Tag Archives: debris flow

Atlas of volcanoes and volcanic rocks

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Volcanoes and the products of volcanism

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

Volcanoes shape our earth: landscapes, the air we breathe, the oceans.  From day one they have had a direct influence and impact on life itself.  They still occupy a central place in our daily lives. Recent events in Hawaii (fissure eruptions, May-June 2018) and the explosive eruption of Fuego with its destructive pyroclastic flows (Guatemala, June 2018), have killed local inhabitants and destroyed property. Social media makes sure that we are kept up to date with these events; the fourth estate makes sure we are regaled with the death and mayhem.

I have never seen an actual eruption; the inquisitive, scientific part of me would love to witness one (at a safe distance). On my doorstep are three extinct stratovolcanoes 2-3 million years old. An hour and a half drive south puts me smack in the middle of the Taupo Volcanic Zone that, historically and prehistorically, is prone to cataclysmic, explosive fits. The same travel time north and I’m in the Auckland volcanic field, the most recent eruption of which was about 600 years ago. But I have seen the products of volcanism – the quiet effusive type, explosive and cataclysmic events; some as old as 2 billion years, others much more recent.

The collection of images here is a sample of these events. Some I have published, others just visited. I include this category in the Atlas because volcanic edifices and eruption products have a significant impact on sedimentary basins, and provide large volumes of sediment that ultimately are distributed throughout terrestrial drainages systems and marine environments; volcanoes contribute to sediments and sedimentary environments.

There are lots of good sites detailing volcanoes and eruptions. One of my favourite sites for volcano updates is Voices of Volcanology – Facebook @VoicesofVolcanology

Its contributors also provide a rational response to some of the silly, sometimes dangerous media hyperbole.

The USGS also provides media, scientific, and activity alert updates for many volcanoes around the world. One that I keep tabs on (for obvious reasons) is the NZ volcano scene

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas. Use the following links to jump to specific image sets:  Precambrian  Arctic

New Zealand  Hawaii  Haleakala

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 Atlas categories.

 

The images:

Paku Rhyolite dome

Paku rhyolite dome, guards the entrance to Tairua Harbour and estuary, east Coromandel Peninsula, New Zealand. The Paku eruptive center was part of widespread rhyolitic and dacitic volcanism from Late Miocene to Early Pleistocene in the Coromandel Volcanic Zone. They are part of the Minden Rhyolite Group. Paku is about 7-8 million years old (Pliocene). The Paku hypersthene-hornblende-biotite rhyolites here are beautifully flow banded and spherulitic,

             

 

             

 

            

 

Garibaldi Volcanic Field

The Garibaldi and Garibaldi Lake volcanic fields contain some of the youngest eruption products in Canada; most of the activity is about mid-Pleistocene through Holocene.  Most common are calc-alkaline andesites and dacites, with some basaltic rocks, associated with subduction of the Juan de Fuca Plate. They are located 60-70 km north of Vancouver. Mt. Garibaldi is a stratovolcano, with foundations as old as 1.3 Ma; Black Tusk also has old volcanic foundations, but its present form is due to much younger activity.  Late Pleistocene activity took place beneath the western segment of the Laurentide-Cordilleran Ice Sheet.

Herein are a few images of Garibaldi, Tusk, and Garibaldi Lake (the latter is a really nice hike).  Images of lava flows are from exposures along Highway 99 between Squamish and Pemberton.

                 

Lake Garibaldi. Sub-glacial andesite flows in the foreground

 

             

Black Tusk is the remains of an eroded stratovolcano whose foundations erupted about 1.3Ma. Rocks forming the present pinnacle were erupted about 200,000 years ago. This dome was subsequently eroded by the Cordilleran Ice sheet.  Left: from the Callaghan River access road. Right: From Brandywine Falls lookout.

 

          

A tiered, columnar jointed flows overlying Coast Mountain granodiorite. About 10 km north of Squamish, on Highway 99

 

          

Jointed andesitic-dacitic overlying pre-existing, sub-glacial topography on eroded Coast Mountains granodiorite and metamorphic rocks. The contact is shown in red on the left image. Right image: detail of the bedrock, that here contains structurally interleaved and sheared granodiorite and amphibolite. About 10km north of Squamish, Highway 99.

