Sedimentology

doi: 10.1111/sed.12895pmid: N/A

Campos‐Soto, Sonia; Benito, M. Isabel; Mountney, Nigel P.; Plink‐Björklund, Piret; Quijada, I. Emma; Suarez‐Gonzalez, Pablo; Cobos, Alberto; Veiga, Gonzalo

doi: 10.1111/sed.12958pmid: N/A

Deciphering the palaeoenvironmental and palaeoclimatic setting of ancient successions that include deposits typical of different climates can be challenging. This is the case in the Late Jurassic succession cropping out in eastern Spain (South‐Iberian and western Maestrazgo basins), where deposits characteristic of both arid to semiarid and humid to subhumid settings have been identified through a detailed analysis of eight stratigraphic sections. These sections comprise shallow marine carbonates changing upward and laterally to a predominantly siliciclastic coastal and alluvial succession, including abundant dinosaur remains. Deposition of coastal and alluvial sediments occurred in flood plains, ephemeral and perennial fluvial channels, aeolian dunes, deltas, distributary mouth‐bars and associated distributary channels, and shallow water bodies influenced by both fresh and marine waters. Some of these deposits, notably those of aeolian and ephemeral fluvial origin, are characteristic of arid to semiarid climates. However, there are also abundant deposits that can be demonstrably shown to have a coeval origin, which are indicative of permanent water courses: (i) sediments of seasonal discharge fluvial channels with perennial to semi‐perennial flow, displaying subcritical and supercritical flow sedimentary structures; (ii) deltaic sediments deposited in permanent freshwater bodies; and (iii) abundant plant and dinosaur remains, especially of herbivorous dinosaurs, which required the presence of permanent water sources and abundant vegetation. These apparently contrasting sedimentary features indicate that deposition occurred under a seasonal climate controlled by monsoonal‐type precipitation. These deposits are analogous to those observed nowadays in the Lençóis Maranhenses National Park (north‐east Brazil), where a subhumid tropical climate with a seasonal precipitation pattern prevails. Thus, this study shows that only through careful facies analysis and interpretation of depositional processes that can be shown to be occurring concurrently in neighbouring and related depositional systems can the detailed palaeoenvironmental and palaeoclimatic setting of complex coastal sedimentary successions be confidently reconstructed in detail.

Allport, Hamilton A.; Davies, Neil S.; Shillito, Anthony P.; Mitchell, Emily G.; Herron, Seán T.; Mángano, M. Gabriela

doi: 10.1111/sed.12947pmid: N/A

A heterolithic tidalite succession yielding spring–neap bundles is newly reported from a mid‐Carboniferous (Serpukhovian) section of the Alston Formation of Northumberland, England. The rhythmite records deposition over an interval that can be confidently calibrated to at least 84 lunar days, and attests to a non‐negligible tidal range in parts of the Northwest European Seaway in the late Mississippian. The tidalite is notable for the presence of a striking crowded Skolithos ichnofabric on both bedding planes and in vertical section. Bedding plane expressions of the ichnofabric reveal true substrates of sand piles excavated during burrow construction, in addition to an apparently remarkable equal spacing between individual burrows that is shown to be genuine through pair correlation function analysis. These characteristics show that the burrowed horizons were registered by contemporaneous ichnocoenoses, with no palimpsesting of burrows. The irregular vertical distribution of burrow horizons, despite a near‐continuous semi‐diurnal record of sedimentation, is suggested to be an artefact of spatial patchiness of burrowing communities in the depositional environment; imperfectly registered in a vertical profile with high‐temporal, low‐spatial resolution. The succession proves that burrow palimpsesting is not an inevitable ichnological conclusion of sedimentary stasis, and attests to intermittent palaeoecological fidelity of the stratigraphic record at the small spatio‐temporal scales recorded at outcrop.

