Depositional Record最新文献

This study aims to interpret and document the depositional architecture styles and sequence stratigraphic framework of the Ash Shumaysi Formation in the Jeddah-Makkah region, the west-central part of Saudi Arabia, and presents an example of rarely discussed, local-scale sub-basins (half grabens). It also shows the relationships between synchronous sedimentary processes and pre, syn and post-rift conditions. The described lithofacies and their facies associations indicate the presence of seven architectural depositional styles: proximal-distal braided fluvial, meandering fluvial (point bar), crevasse splay, floodplain, estuarine and lacustrine. A proposed depositional model for the Ash Shumaysi Formation is drawn. The Ash Shumaysi Formation forms a second order depositional sequence, which is organised into two third order depositional sequences (sequences I and II) bounded by three sequence boundaries. Each third-order sequence encloses the low accommodation systems tract and high accommodation systems tract. The low accommodation systems tract represents the coarse-grained, braided-distal fluvial facies developed during low accommodation space associated with high sediment supply (high discharge). The high accommodation systems tract encloses the fine-grained deposits of point bar, estuarine and lacustrine facies that reflect the creation of significant accommodation space and low sediment supply (low discharge). Vertical and lateral variations of the inferred depositional architectural styles, sequences and systems tracts reflect that tectonic forces and climate are the main controlling factors during deposition of the Ash Shumaysi Formation, although base-level changes in response to sea-level changes cannot be ruled out.

On the Island of Samos (East Aegean region, Greece), two sedimentary basins are filled by thick continental series dated to the Late Miocene to Early Pliocene. A multidisciplinary study has been performed including (1) the definition of 21 sedimentary facies, (2) a review of the biological components and (3) carbon, oxygen and strontium stable isotope analyses. The succession is characterised by various depositional settings and hydrochemical compositions. Five main stages of basin evolution have been identified: (1) The Late Serravallian is marked by the development of alluvial fans and fan delta; (2) during the Lower Tortonian, isolated shallow lakes with variable salinity, from fresh to brackish, developed under warm and relatively humid conditions; (3) the Middle to Upper Tortonian is marked by the development of a large and deep lake with saline and alkaline waters, under colder and drier conditions; (4) the Latest Tortonian to Messinian period is represented by an ephemeral alluvial system, developed under a dry climate; (5) during the Zanclean, a palustrine and paludal wetland system, dominated by tufa carbonates, developed under moderately humid conditions. This succession is of particular interest for the reconstruction of the palaeoenvironmental evolution of the transition zone between the Mediterranean domain, and the Paratethys and circum-Paratethys areas. The geochemical data and the presence of flora (diatoms) and fauna (gastropods) of marine affinity suggest transient ingressions of marine-related water or groundwater inflows as early as the Lower Tortonian. The Samos succession records the complex interaction between the regional geodynamics and climate. The extensional regime of the Eastern Aegean zone generates subsidence, interrupted in the mid-Tortonian (9 Ma) by a brief compressive event and a major exposure of the basins. Furthermore, the Late Miocene progressive aridification, followed by a change to a more humid climate (Pliocene) is also a major driver of the sedimentation.

Deep-water carbonate deposition is relatively poorly understood but an area of vigorous research in academia and industry, where these deposits are a significant component of many unconventional petroleum reservoirs. Recent studies of modern deep-water carbonates have highlighted the wide variety of depositional processes, sediment types and resultant geomorphology; however, well-documented outcrops of ancient systems, their rock types and architecture are relatively sparse. The Mississippian Lake Valley Formation provides world-class exposures of slope-basinal carbonate deposits. The Tierra Blanca and Doña Ana members comprise submarine fans that are >14 to 20 km in length, >5 km wide, and exposed in strike and dip view, affording a unique opportunity to constrain the architecture, rock types and sedimentary processes. Tierra Blanca and Doña Ana sedimentation was dominated by crinoids shed from an up-dip platform and supplemented by sediments sourced locally from Waulsortian mounds. Depositional processes include turbidity flows, debris flows and hybrid sediment-gravity flows. The Tierra Blanca submarine fan thins towards its lateral flanks and distal fringe, where deposits become more mud-dominated, gravelly grain-supported flows are less common, and fewer beds have scoured bases. In proximal settings, bed tracing complemented by measured sections allow mapping of stratal surfaces and identification of stories, elements and complexes. The Tierra Blanca evolved from more unconfined to confined deposition. Point-sourced deposition of the Tierra Blanca fan required a funnelling mechanism, probably due to bathymetry created by Waulsortian mounds or possibly a platform margin re-entrant. Outcrop exposures illustrate that younger Doña Ana submarine fan deposits onlap onto, and compensationally stack with, the thickest portions of the antecedent Tierra Blanca fan. These outcrops illustrate both similarities and differences between carbonate and siliciclastic gravity flow deposits. Similarities include comparable deposit types, depositional processes and architecture; differences relate to hydrodynamics of carbonate grains, funnelling mechanisms for point-sourced deposits and sequence stratigraphic forcing.