 

                       

 

About 15 km north of Squamish on Highway 99; this jointed lava flow has over-ridden dacitic agglomerate. The flow base is lined with abundant vesicles, most of which have been stretched approximately parallel to the basal flow contact.  The flow base is highly irregular, possibly following sub-glacial erosional topography.

 

                           

Same lication as above. The basal agglomerate here appears to be part of the original flow unit, where the lower cooled lava has been fragmented by the moving flow. Some fragments have been reincorporated into the base of the solid flow.

Precambrian volcanics

This set of images is from the Proterozoic Flaherty Formation, Belcher Islands. A small number of Rb/Sr and Pb-Pb ages indicate between 2.0-1.9 billion years. The Flaherty succession consists of interleaved tholeiitic to subalkaline basalt flows, pillowed lavas, pyroclastic and/or density current ash flows, and interflow turbidites, that accumulated mostly in a marine environment, although edifices may have extended above sea level. Metamorphism is prehnite-pumpellyite grade, and preservation of primary volcanic and volcaniclastic structures is excellent.  The volcanics were extruded on Proterozoic continental crust. However it is still uncertain whether they represent some kind of mantle plume, or formed during extension associated with crustal flexure, or are a hybrid of continental arcs.

              

Jointed lava flows and wrap-around piles of pillow lavas on Flaherty Island. Outcrops often permit the 3-dimensional geometry of flows and pillows to be mapped.

An interpretation of interleaving flows and pillowed lavas, and associated volcaniclastics. The figure is from a 1982 paper: Volcaniclastic rocks and volcaniclalstic facies in the Middle Precambrian (Aphebian) Belcher Group, Northwest Territories, Canada. Canadian Journal of Earth Sciences, v. 19, p. 1275-1294. 

 

 

              

Spheroidal, lobate, and lozenge-like pillow lavas. Left: the main pile is overlain and partly amalgamated with a second pile (top to the right), separated by a layer of glassy fragments formed by thermal shock. West Flaherty Island. Right: Oblique section through pillows, shows chilled margins with small vesicle traces (formerly gas bubbles).

 

                

Surface structures on pillows. Left: small ropy festoons. Right: crudely polygonal crusts formed by rapid cooling in the Precambrian ocean.

 

              

Left: Lozenge-shaped pillows with central flat-bottomed gas cavities, now filled with coarse calcite and prehnite crystals. Right: Filling of a pillow interstice with a layer of shards formed by thermal shock, overlain by coarse calcite. Look closely and you will see the first generation calcite grew in radial clusters on the pillow margin.  Both structures (also called geopetal structures) are useful for determining stratigraphic way-up (top).

 

             

Small lava tube that acted as a feeder to the pillow lavas. Lava in the tube must have drained, leaving an open tube that subsequently was filled by very coarse calcite crystals.

 

A recent lava tube breakout on Kamokuna lava delta, November 2017, Hawaii. This example is on land, but serves as a useful analogy to the submarine lava tube in the image immediately above.

 

 

 

              

Two perspectives of the tops of lava flows. Left: A bedding view (looking down on the flow top) of brecciated, folded and generally squished basalt fragments that formed as the flow cooled, and were carried along the top of the flow. The oblique grooves across the rock face were formed by glacial scouring.  Right: Cross-section through a lava flow; the upper part is completely broken up by movement as the lava solidified. More coherent, less fragmented rock occurs below this (lower right), was deeper in the flow and cooled more slowly.

 

More fluid lava flows formed ropy festoons. Here, flow direction was approximately to the lower right.

 

 

 

              

The base of a lava flow will also cool more quickly, especially if sediment or rock beneath is wet. Cooling here has resulted in rapid expansion of gas bubbles, (probably steam) that have risen into the flow forming elongated, sub-vertical, worm-like cavities, or spiracles. On the right, a larger gas bubble has risen above the flow, and subsquently evolved gas into small cavities in a crude radial pattern (arrow).

