Swaley Cross-stratification Research Articles

Depósitos litorâneos neoproterozóicos do Grupo Alto Paraguai no sudoeste do Cráton Amazônico, região de Mirassol d’Oeste, Mato Grosso

Anomalous events of terminal Neoproterozoic have been mostly reported in the cap carbonate sequences exposed in many regions worldwide. Extreme climatic conditions involving global glaciations were associated with significant oceanographic and sea-level changes. Neoproterozoic carbonate successions commonly are succeeded by siliciclastics deposits that in many regions, like in South America, are poorly described and have their sedimentary history still poorly understood. In the southwestern Amazon craton, central region of Brazil, the Alto Paraguai Group is an excellent example of silicilastic deposits that overlie the post-Marinoan cap carbonate sequence of the Araras Group. The Alto Paraguai Group was investigated using facies and stratigraphic analysis in the Mirassol d’Oeste region, State of Mato Grosso, and consists of two successions: 1) amalgamated fine to medium-grained sandstone, cemented by sparry dolomite, with swaley and hummocky cross-stratifications, wave truncated and even-parallel laminations, trough cross stratification, ripple mark, dessication cracks and evaporite molds, interpreted as storm and tidal influenced littoral deposits; and 2) pelite, shale and fine to medium-grained sandstone with ripple marks, even parallel lamination and swaley cross stratification, interpreted as offshore to lower shoreface deposits. Sparry ferrous dolomite cement in sandstone exhibits crystals with frequent curved faces and cleavages, typical of saddle dolomite. Isotopic negative values of carbon (-1,70 ‰ to -3,5 ‰ ) and oxygen (-3,78 ‰ to -5,17 ‰ ) confirm late cementation with influence of meteoric and hydrothermal waters during deep burial diagenesis. The limit between these successions is an unconformity superimposed by marine flooding or transgressive surface, that records na onlap over a littoral zone installed in the southern border of the Amazon craton during the Neoproterozoic.

RECONSTITUIÇÃO PALEOAMBIENTAL DE DEPÓSITOS ALBIANOS NA BORDA LESTE DA BACIA DE GRAJAÚ, MA

The Albian deposits of the Itapecuru Group exposed at the margins of the Itapecuru River, near the town of Coroata, eastern Grajau Basin, were investigated concerning their sedimentological, stratigraphic and palaeontological aspects. The study showed that these strata include sandstones, shales and limestones formed in depositional environments usually found in deltaic systems. They include: 1. delta front, consisting of sandstone lobes, either amalgamated or separated by thin shale layers, and which are internally characterized by massive, deformed or cross-stratified sandstones arranged into thickening- and coarsening upward successions; 2. wave-dominated delta front, consisting of sandstones in a variety of wavy laminations, including swaley cross-stratification, undulating parallel-lamination, low- angle cross- stratification and large- scale scour-and-fill structures; 3. distal bar, characterized by sandstones with sedimentary structures similar to delta front deposits, but forming thinner lenses isolated within shales; and 4. prodelta/restricted shelf/ mud flat, which includes the finest-grained lithologies of the study area, consisting of laminated to massive shales and limestones. Features attributed to subaereal exposure, such as root marks, mudcracks, karstic holes formed by dissolution under influence of meteoric water, and dinosaur footprints, are all locally found in association with these deposits. The analysis of facies architecture revealed three stratigraphic units, defined by discontinuity surfaces marked by one or more of the following elements: calcitic cement, concentrations of fossil fragments, and a lag of coarse-grained sandstone and conglomerate. Despite the internal organization into prograding cycles, typical of deltaic systems, the distribution of the stratigraphic units and facies associations shows that muddier deposits, attributed to low-energy environments ranging from prodelta to restricted shelf and mud flat, become more abundant from southeast to northwest in the study area, as well as towards the top of the sections, recording progressive drowning of the deltaic system. Punctuated floodings took place, being represented by discontinuity surfaces, characterizing a retrogradational stratal pattern, suggesting abandonment of the deltaic system due to an overall transgression.

