Flute Marks Research Articles

Sole marks reveal deep-marine depositional process and environment: Implications for flow transformation and hybrid-event-bed models

ABSTRACTDeposits of sediment gravity flows in the Aberystwyth Grits Group (Silurian, west Wales, United Kingdom) display evidence that sole marks are suitable for reconstructing depositional processes and environments in deep-marine sedimentary successions. Based on drone imagery, 3D laser scanning, high-resolution sedimentary logging, and detailed descriptions of sole marks, an outcrop 1600 m long between the villages of Aberarth and Llannon was subdivided into seven lithological units, representing: a) mudstone-poor, coarse-grained and thick-bedded submarine channel fills, dominated by the deposits of erosive high-density turbidity currents with flute marks; b) mudstone-rich levee deposits with thin-bedded, fine-grained sandstones formed by low-density turbidity currents that scoured the bed to form flute marks; c) channel–lobe transition-zone deposits, dominated by thick beds, formed by weakly erosive, coarse-grained hybrid events, with pronounced mudstone-rich or sandstone-dominated debritic divisions and groove marks below basal turbiditic divisions, and with subordinate amounts of turbidites and debris-flow deposits; d) tabular, medium- to thick-bedded turbiditic sandstones with flute marks and mixed sandstone–mudstone hybrid event beds mainly with groove marks, interpreted as submarine lobe-axis (or off-axis) deposits; and e) tabular, thin- to medium-bedded, fine-grained, mainly turbiditic sandstones mostly with flute marks, formed in a lobe-fringe environment. Both lobe environments also comprised turbidites with low-amplitude bed waves and large ripples, which are interpreted to represent transient-turbulent flows. The strong relationship between flute marks and turbidites agrees with earlier predictions that turbulent shear flows are essential for the formation of flute marks. Moreover, the observation as part of this study that debris-flow deposits are exclusively associated with groove marks signifies that clay-charged, laminar flows are carriers for tools that are in continuous contact with the bed. A new process model for hybrid event beds, informed by the dominance of tool marks, in particular grooves, below the basal sand division (H1 division of Haughton et al. 2009) and by the rapid change from turbidites in the channel to hybrid event beds in the channel–lobe transition zone, is proposed. This model incorporates profound erosion of clay in the channel by the head of a high-density turbidity current and subsequent transformation of the head into a debris flow following rapid lateral flow expansion at the mouth of the channel. This debris flow forms the groove marks below the H1 division in hybrid event beds. A temporal increase in cohesivity in the body of the hybrid event is used to explain the generation of the H1, H2, and H3 divisions (sensuHaughton et al. 2009) on top of the groove surfaces, involving a combination of longitudinal segregation of bedload and vertical segregation of suspension load. This study thus demonstrates that sole marks can be an integral part of sedimentological studies at different scales, well beyond their traditional use as indicators of paleoflow direction or orientation.

Depositional environments of tertiary turbiditic sediments in Metsovo Basin, NW Greece

Tertiary sediments in Metsovo area consist of submarine fan turbidite deposits accumulated in the proximal part of the basin and represent mostly inner fan. The most proximal part of the inner fan could be related to a shelf environment. Palaeocurrent data indicate mainly two directions. The main direction is SW and results mainly from groove marks. The second direction represents an WNW trend which results mainly from flute marks. The Metsovo area seems to have multiple feeders from North and East. The total thickness of the turbiditic deposits in Metsovo area is approximately 6800 m due to tectonic stacking and folding.