 

Larger gas cavities (vesicles) also form within the flow masses, as volcanic gas (water, CO2, SO2) separates from the melt. Here, the cavities are surrounded by much smaller vesicles.

 

 

 

Upper contact of a massive sediment gravity flow. Framework clasts up to a metre are mostly volcanic, but there are a few ripped up sandstone and dolomitic sandstone clasts, presumably from units underlying the Flaherty Formation. Maximum thickness is 85m; the unit covers and area greater than 3000 sq km. There are no obvious depositional or erosional breaks within the unit; there are parallel laminae, and rare crossbeds. The framework is mainly pumice fragments, shards with bubble-wall texture, and lapilli (see photomicrographs below). Pumice generally increases towards the top – density grading, but no obvious size grading. It is not welded.  It is sandwiched between pillowed and jointed lava flows.  It was most likely deposited subaqueously, although it is also possible that the flow originated on a subaerial edifice. Parallel laminae may indicate a degree of surging.  The irregular surface patterns are due to late diagenetic silica replacement of the original volcanic glass.

 

                               

 

Photomicrographs from the massive volcaniclastic flow. From the Left: 1. pumice with elongate gas pores, and various shards (glass replaced by silica). Field of view 8 mm.  2 Shards and lapilli. Both cements are coarse calcite, that has also replaced some matrix. Field of view 8 mm.  3. Bubble-wall ash shards, pumice and matrix in finer-grained part of the flow unit. Field of view 3 mm. 4 Fine, distal flow unit, predominantly bubble-wall shards, some pumice, and basalt fragments.  Field of view 8 mm.

 

              

Inter-flow volcaniclastic turbidites, layers of air-borne ash into water, and hemipelagic mudrock and ash. Right: detail of thin graded volcaniclastic turbidite Tb-d beds, hemipelagic ash, and highly calcareous mudstone – the latter indicating periods of very slow clastic deposition.

 

              

Left: Laminated and rippled volcaniclastic sandstone-siltstone. The middle bed has several load casts. Right: Volcaniclastic sandstone with ripples and climbing ripples. Bedforms in the upper orange-brown layer resemble antidunes.

 

              

Left: Graded volcaniclastic turbidite (Tb-d divisions), in erosional contact with a (steel grey), muddy limestone calcareous mudstone. Right:Laminated and convoluted volcaniclastic Tc-d turbidite intervals. The middle graded bed truncates  the convolutions below.

 

              

Left: Photomicrograph of turbidite volcaniclastic sandstone. Clear bubble-wall textures, some pumice, and crude alignment of shard long directions. Field of view 3 mm. Right: Prehnite in very fine-grained basalt. Field of view 3 mm.

 

Arctic

              

Strand Fiord volcanics, an Upper Cretaceous unit on Axel Heiberg Island, Canadian Arctic. Here an ‘organ-pipe’ array of columnar joints (4m high) have perforated the basalt during cooling. On the right, a bird’s-eye view of 4, 5 and 6-sided columns.

 

              

Curved and radiating entablature joints in Strand Fiord basalt flows, Axel Heiberg Island. Structural dip here is about 60 degrees. Flow thickness is about 35m. The entablature on the left is underlain by bed-normal jointed basalt.

 

              

Left: Cow pat ballistics in bedded lapilli and reworked basalt gravel, attests to eruption material mixing directly with fluvial reworked debris. The right image shows a coalified tree fragment caught up in a lahar. Strand Fiord volcanics, Axel Heiberg Island.

 

              

The Mount Edziza volcanic field in Stikine, northern British Columbia began 7.5 million years ago, and continued into the Pleistocene. It features large stratovolcanoes, calderas and cinder cones. Featured here is a stairway of jointed basalts, and a well preserved cinder cone.

 

New Zealand

                            

 

Upper Miocene Coroglen volcanics north of Whiritoa,  east coast of Coromandel, New Zealand. From the left: 1. Cooling joints in ignimbrite. 2. Detail of ignimbrite fragmental pumice and rhyolite, and fiamme (dark fragments stretched during hot emplacement of the pyroclastic flow).  3. Flow banded rhyolite. Outcrop is about 5m high.  4. A lahar with fragments of rhyolite, ignimbrite and possible andesite, over-ridden by a flow-banded rhyolite.