Origin of hummocky and swaley cross-stratification— The controlling influence of unidirectional current strength and aggradation rate

Research Article| December 01, 2006 Origin of hummocky and swaley cross-stratification— The controlling influence of unidirectional current strength and aggradation rate Simone Dumas; Simone Dumas 1Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Search for other works by this author on: GSW Google Scholar R.W.C. Arnott R.W.C. Arnott 1Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Search for other works by this author on: GSW Google Scholar Geology (2006) 34 (12): 1073–1076. https://doi.org/10.1130/G22930A.1 Article history received: 25 Apr 2006 rev-recd: 27 Jul 2006 accepted: 29 Jul 2006 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Simone Dumas, R.W.C. Arnott; Origin of hummocky and swaley cross-stratification— The controlling influence of unidirectional current strength and aggradation rate. Geology 2006;; 34 (12): 1073–1076. doi: https://doi.org/10.1130/G22930A.1 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 SocietyGeology Search Advanced Search Abstract A series of wave-tunnel experiments was conducted to investigate the conditions under which hummocky and swaley cross-stratification form. Isotropic 3-dimensional (3-D) hummocky bed forms were generated under long wave periods (∼8–10 s) and moderate oscillatory velocities (Uo ∼50–90 cm/s) with very weak (< 5 cm/s) to no unidirectional flow. Hummocks became anisotropic with the addition of only a small unidirectional current (5–10 cm/s), and began to resemble unidirectional dunes when the unidirectional current was increased above 10 cm/s. Synthetic aggradation of the hummocky bed forms at high (4.2 mm/min) and low (1 mm/min) rates generated stratification resembling hummocky and swaley cross-stratification, respectively. Based on these findings, we suggest that hummocky cross-stratification optimally forms above (but near) storm wave base where aggradation rates during storms are high enough to preserve hummocks but unidirectional current speeds are sufficiently low to generate low-angle, isotropic cross-stratification. Swaley cross-stratification is also hypothesized to be deposited by an aggrading hummocky bed between fair-weather and storm wave base, but in shallower water where aggradation rates are low enough to cause preferential preservation of swales. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evidence of lacustrine sedimentation in the Upper Permian Bijori Formation, Satpura Gondwana basin: Palaeogeographic and tectonic implications

The Upper Permian Bijori Formation of the Satpura Gondwana basin comprising fineto coarse-grained sandstone, carbonaceous shale/mudstone and thin coal bands was previously interpreted as the deposits of meandering rivers. The present study documents abundance of wave ripples, hummocky and swaley cross-stratification and combined flow bedforms in the Bijori Formation, suggesting that a significant part of the formation was deposited in a wave-agitated environment. Evidence of near-emergent depositional conditions provided by repeated occurrence of rootlet beds and hydromorphic paleosols, local flooding surfaces denoting rapid fluctuation of water level, occurrences of temnospondyl vertebrate fossils, and absence of tidal signatures and marine fossils suggest a lacustrine rather than marine depositional regime. Five facies associations recognised within the Bijori Formation are inferred to represent fluvial channels and associated floodplains (FA1), lake shorelines (FA2), subaqueous distributary channels and associated levees (FA3), waveand storm-affected delta front (FA4), and open lacustrine/lower shoreface (FA5) deposits. The planoconcave fluvial channel-fill sandbodies with unidirectional cross-beds are clearly distinguishable from the delta front bars that show a convexo-plan or bi-convex sandbody geometry and dominance of wave and combined flow bedforms. Some of the distributary channels record interaction of fluvial and wave-dominated basinal processes. Major distributary sandbodies show a north to northwest flow direction while wave-affected delta front sandbodies show very complex flow patterns reflecting interaction between fluvial discharge and wave processes. Wave ripple crest trends show that the lake shoreline had an overall east-northeast to west-southwest orientation. The lack of documented contemporaneous lacustrine or marine sediments in the Satpura Gondwana basin posed a major problem of basin-scale palaeogeographic reconstruction. The existence of Bijori lake solves the problem and the lake is inferred to have acted as repository for the contemporaneous alluvial drainage. Development of the large Bijori lake body implies generation of accommodation space exceeding the rate of sediment supplied and thus represents locus of high tectonic subsidence. Transition of fluvial sediments with red mudstone and calcareous soil profile in the lower part of the succession to carbonaceous shale and coal-bearing lacustrine sediments in the upper part, denote a change from a warm semi-arid climate with seasonal rainfall to a more humid one.

