Classical Turbidites Research Articles

Stratigraphy and Structure of Early Tertiary Orca Group, Prince William Sound, Alaska: ABSTRACT

The late Paleocene-early Eocene Orca Group records much of the depositional and deformational history of the Tertiary accretionary prism found in the subduction zone of the Pacific plate in south Alaska. The thick (?5,000+ m) quartzo-feldspathic sandstones and interbedded shales were deposited as a submarine fan complex. Paleocurrents show derivation of detritus from a northeastern source. Three facies belts are distinguished. A northwestern belt characterized by massive sandstones and strong deformation is thrust southeasterly over a central melange belt containing originally interbedded basalt and turbidites deposited on an ophiolite basement. To the southeast, the third belt comprises a classic turbidite sequence which may have been deposited upon imbricated offscrapin s of pelagic Upper Cretaceous sedimentary rocks now exposed only in one faulted outcrop on southeast Montague Island. The predominant structures of the Orca Group are steeply northwest-dipping thrusts and folds overturned to the southeast in harmony with northwesterly subduction of the Pacific plate. However, on Montague Island the neotectonic southeasterly overturned structures overprint early major northwesterly overturned folds. Along strike to the northeast, northwest-trending cross structures overprint the early folds. Structural analysis indicates that the structure of the late Tertiary offshore basin is complex. End_of_Article - Last_Page 722------------

Storm-dominated shallow marine deposits: the Fernie–Kootenay (Jurassic) transition, southern Rocky Mountains

The transition from the Passage Beds of the Fernie into the lowermost part of the Kootenay Formation is exposed in 11 sections in the Crowsnest Pass to Banff area. Six distinct facies can be defined. Facies A (thin bedded turbidites) and facies B (thicker bedded turbidites) both have sole marks indicating north-northwestward flow. Facies C is the most important and consists of interbedded sandstones and shales, with sole marks indicating north-northwestward flow, and "hummocky cross stratification" as the characteristic internal sedimentary structure. It is interpreted to be formed by storm waves in depths below fair-weather wave base. Facies D contains low angle intersecting sets of parallel lamination (beach) and facies E is characterized by trough cross bedding (fluvial). Facies F consists of lenticular sandstones and shales, with coals. The facies occur in the sequence listed and indicate a northward prograding beach complex. Following intense storms, water driven toward the beach surges back seaward, entraining sand and developing into a density current. If deposition from the density current takes place in depths stirred by the storm waves, hummocky cross stratification is formed. In slightly deeper, quieter water the density current deposits a classical turbidite. Thus, the eastward-prograding Kootenay "delta" of previous interpretations now appears to be a northward-prograding beach complex fed by rivers, but with sand transported alongshore by waves. There is no evidence of tidal current activity, and shallow marine deposition is dominated by storms.

Deep-Water Sandstone Facies and Ancient Submarine Fans: Models for Exploration for Stratigraphic Traps: DISCUSSION

Five main facies of deep-water clastic rocks can be defined: classic turbidites, massive sandstones, pebbly sandstones, conglomerates, and debris flows (with slumps and slides). The classic turbidites consist of monotonously parallel-interbedded sandstones and shales without channeling; internal sedimentary structures include grading, parallel lamination, and cross-lamination. Massive sandstones are thicker, coarser, and commonly channelized. They lack the sedimentary structures of classic turbidites, but do contain evidence of dewatering during deposition. Pebbly sandstones tend to be well graded, and can contain parallel stratification and large-scale cross-stratification. Conglomerates are characterized by inverse and normal grading, parallel and cross-stratification, nd commonly have a preferred clast fabric (imbrication). Both the pebbly sandstones and conglomerates commonly are channelized. The facies can be fitted into a model of submarine-fan deposition. Modern fans are subdivided into an upper fan (suprafan), characterized by (1) a single deep channel with levees, (2) a middle fan, built up from suprafan lobes that periodically switch in position, and (3) a topographically smooth lower fan. The suprafan lobes have shallow, braided channels on their inner parts, but the outer suprafan lobes are smooth, and grade basinward into the smooth lower fan and basin plain. The smooth suprafan lobes and lower fan are characterized by deposition of the classic turbidite facies, and the braided part of the suprafan lobes by massive and pebbly sandstones. When one lobe is abandoned and another starts to prograde elsewhere, the first lobe is blanketed by mud, forming a potential stratigraphic trap. The upper-fan channel is an area of coarse sediment deposition, or conglomerates where gravel and boulders are supplied to the basin. During fan progradation, thickening- and coarsening-upward facies sequences can be formed in a manner analogous to those of deltas. Fan channels also can be abandoned progressively, forming thinning- and fining-upward sequences similar to those of fluvial or distributary channels. These sequences can be identified on electric logs. Where basin shales act as hydrocarbon-source areas, the classic turbidites can act as conduits, leading the hydrocarbons to the thicker, laterally coalesced massive and pebbly sandstones of the braided suprafan lobes. These bodies can be of the order of 25 km in diameter, and up to 100 m thick. The coarse deposits of the upper-fan channel also might form good reservoirs, being bounded by shales (levee deposits) on either side, and possibly by shales above if the fan-channel system is abandoned. Such channels can be tens of kilometers long, several kilometers wide, and a few hundred meters deep. Reservoirs may be present in all of these environments.