 

              

Late Pliocene flow-banded rhyolite at Mount Maunganui, New Zealand.

 

                              

 

Karioi volcano, Raglan New Zealand.  A Late Pliocene stratovolcano that during eruptions and later formation of lahars, dipped its toes in Tasman Sea. Three images from the left show an upper andesite flow, over airfall lapilli tuffs, and at base a laharic breccia.  On the right, detail of lapilli tuffs with a larger ballistic clast (20cm long).

 

                            

 

Stream reworking and lahars from the flanks of an active Karioi volcano, delivered pebble to boulder sized debris to the Late Pliocene coast. Evidence for this is seen in pockets of fossil molluscs (mostly bivalves) – modern bivalves commonly are wedged between clasts – these are not fossils. Rounding of clasts probably took place in streams and during coastal reworking. A few large boulder rafts are 3-4m across.

 

Auckland volcanic field contains about 50 known eruptive centres; scoria cones, lava flows,  and craters formed by highly explosive, phreatomagmatic eruptions, concentrated in an area of metropolitan Auckland, about 360 sq km. Activity began about 250,000 years ago; the youngest is Rangitoto which erupted 600 years ago, and was witnessed by Auckland Maori.

             

Eruption of Rangitoto in Auckland Harbour produced a circular island. The central scoria cone is flanked by aa basalt flows, air-fall scoria and lapilli. In the left image, In the left image (viewd from Musik Pt.), Motukorea (also known as Brown’s Island) is a small cone, thought to have erupted 7000-9000 years ago. Right image from Motuihi Island.

 

                           

 

Rangitoto features: From the left: Blocky and rubbly aa flows; a lava cave, and a profile of weathered lapilli.

 

                          

Explosion craters, or maars. Lake Pupuke, now filled with a fresh water lake is more than 45,000 years old. Orakei Basin (centre) is tidal – more than 83,000 years, and Panmure Basin (right), also tidal, is older than 17,000 years. The crater entrance is a narrow rocky channel, connected to Tamaki estuary – this is the Pacific Ocean side of Auckland. The stretch of water in the distance is part of Manukau Harbour – the Tasman Sea side of Auckland. A narrow isthmus separates the two ocean water bodies.  All three craters are rimmed by tephras. Lake and tidal basin sediment fill can also be used to date the events, using pollen and ash layers derived from more distant eruptions in Rotorua-Taupo.

 

                          

 

           

This group of images shows well bedded air-fall tephra layering near the outer rim of Maungataketake maar volcano, bordering Manukau Harbour, west Auckland. It is close to Auckland International Airport. Age is uncertain but could be as old as 177,000 years. Layers were also disrupted by ballistics (bomb sag, 3rd from left).  The eruption apparently obliterated a nearby forest – ash layers drape a stump (4th from left), and in the modern shore platform, large tree trunks were flattened.

 

                         

 

Mt Taranaki (Egmont), is an andesitic stratovolcano that began erupting 130,000 years ago. It is  regarded as active. The edifice is surrounded by a spectacular ring plain, dotted with lahar mounds, testament to the destructive nature of volcanoes even when not erupting. The lahars were generated on the volcano flanks during large landslides, including possible sector collapse; loose rock, mud and water form fluid, fast-moving flows. They can travel many kilometres from their source.  Conical lahar mounds commonly form around very large rafts of rock (sometimes many metres across), once the flow has come to rest.  Images on the right show typical, poorly sorted lahar textures.

 

                           

West of Auckland city, Waitakere hills are underlain by Early Miocene andesitic and basaltic flows, pillows, and debris flows, derived from volcanic centres off the west coast of New Zealand. Marine fossils are commonly caught up in the lahars and debris flows. Typical debris flow textures shown on the left; centre cliff with many stacked debris flows) boulders up to 1.5m wide), intruded by a dyke – Karekare Beach. Right image shows two, very large lava tubes, filled with radially jointed andesite (Maori Bay), feeders from nearby Waitakere Volcano.