Phanerozoic shallow marine sole marks and substrate evolution

Research Article| August 01, 2018 Phanerozoic shallow marine sole marks and substrate evolution Lidya G. Tarhan Lidya G. Tarhan * 1Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA *E-mail: lidya.tarhan@yale.edu Search for other works by this author on: GSW Google Scholar Geology (2018) 46 (9): 755–758. https://doi.org/10.1130/G45055.1 Article history received: 19 Apr 2018 rev-recd: 29 Jun 2018 accepted: 04 Jul 2018 first online: 01 Aug 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Lidya G. Tarhan; Phanerozoic shallow marine sole marks and substrate evolution. Geology 2018;; 46 (9): 755–758. doi: https://doi.org/10.1130/G45055.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract The presence of erosive sole structures such as tool and flute marks in the sedimentary record can be used to interpret flow dynamics of ancient storm-dominated settings, as well as seafloor sediment properties. The formation and preservation of these structures require a hydroplastic and cohesive substrate. Therefore, the temporal distribution of these structures can be employed to track secular changes in seafloor rheology, which can significantly alter early diagenetic processes and thus shape global biogeochemical cycling. Herein I present a global compilation of Phanerozoic shallow marine successions bearing erosive sole structures, derived from assessment of the sedimentological, tectonic, and paleontological literature. The frequency of sole mark preservation in shallow marine environments has dramatically declined through the Phanerozoic. This first-order trend suggests that the rheological nature of the seafloor has experienced major secular transformations over the past 541 m.y. This record provides evidence for a marked decrease in sediment cohesion and increase in sediment porosity of the uppermost seafloor sediment column through time. I attribute this change in seafloor sedimentary properties to increases in the extent of bioturbation through the Phanerozoic, because the activity of sediment-mixing infauna dramatically increases the fluid content of the shallow sediment pile, decreasing the hydroplasticity necessary for sole mark formation and preservation. The gradual decline in the frequency of abiogenic sole marks in shallow marine successions through the Phanerozoic indicates that the development of bioturbation has been a similarly protracted process. You do not currently have access to this article.

Ediacaran discs from South America: probable soft-bodied macrofossils unlock the paleogeography of the Clymene Ocean

The origin, affinity and paleoecology of macrofossils of soft-bodied organisms of the terminal Ediacaran Period have been highly debated. Previous discoveries in South America are restricted to small shelly metazoans of the Nama Assemblage. Here we report for the first time the occurrence of discoidal structures from the Upper Ediacaran Cerro Negro Formation, La Providencia Group, Argentina. Specimens are preserved in tabular sandstones with microbially-induced sedimentary structures. Flute marks and linear scours at the base of the sandstone layers indicate deposition under high energy, episodic flows. Stratigraphic, sedimentologic, petrographic and taphonomic analyses indicate that the origin of these structures is not related to abiotic process. Preservational and morphological features, as invagination and the presence of radial grooves, indicate that they resemble typical morphs of the Aspidella plexus. The large number of small-sized individuals and the wide range of size classes with skewed distribution suggest that they lived in high-density communities. The presence of Aspidella in the Cerro Negro Formation would represent the first reliable record of Ediacaran soft-bodied organisms in South America. It also supports the paleogeographic scenario of the Clymene Ocean, in which a shallow sea covered part of the southwest Gondwana at the end of the Ediacaran.

塊状重力流堆積物中の障害物洗掘痕

Scour marks up to 1.5m deep with smooth wall, rich in fine-grained heavy grains are found in the massive division of pyroclastic-flow deposits of the Seiganji Tephra Formation in northern Kyushu. The scour marks are asymmetric with steeper slope on the upstream side, and filled with poorly sorted grains of a(p) a(i) fabric. Some massive turbidite sandstones of the Pleistocene Umegase Formation in Boso Peninsula also exhibit similar scour marks up to 0.7m deep. However, the scour marks are characterized by symmetric shape and high-angle imbrication grain-fabric. The results of open-channel flume experiments indicate that such scours can not be formed as flute marks or pot holes, but by scouring around obstacles. Mud clasts may be favorable for such obstacles in turbidites, and rip-up soil blocks or tree debris are for pyroclastic flows. Shallow flow comparable to the obstacle thickness is responsible for deep sours, especially for scours with secondary hollow and backset fillings. The estimated underflow thickness is 0.7m for turbidity current, and 1.5m for pyroclastic flow, respectively.