Caracterização paleoambiental de Depósitos Albianos na Borda Sul da Bacia de São Luís-Grajaú: modelo de delta fluvial influenciado por tempestade

Exposures of Albian deposits in the Grajau-Arame area, southern Sao Luis-Grajau Basin, are documented for the first time in this paper. In that area, a composite section up to 135 m thick traced over 70 km of horizontal distance, allowed the recognition of six depositional environments, including: delta front (mouth) bar, distal bar/ prodelta, upper shoreface/foreshore, interdistributary bay/crevasse, fluvial channel and distributary channel. A deltaic system influenced by storms is proposed with basin on: 1. facies characteristics typical of deltaic deposits; 2. arquitectural arrangement of facies association into an overall prograding succession; 3. well-developed lobate or sigmoidal sandstones forming bodies up to 6 m thick in the deposits representative of mouth bar setting; 4. abundance of small- and large-scale combined-flow generated structures (such as wave-scour, swaley cross-stratification, undulating parallel lamination, quasi-planar cross stratification, cross strata with undulating set boundaries and abundant reactivation surfaces and mud drapes); and 5. abundance of soft-sediment deformation structures, attesting high rates of sediment deposition. Paleocurrent data show a main northeastward-oriented dip direction for the delta system, which prograded into a marine-influenced basin, as suggested by the ichnological assemblage including Thalassinoides, Skolithos, Ophiomorpha, Planolites, Teichichnus and Taenidium . The interpretation provided in this paper lead to suggest a basinwide incursion of a broad, epicontinental-like sea over the Sao Luis-Grajau Basin during the Albian.

Depositional environments and stratigraphic architecture of the Late Cretaceous Milk River and Eagle formations, southern Alberta and north-central Montana: relationships to shallow biogenic gas

ABSTRACT The Milk River and Eagle formations in southern Alberta and north-central Montana form the core of the first Upper Cretaceous clastic wedge in this part of the Western Interior Foreland Basin. The wedge is a host for shallow biogenic gas in both Canada and the USA. Facies analysis in outcrop and cores shows two overall marine progradational successions with markedly differing depositional environments. The lower, Virgelle succession, is dominated by swaley and hummocky cross-stratification and represents a storm-dominated shoreface deposit. The upper, Upper Eagle succession, is dominated by a high mudstone content and has tidal indicators. Importantly, no abundant swaley or hummocky cross-stratifications can be found within the Upper Eagle succession, indicating deposition under dominantly fair-weather conditions in a restricted, probably deltaic environment. Down-dip correlation with 2760 wireline logs reveals that each succession consists of a series of three progradational allomembers separated by minor transgressions. The three Virgelle allomembers step consecutively farther basinward and were terminated by a regional transgression. The three Upper Eagle allomembers also step seaward in a shingled manner and were partly drowned before being terminated by the regional Pakowki/Claggett transgression. Production data indicates that only the Upper Eagle A, B and, when present, C allomembers are productive in the Bearpaw Mountains area. All Virgelle allomembers commonly test wet, probably due to a large amount of inter-connectivity among the coastal sandstones of the allomembers, which enabled the migration of hydrocarbons up structural dip. Unconventional reservoirs such as the Alderson Member of the southern Alberta Milk River Gas Pool are time equivalent to the Upper Eagle allomembers. Because deltaic sedimentation with little or no storm activity prevailed during Upper Eagle time, the Alderson fine-grained rocks could be the result of a plume of sediment of deltaic origin (pro-delta) being deposited on the shelf during prevailing fair-weather conditions. End_Page 155------------------------

Sequence stratigraphy and paleogeography of a Cenomanian deltaic complex: the Dunvegan and lower Kaskapau formations in subsurface and outcrop, Alberta and British Columbia, Canada

Sequence stratigraphy and paleogeography of a Cenomanian deltaic complex: the Dunvegan and lower Kaskapau formations in subsurface and outcrop, Alberta and British Columbia, Canada

Swaley cross-stratification in medium to coarse sandstone produced by oscillatory and combined flows: examples from the Proterozoic Kansapathar Formation, Chhattisgarh Basin, M.P., India