 

                           

Conical mounds on Mamaku Plateau, north of Rotorua, look superficially like lahar mounds. However, in this case the mounds formed from a hot ignimbrite (240,000 years ago) with preferential cementation and welding by hot gases.  Subsequent erosion has left the harder zones upstanding (also called Tors).  The image on the right shows this hard, central zone. (located on State Hwy 5)

 

                         

A Mamaku Plateau mound with central cemented, resistant hard core (a remnant of the Mamaku Plateau Ignimbrite), draped by air-fall tephra.

 

             

Mt Ngauruhoe (aka Mt Doom) is a stratovolcano, and the youngest component of the Tongariro volcanic complex. It may be as old as 7000 years. It is made up of andesite flows, scoria and ash. Several young lava flows extend into adjacent Mangatepopo Valley.

 

             

Left: Typical scoriaceous aa flow in Mangatepopo Valley. Right: Looking down the path of a pyroclastic flow generated during the 1975 eruption. The flow contains a chaotic jumble of scoria and ash, with blocks up to 2m across. In place there are distinct rubbly levees. The flow rode over part of an older lava flow – the darker grey area, upper middle of image.

 

Hawaii

              

Halemaumau crater, Kilauea as it was in 1983. It looks very different now!

 

             

Left: Lava tree, cast during a 1790 eruption. Right: A nice sea arch eroded into layered flows.

 

             

Left: Spatter on a small lava tube vent, Kilauea. Right: Pelee’s Hair – volcanic glass that has been stretched while molten.  The preservation potential of such structures is very low.

A small flow with ropy central portion and raised levees of fragmented basalt.

 

                          

 

Ropy lavas, festooned, twisted, knot-like, and folded. Kilauea

 

Haleakala crater, Maui

A large stratovolcano that last erupted about 500 years ago

                          

From the summit (a little over 3000m) climb down to the crater floor to a parched landscape of cinder cones, crunchy lapilli underfoot, and a myriad ballistics. The crater walls (centre) exposes multiple episodes of dyke intrusion that fed the overlapping scoria mound conduits.

 

                          

Ballistics: left is a flattened cow pat splatter. Centre a spindle bomb, twisted in flight. Right bread-crust cooling pattern on a flattened bomb.

 

Hardy Silver Swords. Spikey

 

             

Oxidised air-fall lapilli and ash layers on older flanks of the Haleakala Volcano

 

 

 

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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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Lahars; train-wreck geology

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Christmas morning in New Zealand is synonymous with mid-summer barbecues at the beach, deservedly lazy times, perhaps a bit of over-indulgence. That morning, in 1953, Kiwis were expecting to awaken to news of the Royal tour; the newly crowned Queen was doing the rounds of towns and countryside, perfecting that royal wave to flag-waving folk lining the streets. Instead, they awoke to the news of a train disaster near Mt. Ruapehu, one of three active volcanoes in central North Island; a railway bridge on Whangaehu River, near Tangiwai, had been washed out on Christmas Eve.  Train carriages were strewn along the river banks, 151 people were killed.  The culprit was a geological phenomenon known as a lahar. Continue reading

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Submarine landslides; danger lurks in the ocean deep

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Source: Newfoundland and Labrador Heritage Website. You can watch this short video on Youtube

Five pm, November 18 1929 in the sleepy fishing village on Burin Peninsula, Newfoundland (at that time Newfoundland and Labrador were a Dominion of Britain.  They did not become part of the Canadian Federation until 1949).  Most people felt the tremors from the Grand Banks 7.2M earthquake, centered about 260km south of Burin but apparently went about their business as usual.  About 7.30 the same evening, there was a sudden drop in sea level, exposing the local shore and stranding boats.  The follow-up was totally unexpected – three massive waves inundated coastal dwellings, killing 28 people and leaving hundreds homeless.  The waves were 3-7m high in most places, but along some narrow inlets the tsunami energy had focussed into 27m-high monsters.  The tsunami was caused not by the earthquake itself, but by a massive submarine landslide. (Check out some images here). Continue reading

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