Wave-Modified Turbidites: Combined-Flow Shoreline and Shelf Deposits, Cambrian, Antarctica

Sandstone tempestite beds in the Starshot Formation, cen- tral Transantarctic Mountains, were deposited in a range of shoreline to shelf environments. Detailed sedimentological analysis indicates that these beds were largely deposited by wave-modified turbidity currents. These currents are types of combined flows in which storm-generated waves overprint flows driven by excess-weight forces. The interpreta- tion of the tempestites of the Starshot Formation as wave-dominated turbidites rests on multiple criteria. First, the beds are generally well graded and contain Bouma-like sequences. Like many turbidites, the soles display abundant well-developed flutes. They also contain thick divisions of climbing-ripple lamination. The lamination, however, is dominated by convex-up and sigmoidal foresets, which are geometries identical to those produced experimentally in current-dominated com- bined flows in clear water. Finally, paleocurrent data support a tur- bidity-current component of flow. Asymmetric folds in abundant con- volute bedding reflect liquefaction and gravity-driven movement and hence their orientations indicate the downslope direction at the time of deposition. The vergence direction of these folds parallels paleocur- rent readings of flute marks, combined-flow ripples, and a number of other current-generated features in the Starshot event beds, indicating that the flows were driven down slope by gravity. The wave component of flow in these beds is indicated by the presence of small- to large- scale hummocky cross-stratification and rare small two-dimensional ripples. Wave-modified turbidity currents differ from deep-sea turbidity cur- rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves. Such currents are formed in modern shoreline environments by a com- bination of storm waves and downwelling sediment-laden currents. They may also be formed as a result of oceanic floods, events in which intense sediment-laden fluvial discharge creates a hyperpycnal flow. Event beds in the Starshot Formation may have formed from such a mechanism. Oceanic floods are formed in rivers of small to medium size in areas of high relief, commonly on active margins. The Starshot Formation and the coeval Douglas Conglomerate are clastic units that formed in response to uplift associated with active tectonism. Sedi- mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin. The geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso- ciated with storms. The abundance of current-dominated combined- flow ripples at the tops of many Starshot beds indicates that excess- weight forces were dominant throughout deposition of many of these beds.

The Fernie-Kootenay transition, Banff National Park, Canada: depositional characteristics of density-induced and combined flows

The Fernie—Kootenay transition (Late Jurassic-Early Cretaceous) occurs in the east of Banff exit, Canada Highway 1 and consists of deformed sequence of dark gray shale alternated with thin-bedded sandstone. The thin-bedded sandstone comprises sharply based-and-topped massive, laminated, or wavy cross-laminated units that mimic classical turbidites. The sharp base of each sandstone bed is commonly characterized by flute and tool marks of plant debris and fluted burrows that show consistent northwestward paleoflow directions. These observations suggest that deposition of the sandstone/shale couplets occurred in well-aerated shallow water environments adjacent to a landmass where occasional storm-generated density-induced undercurrents were prevalent. The multiple beds that comprise Bouma-type units are perhaps due to multiple strokes of storms that traveled across the shelf.

Sole Marks and Combined-flow Storm Event Beds in the Brixton Formation of the Siliciclastic Archean Witwatersrand Supergroup, South Africa

ABSTRACT Sole marks and associated storm event beds are described for the first time from the middle Archean Witwatersrand Supergroup. The sole marks are excellently preserved and include a variety of marks formed by combined flow: turnabout, curvilinear, and low-angle intersecting groove marks; bipolar prod marks; recurved flute marks; and shallow scour hollows. The sole marks are intimately associated with other sedimentary structures formed by combined flow or oscillatory flow, such as wave ripple marks, undulatory parallel lamination, hummocky cross-stratification, and hummock-and-swale structures. The storm event beds with sole marks were deposited in a transitional shelf setting and grade upwards into swaly and trough cross-bedded orthoquartzite deposited on the inner shelf. Interbedded storm event beds display a variety of sole mark assemblages, reflecting widely varying flow conditions in the storm-generated bottom boundary layer. Oscillatory currents were mainly oriented perpendicular to paleoshore and were combined in some cases with offshore-directed storm-surge relaxation currents. Some sole mark assemblages, specifically those with turnabout and low-angle intersecting groove marks, contain evidence for the presence of shore-oblique or shore-parallel geostrophic currents. The storm event deposits in the Witwatersrand Supergroup are thus compared with models derived from odern storm deposition.