Rocks of coastal to shallow-marine origin are challenging to interpret owing to the complex interplay of various depositional processes. This study reevaluates the relative roles of fluvial, tidal and wave processes in the Upper Cretaceous Sego Sandstone (and subordinately in the underlying Buck Tongue) of the Book Cliffs, USA, a well-studied ancient coastal to shallow-marine succession. Detailed sedimentological and ichnological analyses were used to interpret a previously underemphasised riverine signature, consisting of centimetre- to decimetre-thick alternations of sandstone and heterolithic beds inferred to represent flood–interflood periods of variable river discharge. Recognition of a widespread fluvial-dominated signature across the studied units better agrees with other sedimentological and regional observations in the study area, such as high sandstone–mudstone ratios, largely unidirectional and seaward-oriented palaeocurrents, and modelled weak tidal conditions in the basin. When considering all of the sedimentological, ichnological and stratigraphic observations together with its regional depositional context, the Sego Sandstone/Buck Tongue system is better explained using a mixed-energy but fluvial-dominated deltaic model. This highlights an historical over-interpretation of tidal processes and subordinate wave processes in the Sego Sandstone and likely in similar units. The widely used approach that emphasises only certain sedimentary features in discerning the process regime from analysis of rocks of inferred coastal to shallow-marine origin is unrefined and may therefore underrepresent the actual complexity of these systems.

The size and geometry of river channels play a central role in sediment transport and the character of deposition within alluvial basins across spatiotemporal scales spanning the initiation of grain movement to the filling of accommodation generated by subsidence. This study compares several different approaches to estimating palaeoflow depths from fluvial deposits in the early Palaeogene Willwood Formation of north-west Wyoming, USA. Fluvial story heights (n = 60) and mud plug thicknesses (n = 13) are statistically indistinguishable from one another and yield palaeoflow depth estimates of 4 to 6 m. The vertical relief on bar clinoforms (n = 112) yields smaller flow depths, by a factor of ca 0.3, with the exception that the largest bar clinoforms match story heights and mud plug estimates. This observation is consistent with modern river data sets that indicate unit bar clinoforms do not capture the reach-mean bank-full flow depths except in rare circumstances. Future studies should use story heights (i.e. compound bar deposits) and mud plugs to estimate bank-full flow depths in alluvial strata. Additionally, the thickness of multi-storied fluvial sandbodies (n = 102) and overbank cycles composed of paired crevasse splay and palaeosol deposits (n = 45) were compared. The two depositional units display statistically indistinguishable mean and median values. Building upon previous depositional models, these observations suggest basin rivers aggraded approximately one flow depth prior to major avulsion. This avulsion process generated widespread crevasse splay deposition across the floodplain. Once the main river channel stem was reestablished, overbank flooding and palaeosol development dominated floodplain settings. The depositional model implies river aggradation autogenically generated topography in the basin that was effectively filled during the subsequent avulsion. This constitutes a meso-timescale (10³–10⁴ years) compensational pattern driven by morphodynamics that may account for the high completeness of fossil and palaeoclimate records recovered from the basin.