Mitchell, Neil C.; Jerrett, Rhodri; Langman, Rob; Baas, Jaco

doi: 10.1111/sed.12935pmid: N/A

Repeated multibeam echo‐sounder surveys can provide information on developing stratigraphy over large areas and during periods when environmental conditions are known. In this study, analysis encompasses 13 time‐separated multibeam echo‐sounder surveys between 2002 and 2010 of a tidal sand ridge in a macrotidal estuary: Nash Sands, a banner bank in the Bristol Channel (UK). Over the surveyed period, Nash Sands was S‐shaped in plan‐view, with two en échelon segments separated by a channel (swatchway). Migration of these inflections along the ridge led to deposition of clinoforms up to 5° to 7° steep and 12 to 14 m tall. The clinoforms downlapped onto their substrates or onto cross‐sets formed by dunes migrating clockwise around the lower flanks of the ridge. Clinoform topsets were removed or truncated by repeated erosion and/or dune migrations over the ridge crest and replaced with packages of near‐horizontal laterally discontinuous irregular beds. Flank dune crests were oriented obliquely to nearly perpendicularly to the clinoforms in all three sets, so the clinoforms developed by oblique−lateral accretion locally. Although one area had strongly eroded in the prior decade during elevated wave conditions, the 2002 to 2010 stratigraphic development revealed a different relationship with extreme wave heights; Nash Sands generally accumulated sand during times of more extreme waves and lost sand during more quiescent conditions. Using also single‐beam survey data from 1991 to 2002 to study the ridge morphology over 19 years to 2010, the swatchway was absent in 1991 to 1992 and progressively developed as the ridge sinuosity became more accentuated. Dunes found migrating north‐west through the swatchway are potential evidence of a current caused by tidal height differences across the ridge during ebb conditions. The study illustrates how repeated sonar measurements reveal the processes and timescales that lead to the deposition of stratigraphic units.

Tomašových, Adam; Gallmetzer, Ivo; Haselmair, Alexandra; Zuschin, Martin; Betzler, Christian

doi: 10.1111/sed.12936pmid: N/A

A sequence stratigraphic framework predicts that time averaging and hiatus durations will be long at times of fastest sea‐level rise. This prediction does not necessarily apply to environments where carbonate production keeps up with sea‐level rise and where undetected hiatuses decouple short‐term from long‐term sedimentation rates. The taphonomic clock, however, which measures the residence time of skeletal particles in the mixed layer, can estimate the duration of hiatuses if the rate of skeletal alteration is slow and if skeletal particles endure long‐term exposure in the mixed layer. Here, time averaging is calibrated by using evidence from alteration of bivalves in a metre‐scale Holocene sequence in the Adriatic Sea. In this sequence, transgressive molluscan lags, a maximum‐flooding zone shell bed with bivalves, and highstand bryomol assemblages were all deposited under similar long‐term sedimentation rates (ca 0.01 to 0.03 cm year−1) and exhibit millennial time averaging. Median ages of valves stained by pyrite and cemented by high‐magnesium calcitic micritic envelopes exceeding ca 1000 years indicate that: (i) these authigenic processes are slow in subsurface zones with reducing conditions (with prolonged sulphate reduction and carbonate ions sourced from dissolved shells in the surface zones); and (ii) subsurface micrite precipitation prolongs the disintegration half‐lives of valves exhumed to surface zones from decades to millennia. The high abundance of stained valves, valves with micrite envelopes, and valves with composite alteration (encrusters and borers colonizing stained and cemented grains) thus identifies hiatuses and skeletal concentrations time‐averaged to >1000 years. The upcore decrease in abundance of valves with composite alteration, coupled with temporally‐constant long‐term sedimentation rates and time averaging, indicates that a temporal decline in sediment exhumation was compensated by a decline in burial of skeletal carbonate produced by molluscs.

Engel, Max; Strohmenger, Christian J.; Peis, Kim T.; Pint, Anna; Brill, Dominik; Brückner, Helmut; Eberli, Gregor

doi: 10.1111/sed.12938pmid: N/A

Sabkhas are key landforms along the southern coast of the Arabian Gulf and represent modern analogues for depositional and diagenetic processes controlling properties and quality of ancient hydrocarbon‐bearing carbonates. While previous investigations of coastal sabkhas in Qatar have mainly focused on dolomitization processes, presented here is one of the first studies reconstructing facies changes and coastal formation in great detail. In the sabkha of Al‐Kharayej (Gulf of Salwa), fifteen different facies types were distinguished based on twelve sediment cores, two trenches, as well as grain‐size distribution, X‐ray powder diffraction, thin section and microfossil analyses. Age estimates were based on seventy‐eight 14C‐AMS and optically stimulated luminescence data. The sabkha parasequence comprises pre‐transgressive dune sands, a thin, transgressive layer of reworked dune material, a mid‐energy open‐coast to open‐lagoon facies, a low‐energy lagoon facies, saline lake facies (salina: swallow‐tail gypsum and gypsum mush) and the supratidal sabkha characterized by diagenetic overprinting (buckled gypsum crusts and halite crust). Transgressive marine flooding created open‐coast to open‐lagoon sedimentation after ca 7000 cal yr bp, followed by initial spit formation at the northern sabkha end at the beginning of the relative sea‐level highstand (6000 cal yr bp). This main outer spit prograded southward and a more narrow, low‐energy spit, diverted landward, closing a small lagoon in the northern sabkha 4500 to 4000 cal yr bp. The falling relative sea‐level and longshore drift intensified the southward extension and widening of the main spit, and the main lagoon became more shallow. At 2000 to 1500 cal yr bp, the outer spit had almost closed the main lagoon, leading to salina and, finally, sabkha conditions. It is shown how specific local conditions (coastline orientation; wind, wave, tidal energy, longshore drift; depositional relief; sediment sources) created a spit‐controlled sabkha that is genetically distinct from the classical model of shore‐perpendicular accumulation of coarser sediment during high tides or storms.