Paleoclimatic significance of the lacustrine Carboniferous deposits in northwest Argentina

The origin and probable paleoclimatic significance of Carboniferous lacustrine deposits in northwest Argentina are discussed. These rocks are developed extensively throughout northwestern Argentina, but are described in the present paper only from the region of the Sistema del Famatina. Five lithofacies have been identified: (A) laminated siltstones and claystones; (B) thinly interbedded mudstones and sandstones; (C) interlayered mudstones and sandstones with sole structures (flute marks and tool marks); (D) conglomerates and fine sandstones; (E) sandstones and diamictites. These lithofacies are interpreted as lacustrine deposits formed during the final episode of the Carboniferous glaciation. Lithofacies A originated as a suspension-load deposit in the central and intermediate parts of the lake; the presence of dropstones in this lithofacies suggests coeval ice-rafting processes. Underflow currents probably formed lithofacies B, and lacustrine turbidites lithofacies C. Lithofacies D and E were deposited in marginal environments and represent in some cases shoreline bars and small deltas. The age of the lacustrine episode is deduced from paleobotanical information. Both palynological and megafloristic data suggest that these deposits were formed during the middle Carboniferous. A palynological assemblage dominated by Cristatisporites-like spores characterizes these sediments.

Nivation and Local Glaciation in N. and S. Sweden. A Progress Report

An ongoing geomorphological research project on nivation and local glaciation is testing the following working ./ hypothesis. After the main Weichselian deglaciation in south Sweden, periods of cold tundra conditions returned, e.g. in the Younger Dryas period. Fossil ice-wedge casts and rocks with wind-polis- ^// hed facets and flute marks indicate permafrost climate and _ strong winds with snow-drifting from easterly and westerly direc- tions. Large masses of drifting snow were trapped in the canyon valleys of Soderasen and other horst ridges, where nivation / hollows and small glacial cirques were slightly rejuvenated. Ni- / veo-eolian sand was deposited in some hollows and also incorpo- // rated in some moraine-like deposits. The bedrock hollows and cirques were mainly excavated during the much longer tundra 64// periods before the advances of the Quaternary main ice sheets over Scania. But the hollows survived the erosional impact of the ice sheets. The work to test the hypothesis is making progress, by analysis/// of the Late-Glacial forms and material in south Sweden, by comparisons with landforms, material and processes in the actual , // mountain tundra of Abisko in the north, and by comparisons - /

Setulflike Scour-Remnant Features in a Neogene Volcaniclastic Flood Deposit, Central Oregon

ABSTRACT Positive relief structures, resembling the inverse of flute marks, occur on a cohesive siltstone bed in nonmarine volcaniclastic sediments of the Deschutes Formation in central Oregon. Unlike setulfs and previously described scour remnant ridges, these structures increase in relief, rather than decrease, in the presumed downcurrent direction. The origin of these structures by sheet flow during floods is preferred over a wind origin, but the evidence is equivocal. The nature of flow necessary to produce these features is not known but may be related to unusual fluid boundary-layer conditions associated with high sediment-load transport.

Shelf and Deep-Sea Sedimentation in Eocene Forearc Basin, Western Oregon--Fan or Non-Fan?

The lower Eocene Tyee and Flournoy Formations in the southern and central Coast Range of Oregon contain unusual associations of shelf and deep-sea facies deposited in an elongate portion of a forearc basin. The sandstone-rich nature and abrupt transition of sedimentary facies suggest that a variation of existing deep-sea fan models is present. The 6,500 ft (2,000 m) thick sequence calls for shelf sandstone virtually cascading into deep water along a line (shelf edge) rather than from a point source (submarine canyon) to feed a sand-rich system. Facies are described in terms of a sand-rich fan system. From south to north these facies include: deltaic, shelf, inner fan, mid-fan, outer fan, and basin plain. The deltaic facies contains coarse-grained, cross-bedded, distributary channel sandstone and interbedded coal. Some prograding deltaic sand extended to the edge of the narrow Eocene shelf and cascaded into the basin. Many complex, nested channels up to 1,200 ft (350 m) wide and 130 ft (40 m) deep were incised into fine-grained shelf and slope sediments at slope depths (line source). These channel and shelf-slope deposits comprise an inner fan facies. A single large canyon or feeder channel (point source) typical of some fan systems was not recognized. Mid-fan facies consist of broad, coalescing channels filled with thick, am lgamated sandstone beds. Sandstone/shale ratios in the mid-fan facies are from 85 to 15 up to 95 to 5. Outer fan facies have sandstone/shale ratios of 60 to 40 and contain well-developed, graded turbidite beds about 5 ft (1.5 m) or less in thickness, interbedded with shale. Basin-plain facies consist of fine-grained strata (sandstone/shale ratios of 30 to 70) deposited at the north end of the basin. These thin-bedded deposits also contain graded fine-grained turbidite sandstones. Paleogeographic reconstructions based on paleocurrent data, facies distribution, and sandstone composition indicate sediment derivation from the pre-Cenozoic Klamath Mountains terrane and a volcanic arc system to the south and southeast. The average sandstone composition is 19% quartz, 27% feldspar, 19% volcanic rock fragments, 7% other rock fragments, and 28% accessories and cement. Paleocurrent data from cross-bedding, and groove and flute marks indicate transport directions from south to north instead of a radially dispersed fan pattern. The stratigraphic succession of the Tyee-Flournoy system indicates a general shoaling trend and a progradation of deltaic facies across a narrow shelf and out over the basin fill during the early Eocene. The predominance of sandstone and confinemen within a narrow basin account for a turbidite facies differing from more familiar mixed sandstone and shale submarine-fan facies models.