Hamelin Pool, Shark Bay, Western Australia hosts the world's largest and most extensive assemblages of living marine microbialites, comparable in size and shape to ancient structures found throughout the fossil record. Documented here are the internal fabrics of modern microbialites collected throughout Hamelin Pool. Mesoscale and microscale observations of microbialite polished slabs and thin section scans, optical microscopy and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy formed the basis for a fabric classification system that combines accretionary mat type with microfabric. Accretionary mat types included pustular, smooth, colloform, as well as ‘transitional’ mats that are a cross between pustular and smooth mats. Mapping of fabrics in 45 microbialite heads indicated bidirectional evolution. An upward progression of fabrics corresponded to changes in mat type as the head grew upward into shallower water. A downward evolution of microfabrics occurred as surface mats transitioned into the subsurface of the microbialite structure. Downward microfabric evolution occurred as a result of early taphonomic processes, and involved a progression from the original depositional architecture to subsequent stages of “Micritic Thickening”, and finally, “Cement Infilling”. The observed bidirectional evolution of microbialite microfabrics within Hamelin Pool offers a conceptual framework for the study of modern microbialites, not simply as the sole product of accretionary mat types but rather as the combined result of the activity of surface mats and their taphonomic evolution. Early taphonomic processes induce further lithification of the microbialites which may enhance preservation potential in the geological record.

Holocene evolutionary history of the Banni Plain in the Great Rann of the Kachchh Basin is reconstructed from a subsurface sediment core of ca 50 m. Detailed data on textural and lithofacies variations, grain-size analysis, environmental magnetism and accelerator mass spectrometry ¹⁴C dates on seven samples were generated on the sediment core retrieved from the Banni Plain near Berada. A high-resolution record extending back to 10 ka has been reconstructed from the top ca 40 m of the core section comprising shallow marine sediments. The core is divisible into five depositional units. The basal part is a fluvial depositional unit followed upward by estuarine, sub-tidal, intertidal and supra-tidal environments. The sediment accumulation rate is highest in the sub-tidal to intertidal facies (1.9 cm year⁻¹) and decreases towards the supra-tidal facies to 0.09 cm year⁻¹. Environmental magnetic analysis, χlf coupled with the S-ratio, indicates high magnetic mineral concentrations during the Early Holocene, suggesting a wet period accompanied by high monsoon precipitation. This is followed by the onset of semi-arid conditions in the Great Rann of the Kachchh Basin as indicated by the low values of the χlf and S-ratios. A westward and northward shift in the shoreline towards the deeper part of the basin is suggested during the Late Holocene, which is coupled with aridity and reduced monsoonal conditions. The change in depositional pattern from the retrogradational deposit of fluvial (Unit 1) to estuarine sediment (Unit 2), progressing to sub-tidal (Unit 3), is attributed to sea-level transgression followed by regressive intertidal (Unit 4) to supra-tidal deposition (Unit 5), culminating in complete withdrawal of the sea, aided by tectonic uplift, during the Late Holocene. The results reveal that the sediment accumulation rates and depositional environments changed over time in response to changes in sea level from minima to maxima and then eventually to the present level.

Coral reef islands are vulnerable landforms to environmental change. Constructed of largely unconsolidated reef-derived sediments, they are highly sensitive to variations in metocean boundary conditions, raising global concern about their future resilience and stability in the face of increased natural hazards, sea-level rise and anthropogenic climate change. This study examines the evolution of an inshore turbid reef island from the southern Pilbara region of Western Australia (Indo-Pacific) using detailed analyses of island chronostratigraphy (composition, texture) and geochronology (21 in-situ radiometric dates) from Eva Island. Downcore, composition of island-grade (reef-derived) sediments were homogenous, dominated by molluscan (37%–42%) and coral (32%–37%) constituents. The ¹⁴C radiometric dating of island sediments, beachrock and coral microatolls identified five stages of island formation across changing sea-level regimes over the mid to late Holocene: (1) limestone platform accretion at ca 6,000 cal yr BP, coinciding with reef decline or ‘give-up’ on neighbouring Exmouth Gulf reefs; (2) sand cay (i.e. core) initiation and vertical aggregation at ca 5,000 cal yr BP during the point of sea-level regression to current levels; (3) major accretion and lateral progradation of the island between 3,500 cal yr BP and 2,500 cal yr BP including the modification of island shorelines through alongshore reworking of sediment; (4) lateral accretion and minor expansion to the north and formation of beachrock pavement between 2,500 and 650 cal yr BP; and (5) planform adjustment (erosion of the north-west island) and backstepping under stabilised sea levels over the past 650 years. While this model is comparative to studies on island formation following incremental sea-level fall following the mid-Holocene highstand, it demonstrates active landform readjustment under stabilised sea levels over the past 2,000 years, probably the influence of local-scale metocean boundary conditions within climate windows across the mid to late Holocene period (i.e. independent of sea-level fluctuations). Importantly, while sediment production rates are predicted to be lower in turbid-water reef systems than clear water, Eva Island shows no change in carbonate producers (i.e. proportion of mollusc and coral) over the course of island building, indicating the carbonate factory has not experienced significant adjustments in reef ecology, but has remained stable despite low water quality.