Gugliotta, Marcello; Saito, Yoshiki; Ta, Thi Kim Oanh; Nguyen, Van Lap; La Croix, Andrew D.; Wang, Zhanghua; Tamura, Toru; Nakashima, Rei; Lieu, Kim Phuong; Fielding, Christopher

doi: 10.1111/sed.12940pmid: N/A

To better understand the sedimentary facies of tide‐dominated deltas, a core dataset from the Ba Lai palaeochannel in the Mekong River Delta was obtained and studied. Nine sedimentary facies were identified and interpreted as representing the Late Holocene evolution of the Ba Lai palaeochannel, including its pre‐abandonment and post‐abandonment phases, as well as pre‐channel phases. The channel formed at 2.6 ka as a distributary channel connected to the deltaic network and was abruptly abandoned and rapidly infilled with sediment at 0.7 ka. The channel deposits are up to ca 11 m thick and overlie shelfal shell layers, which, in turn, overlie Mid‐Holocene and Pleistocene deposits. The active‐channel fill and mouth‐bar deposits consist of sand and mud with cyclical patterns, bidirectional lenses and abundant mud layers, suggesting deposition mainly driven by river and tidal processes. The abandoned‐channel fill consists entirely of organic‐rich mud, suggesting a predominance of tidal processes. Other sedimentary facies include tidal‐flat and marsh deposits; they mostly consist of mud and formed in shallow to subaerial areas near the channel margins or on barforms. Depending on the exact location of the core in this depositional setting, three possible stratigraphic successions and facies models are presented herein. Near channel margins, the deposits show an upward gradual change from heterolithic to mud with a well‐developed fining‐upward trend. Away from the channel margins, the deposits show an upward sharp change from heterolithic to mud due to the channel abandonment. The mouth‐bar‐area facies model shows an upward gradual change from heterolithic to heterolithic/sand to mud deposits with a coarsening‐upward to fining‐upward trend. Although differences among systems likely exist, the authors suggest that the sedimentary facies described in this study and the resulting facies models should be used to better understand tide‐dominated deltaic systems and to improve their interpretation in the geological record.

Cosma, Marta; Lague, Dimitri; D’Alpaos, Andrea; Leroux, Jerome; Feldmann, Baptiste; Ghinassi, Massimiliano; Plink‐Björklund, Piret

doi: 10.1111/sed.12942pmid: N/A

Intertidal meanders developed on salt marshes are known to expand and produce inclined heterolithic stratification rich in fine‐grained sediments and to bear evidence for rhythmic deposition in the upper part of the inner meander bend (i.e. the upper part of the point bar). This occurs because the lower point‐bar deposits are washed by strong currents, which remove mud drapes and develop discontinuous record of tidal cycles. Although these criteria are widely accepted, facies models for tidal point bars still lack a three‐dimensional perspective and overlook the along‐bend variability of sediment distribution. The present study focuses on a hypertidal point bar belonging to the upper‐intertidal domain of the Mont‐Saint‐Michel Bay (France), and it analyses the sedimentology of a 3D time‐framed accretionary package formed between 28 March 2012 and 29 November 2012 by means of lidar topographic data, geomorphological field surveys and sedimentological core data. To define the 3D time‐framed accretionary package, data from thirteen lidar (light detection and ranging) topographic surveys were used to create the point‐bar synthetic stratigraphy. Data show that over the study period the point bar expanded alternating deposition along its seaward and landward sides, pointing out the occurrence of depositional patterns more complex than a simple progressive expansion of the bend. The thickest deposits were accumulated in the point‐bar‐apex zone, where the largest amount of mud was also stored. High sediment accretion in the bend‐apex zone is ascribed to the development of low‐energy conditions due to flow and bed configuration. High accretion rate of the point‐bar‐apex zone promoted also a better preservation of rhythmites, which are almost missing from deposits accumulated along the point‐bar sides. This study remarks that preservation of mud and tidal rhythmites within intertidal‐point‐bar deposits is controlled, not only by their elevation with respect to the tidal range, but also by their location along the point bar.

