Description: Rocky shorelines are relatively common features along modern coastlines, but few have been recognized in the geologic record. The hard substrates of rocky shorelines telescope the width of offshore marine environments, thus the diagnostic deposits observed in such settings today have a low preservation potential due to small accommodation space and high-energy conditions. This study recognized previously overlooked, laterally extensive Lower(?) Devonian rocky shoreline deposits in the San Juan Mountains of southwestern Colorado. The newly defined lithostratigraphic unit, the East Lime Creek Conglomerate (ELCC), is 0-23 m thick, unconformably overlying Proterozoic crystalline rocks and unconformably overlain by the Upper Devonian Ignacio Formation and/or Elbert Formation. The unit mostly consists of clast-supported cobble-boulder conglomerate with rounded quartzite clasts up to 1.4 m in length interbedded with thin sandstone layers and lenses. Sandstones in the ELCC are distinguished from unconformably overlying Upper Devonian sedimentary rocks because they have sericite cements. Most importantly, there are buttressing relationships between the ELCC and underlying Proterozoic crystalline rocks interpreted as palaeo-sea cliffs, palaeo-wave-cut platforms, and palaeo-tombolos. A proposed rocky shoreline facies model includes headlands with upper shoreface-beachface tabular cobble-boulder gravels sourced from rock fall talus, nearshore subaqueous debris flow deposits, and intervening pocket beaches with imbricated, stratified pebble-cobble gravel sheets. Palaeocurrent data (n = 338) from clast long-axis orientations, imbrication, and cross bedding indicate south-to-north transport roughly onshore-offshore to a coastline consisting of alternating rocky headlands and pocket beaches. This Lower(?) Devonian unit documents a previously unrecognized episode in the geologic history of southwestern Colorado. This article is protected by copyright. All rights reserved.

Description: The southern Central American active margin is a world-class site where past and present subduction processes have been extensively studied. Tectonic erosion/accretion and oblique/orthogonal subduction are thought to alternate in space and time along the Middle American Trench. These processes may cause various responses in the upper plate, such as uplift/subsidence, deformation, and volcanic arc migration/shut-off. We present an updated stratigraphic framework of the Late Cretaceous–Cenozoic Sandino Forearc Basin (SFB) which provides evidence of sedimentary response to tectonic events. Since its inception, the basin was predominantly filled with deep-water volcaniclastic deposits. In contrast, shallow-water deposits appeared episodically in the basin record and are considered as tectonic event markers. The SFB stretches for about 300 km and varies in thickness from 5 km (southern part) to about 16 km (northern part). The drastic, along-basin, thickness variation appears to be the result of (1) differential tectonic evolutions and (2) differential rates of sediment supply. (1) The northern SFB did not experience major tectonic events. In contrast, the reduced thickness of the southern SFB (5 km) is the result of at least four uplift phases related to the collision/accretion of bathymetric reliefs on the incoming plate: i) the accretion of a buoyant oceanic plateau (Nicoya Complex) during the middle Campanian; ii) the collision of an oceanic plateau (?) during the late Danian–Selandian; iii) the collision/accretion of seamounts during the late Eocene–early Oligocene; iv) the collision of seamounts and ridges during the Pliocene–Holocene. (2) The northwestward thickening of the SFB may have been enhanced by high sediment supply in the Fonseca Gulf area which reflects sourcing from wide, high relief drainage basins. In contrast, sedimentary input has possibly been lower along the southern SFB, due to the proximity of the narrow, lowland isthmus of southern Central America. Moreover, two phases of strongly oblique subduction affected the margin, producing strike-slip faulting in the forearc basin: i) prior to the Farallon Plate breakup, an Oligocene transpressional phase caused deformation and uplift of the basin depocenter, triggering shallowing-upward of the Nicaraguan Isthmus in the central and northern SFB; ii) a Pleistocene–Holocene transtensional phase drives the NW-directed motion of a forearc sliver and reactivation of the graben-bounding faults of the late Neogene Nicaraguan Depression. We discuss arguments in favor of a Pliocene development of the Nicaraguan Depression and propose that the Nicaraguan Isthmus, which is the apparent rift shoulder of the depression, represents a structure inherited from the Oligocene transpressional phase. This article is protected by copyright. All rights reserved.