Abstract The Proterozoic Kansapathar Formation is the topmost unit of the siliciclastic Chandarpur Group of rocks which records a fluvial to marine transition and comprises a fining-upwards transgressive succession overlain by a coarsening-upwards prograding succession. The Kansapathar Formation around Panduka, M.P., is composed of medium- to coarse-grained, well sorted quartzarenite and is replete with swaley cross-strata (SCS), trough cross-strata and other structures of intermediate morphology. In laterally continuous exposures SCS are more common in the northeastern part of the study area, whereas trough cross-strata are more dominant in the southwestern part. We have recognised the SCS on the basis of the stratal geometry, originally described by Leckie and Walker (1982) . SCS occur as nested sets of elliptical swales which are commonly symmetrically filled. The trough cross-strata are variable in scales and dominantly SW-directed. The major axes of swales show a strong (NNW–SSE) preferred orientation orthogonal to the orientation of trough cross-strata and at a low angle to the shoreline orientation determined from the megaripple orientation in the immediately underlying unit. SCS described herein, in contrast to many examples reported from elsewhere, formed in medium to coarse sand and the maximum inclinations of their lower bounding surfaces and of the infilling laminae are greater than those of fine-grained hummocky cross-strata (HCS) and SCS. This is explicable since medium- to coarse-grained sediments did not experience significant suspension transport and, thus, the development of low-angle lamination was suppressed. The symmetrically-filled swales do not show any evidence of lateral migration and might have been formed by bidirectional oscillatory flow. The trough cross-strata could have been formed by translatory current-dominated combined flow. The spectrum of structures may owe their origin to symmetrical oscillatory flow transforming into asymmetric flow during onshore propagation of waves. This is expected to be common in shoreface environment. The spatial variation in relative frequency of occurrence of various types of cross-strata is also consistent with the palaeogeographic reconstruction of the study area. The trough cross-strata seem to be more common towards the constantly shifting sands of nearshore. Thus the relative frequency of trough cross-strata and SCS has been used as a clue to determine relative proximity to shoreline.

Sedimentary structures: Sorby and the last decade

Abstract Henry Clifton Sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures, together with their use as palaeocurrent indicators. Research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant, particularly as regards the physical explanation of structures, and to present significant challenges and opportunities. Perhaps the most pressing needs are for a better understanding of (1) bedforms in gravels, silts and carbonate sediments, (2) tidal and especially sand-wave bedding, (3) hummocky and swaley cross-stratification, and (4) soft-sediment and dewatering structures in turbidites. Many sedimentary structures present a little-exploited opportunity to quantify process-rates and define short time-periods from the rock record.

Sedimentary structures: Sorby and the last decade

Henry Clifton Sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures, together with their use as palaeocurrent indicators. Research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant, particularly as regards the physical explanation of structures, and to present significant challenges and opportunities. Perhaps the most pressing needs are for a better understanding of (1) bedforms in gravels, silts and carbonate sediments, (2) tidal and especially sand-wave bedding, (3) hummocky and swaley cross-stratification, and (4) soft-sediment and dewatering structures in turbidites. Many sedimentary structures present a little-exploited opportunity to quantify process-rates and define short time-periods from the rock record.

Ripple cross-stratification in swaley cross-stratified sandstones of the Chungo Member, Mount Yamnuska, Alberta

Swaley cross-stratified sandstones are exposed in the Late Cretaceous Chungo Member of the Wapiabi Formation at Mount Yamnuska, Alberta. At this site, many of the individual flaggy-weathering, thin beds of sandstones that make up the swaley cross-stratified structure are internally ripple cross-stratified. The association of swaley and ripple cross-stratification has not previously been described and is interpreted to indicate compound cross-stratification generated under combined flow condi tions. Furthermore, this association brings into question the possible genetic relationship between swaley cross-stratification and hummocky cross stratification, a sedimentary structure generated under purely oscillatory conditions.

Grain fabric of hummocky and swaley cross-stratification

Bed-form and grain fabric of two types of hummocky and swaley cross-stratification from the Pleistocene formations in the Kanto Plain, cental Japan, display clearly distinctive flow conditions of storm (typhoon)-generated combined flow. The grain alignment in the vertical profile of a domal structure of hummocky cross-set, together with variable strike directions of a lamina, suggest that the cross-stratification was produced under the waning stage of oscillatory-dominant combined flow. Elsewhere, the preferred grain orientation along the maximum dip direction of a large swaley structure shows a small angle of upcurrent imbrication both on the stoss and lee sides of the laminae. The stratification within the swale is asymmetric, but the strike directions of a lamina are persistently oriented in a preferred direction. The fabric and the structure of the swale indicate that the cross-stratification was formed as a very low-angle mega ripple under unidirectional-dominant combined flow. The grain fabric of both cross-sets clearly shows that in spite of a superficial resemblance to antidunes, neither of the two structures was formed in flow conditions for antidunes.