Methane-derived authigenic seep carbonates occur globally along continental margins. These carbonates are important archives to identify seep dynamics, the source of the ascending methane-enriched fluids together with their timing, and are an important carbon sequestration mechanism. Recently, seep carbonates were discovered in the Levant Basin in the south-eastern Mediterranean Sea. To elucidate past seepage activity and dynamics across the basin, different seep carbonate morphologies (chimneys, crusts and pavements) retrieved from the Levant Basin were mapped based on remotely operating vehicle data and analysed using standard sediment petrographic techniques, X-ray diffraction and stable carbon and oxygen isotope analyses. Carbonate chimneys consist of micrite (δ¹³C_VPDB of −10‰ to +5‰) with dispersed baryte and dolomite crystals, fan-shaped aragonite (δ¹³C_VPDB of −52‰ to −30‰) and high-magnesium calcite cements, with the latter often growing from low-magnesium calcite spherules. Botryoidal low-magnesium calcite cements are forming in small cavities. Carbonate crusts consist of micrite with low-magnesium calcite breccias, high-magnesium calcite nodules (δ¹³C_VPDB of −35‰ to −20‰) and cements, and partially replaced fan-shaped aragonite cements. Carbonate pavements consist of low-magnesium calcite microsparite, micritic dolomite and high-magnesium calcite. Fan-shaped aragonite is locally present as pore-lining cement. Iron oxides are often seen coating the low-magnesium calcite, high-magnesium calcite and dolomite cements. Chimneys and crusts, characterised by high amounts of high-magnesium calcite and aragonite, are interpreted to have formed through advective methane fluxes. Pavements, with high quantities of dolomite, are explained as the product of diffusive methane flux. Sediment petrographic and geochemical analysis of the different carbonate morphologies and cement phases therein witness distinct modes of ascending fluid fluxes and their mixing with marine pore water and/or sea water during precipitation of the individual phases.

Late Oligocene (ca 25 Ma) volcano-sedimentary successions exposed on the western periphery of the Doupovské Hory Volcanic Complex reveal a complex sedimentation history influenced in various ways by decay of the alkali basalt volcanic edifice. Weathering of the volcanic rocks supplied abundant reactants that promoted carbonate precipitation in the peripheral palaeolakes—as evidenced by strongly non-radiogenic ⁸⁷Sr/⁸⁶Sr values (0.7038–0.7041). On the other hand, the sediments of the initial shallow lake became deformed by the bulldozing effect of a debris avalanche. The debris flow and avalanche deposits filled up the original depression, modified the basin morphology and shifted the peripheral lacustrine setting further away from the volcano. At this stage, surface water influx from the surrounding granites conferred a more radiogenic character (⁸⁷Sr/⁸⁶Sr values 0.7046–0.7049) to the calcrete deposits. Fossil assemblages as well as limestone textures suggest significant seasonal water-level fluctuations, possibly reflecting the alternating rainy and dry-seasons of a prevalently humid Central-European Late Oligocene climate. The seasonal drying out of the ponds resulted in significant ¹⁸O enrichments. Although the ca 0‰ δ¹³C values might suggest mixing of atmospheric and volcanic CO₂ during carbonate precipitation, no active volcanic conduits of relevant age are known in the close vicinity. The lower δ¹³C values are likely a result of mantle degassing through rift faults, a phenomenon observed in the magmatically extinct Ohře Rift until present. This paper demonstrates that limestones derived from weathered alkaline basalts are characterised by highly non-radiogenic Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr ca 0.704), suggesting a magmatic origin for the Ca within these carbonates. Contrary to the notion of carbonatites being present when highly non-radiogenic Sr isotopes are found, these results show that Sr isotopes in carbonates formed in alkali basalt-sourced environments only reveal the source of the Sr (and Ca) ions, not necessarily the presence of carbonatite.