Storm-Wave Genesis of Bioclastic Carbonates in Upper Jurassic Epicontinental Mudstones, East-central Wyoming

ABSTRACT The offshore mud facies of the Late Jurassic (Callovian-Oxfordian) North American epicontinental seaway is represented in east-central Wyoming by the Redwater Shale Member of the Sundance Formation. This member contains stratigraphically discontinuous concentrations of carbonate skeletal debris and fossiliferous mudstone within an otherwise siliciclastic lithologic association. Individual limestone beds thin and thicken laterally and range in thickness from a few centimeters to several tens of centimeters. Detailed petrographic analysis reveals several distinct carbonate depositional textures; grainstones (containing no mud-sized matrix grains other than remnant grain-sheltered patches), packstones and wackestones. These carbonate textures indicate multiple modes of formation. Fossil rains similar to those from carbonate lithologies occur, in much lower concentration, within surrounding siliciclastic mudstones. Muddy fabrics suggest that original skeletal accumulation took place under non-turbulent conditions. Subsequent winnowing during passage of greater-than-normal magnitude waves resulted in varying degrees of grain-support. Winnowing is attributed to turbulence generated near effective wave base, resulting in entrainment of grains and/or pressure gradients created by the passing wave form, which gently flush mud and interstitial waters from saturated bottom sediments. Wackestone and packstone fabrics are the result of incomplete winnowing of mud-sized matrix grains and form at greater distances from the effects of wave energy. Grainstone fabrics result from wave energies that either entrain bioclasts or cause a thorough flushing of mud from near-surface sediments. Progressively shoaling regressive conditions during upper Redwater Shale deposition facilitated the winnowing and concentrating effects of storm-wave interaction with the fossiliferous mud facies of topographically higher areas in the Late Jurassic sea. It is proposed that a variety of carbonate fabrics result from wave-caused winnowing effects, which vary with water depth and wave magnitude. Such fabrics are present in thin bioclastic carbonate units of the Redwater Shale Member of the Sundance Formation, and it is suggested that their origin is due to storm-wave winnowing of a slightly irregular muddy sea bottom. This mechanism and the depositional textures it produces are significant in the distinction between diastemic skeletal grain accumulations and wave-winnowed grainstones. The storm-wave process may also prove applicable in the re-evaluation of certain epicontinental marine deposits previously interpreted as classical turbidite lithologies. Given the vast expanse of geologic time, it is likely that catastrophic events, suc as intense storms and their related processes, have contributed significantly to the preserved sedimentary record.

Deposition of upper Mesozoic resedimented conglomerates and associated turbidites in southwestern Oregon