Development of flute-mark assemblages, 2. Evolution of trios of defects

Experiments using a bed of hardened Plaster of Paris dissolved beneath a turbulent water stream (practical analogue of natural mud-bed erosion) show that trios of defects evolve into compound flute marks which, at a critical age, become indistinguishable from structures evolved from single defects. The critical age is comparable with 0.6, defined in terms of the erosion velocity of the bed surrounding the mark, duration of erosion, and the overall size of the assemblage of defects. Marks formed from single defects (Allen, 1971) and from paired defects (Allen, 1973) also lose evidence of their parents broadly at this value of the age. Marks grown from trios increase in maximum depth on a similar law to the marks formed from single and paired defects. A natural assemblage of flute marks may be expected during an early stage of evolution to show a progressive reduction in the number-density of structures, as a consequence of the merging of marks produced at sufficiently closely spaced unstable defects.

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.

Reaction, relaxation and lag in natural sedimentary systems: General principles, examples and lessons

Natural sedimentary systems are process-response systems in which hierarchical configurations on the sedimentary surface are given character, maintained and translated because some of the energy supply is expended, that expenditure resulting in material transfers and transports. The energy supply to most types of system generally varies on several different time scales. Consequently, a complex of reaction and relaxation effects marks the operation of these systems, the equilibrium of which can be defined only in terms of explicit time scales. Mathematically, the existence of these lag effects introduces non-uniqueness into the operation of natural systems, which is best represented and assessed in terms of phase diagrams. Many aspects of the behaviour of natural systems are in conformity with this theoretical model. The transport of suspended sediment in rivers and tidal flows ordinarily differs in phase from the aqueous discharge. Bed forms in tidal and river currents lag the changing flow conditions, by extents to a modest degree predictable by the model. Lag effects also mark wave-dominated sedimentary environments. Spatial lag effects, in which flow properties differ in phase from bed configurations, are critical to the dynamics of many kinds of bed form, including dunes, flute marks, and stream meanders. The ubiquity of lag effects, and the multiple time-scales of natural sedimentary systems, call into question aspects of current practice in the study of sedimentary environments and, as well, much of the way in which experimental and analytical studies are designed and used in sedimentology.

Development of flute-mark assemblages, 1. Evolution of pairs of defects

Experiments using a bed of hardened Plaster of Paris dissolved by a turbulent water stream (practical analogue of natural mud-bed erosion) show that paired defects evolve into compound flute marks which at a critical age become marks indistinguishable from structures developed from single defects. The critical age increases as the relative spacing of the paired defects grows larger but decreases as their alignment approaches the mean flow direction. Growth of the compound marks follows similar laws to single flutes. A natural assemblage of flute marks may be expected during an early stage of development to show a progressive reduction in the number-density of the structures, as a consequence of the merging of marks generated from defects sufficiently closely spaced to have interacted during the time available.