Mitchell, Stephen; Pilarczyk, Jessica E.; Spiske, Michaela; Jaffe, Bruce; Reid, Catherine

doi: 10.1111/sed.12944pmid: N/A

Modern microfossil distributions reflect site‐specific habitats and provide an opportunity to assess sediment transport pathways in the nearshore environment. When applied to overwash deposits in the geological record, they provide insight into sediment provenance and transport, factors important for understanding patterns of frequency and intensity of past storms and tsunamis. Modern distribution studies are rare and often the first established ones occur immediately after an overwash event as part of a post‐event field survey. This is problematic because it is unclear what effect overwash events have on nearshore microfossil assemblages and what time interval is necessary for them to return to pre‐event conditions. This study documents the impacts of Hurricane Irma on nearshore sediments off the coast of Anegada, British Virgin Islands, using distributions of Homotrema rubrum, an encrusting foraminifer with a defined provenance in coral reefs. At four sampling intervals spanning two years, from six months pre‐Hurricane Irma to eighteen months after, surface sediment was collected from three transects on the northern and southern shores of the island. Partitioning Around Medoids cluster analysis revealed that Hurricane Irma introduced an influx of well‐preserved fragments into the reef flat and made the sediments more uniform, limiting the foraminifer’s utility as a known sediment transport indicator. The mixing of sediments along the two northern transects (reef proximal) persisted for seven to eighteen months before returning to near pre‐hurricane conditions. However, the southern transect (absence of reef), where Homotrema rubrum concentrations are significantly less, failed to recover within the time period assessed by this study, indicating a variable recovery period between Atlantic Ocean and Caribbean Sea facing shorelines. Results from this study suggest that a waiting period of at least eighteen months after a major storm is recommended before collecting surface sediment from the nearshore environments of reef‐dominated coastlines.

Ash‐Mor, Anat; Almogi‐Labin, Ahuva; Bouchet, Vincent M. P.; Seuront, Laurent; Guy‐Haim, Tamar; Ben‐Avraham, Zvi; Bookman, Revital; Dey, Subhasish

doi: 10.1111/sed.12945pmid: N/A

Transport of continental shelf sediments to the deep ocean can be studied from displaced symbiont‐bearing larger benthic foraminifera found in turbidity current deposits. The larger benthic foraminifera habitat depth, physical characteristics and preservation serve as indicators for understanding sediment transport dynamics near the seabed and in the water column. Here, an experiment was designed to explore sediment transport in a closed flume system using simulated high current velocities. Shelf sediments from the Gulf of Eilat/Aqaba, dominated by Amphistegina papillosa and Operculina ammonoides, were subjected to 60 cm s−1 and 80 cm s−1 current velocities while collected in a 10 cm vertical sediment trap. Larger benthic foraminifera abundance, shell physical properties and preservation were analyzed and compared with the original bulk sediments. The experiment results showed that at 80 cm s−1 velocity, larger benthic foraminifera shells of all sizes and preservations are efficiently resuspended and transported in large quantities throughout the water column, as opposed to their transport as bedload by the lower velocity current. Larger benthic foraminifera shape also has a role in the transport distances and accumulation depths. Operculina ammonoides shells were found to be more portable, compared to Amphistegina papillosa, due to their flatter discoid shape. The results suggest that a threshold velocity of ca 80 cm s−1 was needed to generate the thick coarse deposits found in the Gulf of Eilat/Aqaba slope sedimentary record, which were previously suggested to be triggered by large magnitude seismic events. Lower velocities probably winnowed minor amounts of larger benthic foraminifera shells (with little or no coarser sediments) that were deposited as a thin sand layer may point to lower magnitude seismic triggers. In conclusion, larger benthic foraminifera shells are transported and deposited in accordance with their hydrodynamic properties, resulting in assemblage differentiation along the transport pathway. This study shows that the fossil biogenic composition in slope sediments includes valuable information on current velocities, transport dynamics and possible triggers in the geological record.