Description: Existing dimensionless expressions that represent the incipient motion of sediments are based on studies of non-cohesive sediments. Because of the complex behaviour of cohesive sediments, many simulators also assume non-cohesiveness when simulating the erosion of cohesive sediments. However, studies show that the critical shear force needed for entrainment is much higher for consolidated cohesive sediments than for similarly sized non-cohesive sediments. Treating cohesive sediments as non-cohesive sediments thus will introduce a significant error with regard to quantifying the eroded sediment mass. On the other hand, the existing expressions of non-cohesive sediments require relatively detailed hydraulic calculations to estimate the shear velocity or the bed shear stress and thus cannot be used with simplified simulators. Therefore, it is essential to have a versatile simple explicit method that estimates the incipient motion condition of both the consolidated cohesive and non-cohesive sediments whenever needed. In this paper, explicit analytical expressions are proposed that simulate the incipient motion of consolidated cohesive and non-cohesive sediments, based on the critical erosion curves of the Hjulström-Sundborg-Miedema diagram. The new method reproduces the latter diagram with high precision. It also mimics the critical incipient condition of non-cohesive sediments determined by a well-known analytical method for other experimental data sets and for the East Fork River without the need of an iterative solution. The new approach provides essential information for estimating the entrainment condition of pebbles or finer sediments. Besides, the use of the mean flow velocity and the flow depth as predictors of incipient condition allows for its easy and efficient implementation in conceptual simulators that do not perform detailed hydraulic calculations and for use by modelers that are not familiar with the hydrotechnical literature. It also reduces the computation time required for simulation. This article is protected by copyright. All rights reserved.

Description: Tsunamis are marked by distinct phases of uprush during coastal inundation and backwash when tsunami water recedes. Especially in the case of a steep coastal profile, the return flow may operate in a Froude-supercritical regime, eroding the flooded area and transporting large volumes of sediment seawards. Important sediment accumulation occurs when the supercritical flow goes through a hydraulic jump where it becomes subcritical upon deceleration. An inferred example in coarse-grained, mixed carbonates from the Lower Pleistocene on Rhodes (Greece) is described, with offshore bars up to 10 m long with scour-and-fill structures and steep antidune stratification. In finer-grained sandy depositional systems such structures may be much longer, up to hundreds of metres. It is suggested here that, analogous to some turbidite beds, the apparent lack of structures or the presence of faint stratification that is common for graded sand layers within marine tsunamiites may in fact consist of extremely low-angle, landward-dipping backset-strata that formed under a landward-migrating hydraulic jump during the basinward retreat of tsunami water. Numerical simulations that focus on the internal stratification of backwash-generated offshore bars support this hypothesis. The recognition of such deposits in the sedimentary record enlarges the toolbox for assessing the past frequency of tsunamis in coastal areas. This article is protected by copyright. All rights reserved.