FLOOD DEPOSITS OF THE SAND‐BED EPHEMERAL STREAMS OF CENTRAL AUSTRALIA

ABSTRACTThis study investigates the deposits of sand‐bed ephemeral streams within an area of 250,000 km2 in the western Lake Eyre basin, central Australia, being initiated after the record floods of February–March 1967.Large‐scale ripples were the most common bed forms preserved in the majority of channels, covering 30–40% of the depositional area. Other bed forms included longitudinal, transverse and linguoid bars (20%), upper‐regime plane beds (<5%), small‐scale ripples (25–30%), and flute marks (<1%). Major bed forms could be related to flood stage: plane beds (upper regime), large‐scale ripples, and longitudinal bars usually were generated by high‐stage flow, and transverse and linguoid bars by waning and low‐stage flow.Trough cross‐stratification was produced by migrating small‐scale and large‐scale ripples, large‐scale sets comprising about 60% of the deposits. Tabular cross‐stratification (25% of deposits) resulted from the downstream and lateral growth of channel bars. Flat‐bedding with parting lineation (<5%) was dominant only along the middle reach of the Finke, the major stream in the western Lake Eyre basin. Disturbed bedding was almost totally absent. Bed forms, in order of increasing overall mean grain size (?) of deposits, are plane beds (upper regime), small‐scale ripples, transverse and linguoid bars, and large‐scale ripples. Sorting (σI) and skewness (SkI) are dependent on mean grain size (Mz).Lower‐regime flow prevailed for the majority of streams, upper‐regime flow being dominant only in the middle reach of the Finke. Using estimated Froude numbers and Darcy‐Weisbach resistance coefficients, the greatest mean velocities of flow have been calculated at about 0.8–3.7 m/sec.

Downcurrent Changes in Sedimentary Structures in Ordovician Turbidite Greywackes

ABSTRACT Sedimentary structures were studied in detail in eight turbidite greywacke beds from the Cloridorme Formation (Middle Ordovician), Gaspe, Quebec. The beds are exposed on the wave-cut platform near Grande Vallee and were traced for about two miles along the strike (276°), which is parallel to the average current direction (274°) as indicated by sole mark directions. The beds under study show the following systematic downcurrent changes along the strike from east to west: (i) Sole mark types change as follows: (1) Tool marks such as groove marks, prod marks and isolated flute marks, to (2) longitudinal ridges, longitudinal ridges with overlapping flute marks and closely spaced flute marks, to (3) poorly developed shallow longitudinal ridges with occasional cuspate crossing bars, to (4) smooth bottom with occasional groove marks. (ii) There is an increase in variance of sole mark directions in the downcurrent direction. The sole mark direction changes by nearly 90° as one of the beds is traced from east to west along the strike. (iii) Most beds are massive and are divided into two parts by a bedding joint. However, in the proximal regime, if a bed is unusually thin, the bedding joint is absent and the bed shows slightly wavy plane-laminated structure. It is considered that beds were deposited by relatively low concentration, highly turbulent currents. Deposition of the beds took place in two distinct phases separated by a small time interval and the depositing currents tended to meander greatly on slowing down in the distal region.

Erosional Current Marks of Weakly Cohesive Mud Beds

ABSTRACT The manner in which a bed of weakly cohesive mud is eroded by a plain, turbulent flow of water is found experimentally to depend on the severity of the flow over the bed. The major structures left behind on a weakly cohesive mud bed after a prolonged period of erosion also depend on the severity of flow. With as ascending severity, we obtain: (1) longitudinal rectilinear grooves, (2) longitudinal meandering grooves, (3) flute marks, and (4) transverse erosional markings. The more severe flows also give rise to shear wrinkles and to erosional structures of a less well defined nature. Of the four structures enumerated, all but longitudinal rectilinear grooves have counterparts in the fossil record. Of particular importance is evidence confirming that flute marks are associated with sepa ated flows and are due to processes of flow separation and reattachment.

Flute Marks and Flute Separation

FLUTE marks abound as moulds in rocks attributed to turbidity current action1 and are not uncommon in beds assigned a fluviatile origin2. It was early recognized that flute marks are caused by the eroding action of currents of water on cohesive mud beds. Much later it was found experimentally1 that the growth of flutes depended in some way on eddying motions in the current of fluid acting on the bed, although no clear idea emerged at the time as to the physical character of these eddies nor as to how they were involved in the bed erosion.