Research Article| February 01, 1977 Deposition of upper Mesozoic resedimented conglomerates and associated turbidites in southwestern Oregon ROGER G. WALKER ROGER G. WALKER 1Department of Geology, McMaster University, Hamilton, Ontario, Canada L8S 4M1 Search for other works by this author on: GSW Google Scholar Author and Article Information ROGER G. WALKER 1Department of Geology, McMaster University, Hamilton, Ontario, Canada L8S 4M1 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1977) 88 (2): 273–285. https://doi.org/10.1130/0016-7606(1977)88<273:DOUMRC>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROGER G. WALKER; Deposition of upper Mesozoic resedimented conglomerates and associated turbidites in southwestern Oregon. GSA Bulletin 1977;; 88 (2): 273–285. doi: https://doi.org/10.1130/0016-7606(1977)88<273:DOUMRC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The upper Mesozoic sedimentary rocks of southwestern Oregon include turbidites, dark shales, and great thicknesses of resedimented conglomerates. At two localities, these lithologies are arranged in thinning- and fining-upward sequences that begin abruptly with graded conglomerates (individual beds as much as 50 m thick) and pass upward into massive sandstones, classical turbidites, and dark mudstones. Detailed study of the fabric of the conglomerates indicates paleoflow roughly toward the southeast; this direction is confirmed by sole marks on the associated turbidites. The highest angles of imbrication occur at the bases of beds; upward, the imbrication angle flattens, and the preferred alignment of clasts is better developed.The angle of imbrication is not related to clast size (as expressed by D/10, the mean of the ten largest clasts), but D/10 is strongly related to conglomerate bed thickness, which indicates that the conglomerates (thickness, 1.85 to 50.0 m; mean, 17.5 m) are probably single flows.Some of the thinner conglomerates belong to the graded-stratified model described by Walker. The thicker conglomerates are characterized only by normal grading (inverse grading is rare or poorly developed; no stratification), and a new model, the graded bed, is suggested as an intermediate form between the inverse-to-normally graded and graded-stratified models.The thinning- and fining-upward sequences range from 3.40 to 85.0 m (mean, 31.5 m) and are tentatively interpreted to be the result of progressive channel abandonment on a submarine fan. In this interpretation, the conglomerates, although prominent in the section, are unusual events, transported by flows that were far too big for the scale of the channel. Thus, conglomerates plugged the channel bases, subsequently causing diversion of the normal flows into other channels. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Upper Cretaceous Resedimented Conglomerates at Wheeler Gorge, California: Description and Field Guide

ABSTRACT A field map and measured stratigraphic sections revise the geology of Wheeler Gorge. There are three layers of conglomerate, each of which passes upward into massive sandstones, classical turbidites, and/or dark mudstones with thin distal turbidites. Clast long axis orientation on conglomerate layers 1 and 3 gives vector means of 282 and 287 degrees respectively. This agrees closely with flutes (285) and grooves (279) on layer 1, and hence gives a paleocurrent direction for layer 3 for the first time. The conglomerates do not contain as much inverse grading as has previously been suggested, and none of the conglomerates is stratified. Inverse grading was measured in one bed, indicating a continuous upward coarsening of both the smaller and larger clasts in the population. The Wheeler Gorge conglomerates do not fit precisely with the three generalized models of conglomerates recently proposed by Walker, but are closest to the disorganized-bed model.

Density-current Strata in Lower Cretaceous Washita Group, North-central Texas

ABSTRACT Distinctive sedimentary structures and textures characterize flaggy siltstone interbeds sporadically distributed within the predominantly terrigenous clastic strata of the Washita Group, such as the Kiamichi, Denton, and Pawpaw formations. The 1 to 10 cm thick strata consist mainly of silt- and sand-sized grains; and the grain-size distribution is normally graded. Both basal and uppermost surfaces of these strata are in sharp contact with overlying and underlying shale. Burrow casts, tool marks and flute marks are common on the basal surfaces. Upper surfaces display low-amplitude interference ripples crossed by burrows. Internal structures are parallel laminae at the base overlain by climbing ripple or rarely trough cross laminae. Most strata persist at a given stratigraphic position or less than a few kilometers. These siltstone strata resemble classic distal turbidites. However, the small scale of their properties and the absence of thick, rhythmically bedded sequences indicate shallow-water deposition by rather sporadic agents. Some form of advective density current generated by storms, tides, rip flow, or floods transported this clastic debris from a nearby shelf zone.

Characteristics of the Pennsylvanian Lower-Middle Haymond Delta-Front Sandstones, Marathon Basin, West Texas

The lower-middle part of the Pennsylvanian Haymond Formation in the Marathon Basin, west Texas, consists of interbedded sandstone, siltstone, and shale, which have been traditionally interpreted as turbidites deposited in deep water. Comparisons of the internal structures and mean grain size of the turbiditelike sandstone to Bouma9s (1962) classical turbidite sequence suggest important dissimilarities. The majority of the Haymond turbidite-like sandstone beds displays only one kind of internal structure — cross-lamination, which can be subdivided into trough and flaser subtypes. Other turbiditelike sandstones in which two kinds of internal structure are found appear to have some similarities to Bouma9s sequence; however, their general lack of normal grading indicates that turbidity currents played only a very minor depositional role. On the bases of the internal structures, grain-size distributions, and mineralogic and bed-thickness variations, it is suggested that the sandstone was probably deposited on a sloping, delta-front platform on which currents generated by waves, tide, and wind have constantly acted upon the sediments that were periodically flushed from nearby distributary channels. During flood stages in distributary channels or during storms, high-velocity currents and turbulence may have developed, thus modifying depositional processes on the platform to produce turbiditelike sedimentation.