

Paleocene turbidites, Point San Pedro, California.

 

 

 

 

                  

Left: Successive cycles of thinning upward and thin bedded, distal fan turbidites, Point San Pedro, California.  Right: Cyclic, thinning upward interchannel facies, Paleocene Point San Pedro, California.

Small slump package in thinly bedded distal fan facies, Point San Pedro, California.

 

 

 

Submarine channel sandstone overlain by thin sandy turbdites and overbank mudstone. Point San Pedro, California.

 

 

 

Thick submarine fan channel and overbank, Point San Pedro, California.

 

 

 

                  

Classic outcrops of pebbly mudstone – matrix-supported debris flows, that probably accumulated in proximal fan channels. Upper Cretaceous Pigeon Point, California.

A variation on the debris flow theme, with well stratified conglomerate and commonly clast-supported frameworks, that are inferred to have formed from more fluid flows than their pebbly mudstone counterparts. Upper Cretaceous Pigeon Point, California.

 

 

A broader view of stratified, possibly surging debris flows in proximal fan channels. Upper Cretaceous Pigeon Point, California.

 

 

 

Slump folded, and partly fluidized turbidites in Upper Cretaceous Pigeon Point, California.

 

 

 

Thin Bouma Tb-c flow units, Pebble Beach, California. the middle unit has developed some excellent flame structures. the lower unit contains sand-filled burrows, and detached load casts.

 

 

Dish structures and pillars indicating dewatering (fluid expulsion) during early burial by the overlying sandy turbidites. Rosario Group, San Diego.

 

 

Stacking of sandstone and conglomerate-filled submarine channels in the Miocene Capistrano Formation, Dana Point, California.

 

 

 

                  

Submarine channel sandstones and overbank facies exposed at Wheeler Gorge, California.

 

                   

Bedding style in the Omarolluk Fm. turbidite succession, Proterozoic, Belcher Islands (about 1800-1900 Ma). On the left, mid fan channel sandstone and overbank; on the right more proximal sandstone facies.

A paper on the Omarolluk Formation: Ricketts, B.D.  1981: A submarine fan – distal molasse sequence of Middle Precambrian age, Belcher Islands, Hudson Bay; Bulletin Canadian Petroleum Geology, v. 29, p. 561-582.

Channel overbank facies containing thin graded sandstone, thin sandstone beds with ripples and starved ripples, and Bouma Td-e mudstones. Omarolluk Fm. Proterozoic, Belcher Islands

 

 

 

Four incomplete Bouma cycles, each Tb with thin Tc.  The whitish patches are very early diagenetic concretions.  Omarolluk Fm. Proterozoic, Belcher Islands.

 

 

 

Thin Bouma Tc-d mid-fan cycles, with ripple drift, flame structures, and a small scour. Omarolluk Fm. Proterozoic, Belcher Islands.

 

 

 

A Bouma Tb-c cycle with  well developed and oversteepened ripple drift, overlain by a thicker Tb cycle with only a thin Td cap. Omarolluk Fm. Proterozoic, Belcher Islands

 

 

 

 A view of Bouma Tc-d intervals and convoluted laminae. Omarolluk Fm. Proterozoic, Belcher Islands

 

 

 

 

                  

Sole structures beneath sandy turbidites. On the left, flute casts are superposed on grooves. On the right, large flute casts are slightly deformed (block is about a metre across). Omarolluk Fm. Proterozoic, Belcher Islands.

Large flute cast, paleoflow to top right. Omarolluk Fm. Proterozoic, Belcher Islands

 

 

 

 

                  

Dewatering of turbidites, soon after deposition, produced thin fluid-escape pillars (left, cross-section view), and on bedding planes, small sand-mud volcanoes. Right image is a bedding view. Omarolluk Fm. Proterozoic, Belcher Islands

 

                                

Oblique views of thick Bouma Tb units, and sheets of dewatering pillars formed during very early burial and compaction. Segregation of sheets through the sandstones is a function of different permeabilities between successive flow layers.  Dark globular shapes on left image, and white patches in the middle image, are early diagenetic calcite concretions (see images below). Omarolluk Fm. Proterozoic, Belcher Islands

                  

Left: Proximal submarine channel conglomerate consisting almost entirely of reworked calcite concretions. Right: Detail of the channel conglomerate clasts. Elongate clasts are concretions that formed in laminated and rippled Tc intervals; the ovoid and spherical concretions are coarser grained and formed in Ta or Tb Bouma intervals. Omarolluk Fm. Proterozoic, Belcher Islands.

Mostly Td intervals in this view, with significant detachment of convoluted-folded very thin sandstone beds. Subvertical, wrinkled conduits, 2-3 mm wide, are dewatering pillars formed by escaping fluids during early compaction.  These units are associated with inter- lava flow turbidites in the volcanic Flaherty Fm, Proterozoic Belcher Islands (Flaherty volcanics overlie the Omarolluk Fm.),

 

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