Discussion on swaley cross-stratification produced by unidirectional flows, Bencliff Grit (Upper Jurassic), Dorset, UK

Discussion on swaley cross-stratification produced by unidirectional flows, Bencliff Grit (Upper Jurassic), Dorset, UK

Swaley cross-stratification produced by unidirectional flows, Bencliff Grit (Upper Jurassic), Dorset, UK

Unusual bedforms occur in the Bencliff Grit at Osmington Mills, Dorset. The geometry of cross-strata in stacked, predominantly fine-grained sandstone units is dominated by low-angle troughs with (locally) perfectly-preserved antiforms which wedge-out onto climbing low-angle erosion surfaces. Many of the antiforms (hummocks) and troughs (swales) appear to have grown at similar rates of sediment fall-out. Others show draped antiforms and thickening into swales. In both cases laminae are concordant with the lower bounding surfaces of the cross-sets, demonstrating that vertical accretion was the dominant process. This geometry together with the conspicuous absence of angle-of-repose cross-strata give the bedforms a resemblance to amalgamated hummocky cross-stratification or swaley cross-stratification. Many workers consider amalgamated hummocky cross-stratification and swaley cross-stratification to be identified with storm sedimentation on the shelf to shoreface, thereby bracketting the structures both in terms of process and environment. However, the structures in the Bencliff Grit suggest that such a precise interpretation of amalgamated hummocky cross-stratification or swaley cross-stratification is inadvisable. The geometry of the cross-stratification suggests deposition under a highly sediment charged but essentially unidirectional flow. This study therefore supports recent views that hummocky and swaley styles of cross-stratification are in reality a form of low-angle trough cross-stratification produced primarily by unidirectional flows. Clearly, since undulatory forms of stratification are substrate- and process-dependent and not environmentally-specific, great care must be taken in the interpretation of palaeoenvironmental settings, especially where exposures are limited, as for instance in borehole cores. It is unlikely that core data can be used to identify bedforms analogous to hummocky cross-stratification or swaley cross-stratification unless boreholes are highly deviated.

Hummocky Cross-Stratification Vs. Swaley Cross-Stratification Sequences in South-Central Wyoming, Mesa Verde Sandstone and Haystack Mountains Formation Outcrops: ABSTRACT

Progradational shoreline sandstones, showing evidence of combined flow (waves and currents), and distributary-channel sandstones were deposited in Cretaceous (Campanian) time as part of a wave-dominated delta of the upper part of the Haystack Mountains Formation in the Rawlins, Wyoming, area. Depositional processes, water depths, and flow directions were interpreted utilizing physical structures and trace-fossil assemblages in two progradational inner-shelf to upper shoreface sequences. The uppermost formal member of the Haystack Mountains, the Hatfield Sandstone Member, where it crops out northeast of Rawlins, is a shoreline deposit. A typical suite of well-developed shoreface trace fossils is recognized, the upper units including Ophiomorpha, Asterosoma, Rosselia, and Teichichnus. Hummocky Cross-stratification (HCS) and swaley cross-stratification (SCS) are abundant in two of the upper members of the Haystack Mountains Formation. HCS and SCS, where they occur in the same sequence, differ in form (synform, antiform); grain size ( =/ 125 /mu/); bed thickness (0.1-1 m, 0.5-4 m); amalgamation (uncommon to common, very common); stratigraphic position (below SCS, above HCS and below foreshore); and environments of deposition (inner shelf and lower shoreface, middle and upper shoreface). In SCS, antiforms are essentially absent (<5%) and horizontal laminae are rare (<10%), In HCS, antiforms usually occur inmore » amounts greater than 20% by volume. Within a single bed of SCS, synforms usually occur almost to the exclusion of antiforms. The thickness of individual lamina sets is highly variable and is strongly dependent on the lateral dimensions (up to 3 m) of individual HCS or SCS.« less

Storm sands with swaley cross-stratification in the Lower Miocene Taliao Formation, Taiwan

Storm sands with swaley cross-stratification in the Lower Miocene Taliao Formation, Taiwan

Teredinid-bored Araucariaceae logs preserved in shoreface sediments, Wangaloa Formation (Paleocene), Otago, New Zealand

Abstract Bored, calcite-cemented, and partly carbonised Araucariaceae logs (?Agathis sp.) occur within a concretionary shellbed in Wangaloa Formation shallow marine sandstone exposed near Mitchells Rocks, southeast Otago. Dense concentrations of carbonate-lined and sand-infilled teredinid tubes occur in the sapwood of four logs. Conspicuous sedimentation features such as shell lags, truncated and reworked Ophiomorpha burrows, and low-angle hummocky and swaley crossstratification exposed in strata at Mitchells Rocks, indicate shoreface or storm deposition close to fairweather wave base. The logs probably endured only a brief period of sea-floor exposure and decay before final burial and cementation.

Significance of hummocky and swaley cross-stratification in late Pleistocene lacustrine sediments of the Ontario basin, Canada

Research Article| August 01, 1986 Significance of hummocky and swaley cross-stratification in late Pleistocene lacustrine sediments of the Ontario basin, Canada Nicholas Eyles; Nicholas Eyles 1Glaciated Basin Research Group, Department of Geology, University of Toronto, Scarborough Campus, Toronto, Ontario M1C 1A4, Canada Search for other works by this author on: GSW Google Scholar Bryan M. Clark Bryan M. Clark 1Glaciated Basin Research Group, Department of Geology, University of Toronto, Scarborough Campus, Toronto, Ontario M1C 1A4, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Nicholas Eyles 1Glaciated Basin Research Group, Department of Geology, University of Toronto, Scarborough Campus, Toronto, Ontario M1C 1A4, Canada Bryan M. Clark 1Glaciated Basin Research Group, Department of Geology, University of Toronto, Scarborough Campus, Toronto, Ontario M1C 1A4, Canada Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1986) 14 (8): 679–682. https://doi.org/10.1130/0091-7613(1986)14<679:SOHASC>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 Nicholas Eyles, Bryan M. Clark; Significance of hummocky and swaley cross-stratification in late Pleistocene lacustrine sediments of the Ontario basin, Canada. Geology 1986;; 14 (8): 679–682. doi: https://doi.org/10.1130/0091-7613(1986)14<679:SOHASC>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 SocietyGeology Search Advanced Search Abstract In the Ontario basin, Canada, last interglacial, glacial, and postglacial lacustrine sands, deposited at times of high lake level, commonly show hummocky and swaley cross-stratification, inferred to form beneath storm waves. Calculations using linear wave theory show that storm-deposited cross-strata formed in water depths as shallow as 2 m and as deep as 20 m. Storm-deposited bed forms, though widely known, are almost exclusively reported from shallow marine rocks. Their common presence in lacustrine sediments of the Ontario basin shows that these facies can be a major sedimentary component of the nearshore depositional record of lake basins. 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.

Swaley Cross-Stratification and Associated Features, Upper Cretaceous Western Interior Seaway of United States: ABSTRACT

Swaley Cross-Stratification and Associated Features, Upper Cretaceous Western Interior Seaway of United States: ABSTRACT

Gravel‐topped offshore bar sequences in the Lower Carboniferous of southern Ireland

ABSTRACTWithin the Kinsale Formation (Lower Carboniferous) of southern Ireland are pebbly sandstones and conglomerates contained in what is known locally as the Garryvoe conglomerate facies. In this facies there are three main groups of lithologies: (a) heterolithic mudrocks and sandstones characterized by a wide variety of wave‐produced structures; (b) sandstones dominated by swaley cross‐stratification (SCS), parallel lamination, and rare hummocky cross‐stratification (HCS); and (c) pebbly sandstones and conglomerates occurring as discrete beds or as gravel clasts dispersed through SCS sets. Successions of the facies comprise units of heterolithic mudrock and rippled sandstone alternating repeatedly with coarsening‐upward units of SCS pebbly sandstone capped by top‐surface granule and pebble lags.The Garryvoe conglomerate facies accumulated in a system of offshore bars on a muddy shallow‐marine shelf that was dominated by waves and currents generated by storms. Sands and gravels were bypassed from a contemporaneous northerly coastal zone to the shelf, where they were moulded by the storm‐generated flow into low, broad, sand ridges (offshore bars). The elongate bars were spaced kilometres apart, oriented obliquely to the coast, and separated by muddy interbar troughs. Their surfaces were largely covered by hummocky and swaley forms. Long‐term, gradual seaward migration of the offshore bars concentrated gravels on landward flanks from the dispersed pebbly sands that were on the crests and seaward flanks. Exceptionally intense storms could form laterally extensive winnowed gravel lags above thinned bar sequences. Such storms could also flush gravel‐bearing turbidity currents into muddy interbar trough areas.