Description: In north-central Alberta and adjacent British Columbia, clastic strata of the middle to late Albian Peace River and Shaftesbury formations were deposited in alluvial to shallow marine environments across the foredeep of the Western Canada Foreland Basin. A high-resolution, log and core-based allostratigraphic framework for the Paddy Member of the Peace River Formation established nine allomembers, PA to PI, bounded by flooding surfaces and apparently equivalent non-marine surfaces. Within the estimated 2 Myr. duration of the Paddy, allomembers allow the evolving palaeogeography and changing relationship between accommodation and sedimentation rates to be analysed on time-steps on the order of 10 5 yrs. Paddy strata fill an arcuate depocentre ~ 300 km wide, across which the rocks thin eastward from 125 m to ~ 5-10 m. The northern part of the basin is occupied by muddy, offshore marine deposits that pass abruptly southward into a linear, WSW-ENE-trending body of sandstone deposited in a wave-dominated barrier-strandplain, at least 350 km long. Extending 〉 200 km to the south of the strandplain was a region of shallow brackish to freshwater lagoons and lakes that graded to the SW into alluvial facies. Within the lagoon region, few m thick, elongate and patchy sandstones represent river dominated deltas. In allomembers PA to PG, these sandstones are concentrated in the west and south, implying supply from the western Cordillera. In allomembers PH and PI, sandstones are mainly in the east and have a distinctive, quartz-rich composition. They can be correlated eastward into the coeval Pelican Formation, and were probably sourced from the Canadian Shield on the opposite side of the basin. In the western foredeep, alluvial rocks comprise aggradational, unconfined floodplain deposits with ribbon sandstones, dissected, on at least nine separate levels, by paleovalleys that are confined to the proximal foredeep. Valleys are 10-20 m deep, few km-wide, and filled with multi-storey channel-bars of pebbly coarse sandstone or conglomerate. Valleys cut down from well-developed interfluve palaeosols that record a falling and then rising water table. Alternating aggradation and degradation, and advance and retreat of the alluvial gravel front is attributed to cycles of varying rainfall intensity, rather than tectonism or eustasy. Apparently coeval transgressive-regressive successions in the lagoon and marine regions are attributed to few-m scale eustatic changes. On the NE margin of the basin, tidal sandstone fills a northward-opening estuary cut on the basal Paddy unconformity. This sandstone contains the first well-documented specimens of Gnesioceramus comancheanus (Cragin), proving contemporaneity with at least part of the marine Joli Fou Formation to the east. Paddy allomembers change shape upward from short blunt wedges, through more acutely tapered wedges, to sheets. This change reflects initially rapid flexural subsidence, attributed to active thickening of the adjacent orogenic wedge. A waning rate of deformation permitted wider dispersal of sediment across the basin, driving broad isostatic subsidence beneath increasingly sheet-like rock bodies. A major hiatal surface, VE3, records non-deposition or subtle erosion attributed to erosional unloading and uplift of the adjacent orogen. A subsequent marine transgression is attributed to renewed thickening of the tectonic wedge that triggered deposition of marine mudstone that thickens westward from 0 to 〉 110 m over 300 km. A postulated Milankovitch-band climatic control on both local gravel supply (via fluctuating rainfall), and shoreline movement (via ?Antarctic glacio-eustasy or groundwater storage), might account for cycles of alternating incision and aggradation in the alluvial realm. The same mechanism may also explain why shallow marine units such as the Cretaceous Viking and Cardium formations contain abundant conglomerate in lowstand shoreface deposits (higher river discharge), yet have highstand shorelines dominated by sandstone (lower river discharge). This article is protected by copyright. All rights reserved.

Description: Chemical sedimentary rocks in the Red Lake-Wallace Lake area of the Canadian Shield form the Earth's oldest known carbonate platform and, as such, provide a unique opportunity to explore the floor of a 2930 Myr old warm, shallow sea. Peritidal depositional features dominate the platform top, ranging from colloform crusts, teepee structures, and evaporate pseudomorphs in the supratidal; pseudomorph crystal fans and laterally linked domal stromatolites, associated with stromatactis-like structures, sheet cracks, and liminoid fenestral fabrics extending from the lower supratidal through the intertidal; and herring-bone cross-stratification, isolated domal stromatolites, and herring-bone calcite cement in tidal channels. These lithofacies indicate a low energy, restricted evaporitic environment. Limited subtidal platform top deposits are characterized by laterally linked domal stromatolites and pseudomorph crystal fans on a larger scale than those found in the intertidal areas. A transitional lithofacies to deeper water was deposited further offshore. It consists of ribbon rock (mixed laminae and beds of carbonate, slate, and iron oxide sediments), slump structures composed of intraclastic carbonate lithoclasts in a marl matrix, and carbonate associated iron formation. Basinal deposits consist of chert and chert-oxide facies iron formation. Peritidal lithofacies are composed of ferroan dolomite, whereas deep subtidal to upper slope lithofacies are composed of calcite. The dolomites have O and Sr isotopic ratios which were probably reset during dolomitization, whereas only O is altered in the limestones. The δ 13 C values for all the carbonate samples were 0 ± 1.1‰ V- PDB , with samples formed in deeper water having lighter δ 13 C values, suggesting the deeper open ocean had a lighter δ 13 C budget than water in the platform interior. Post Archean Australian Shale normalized rare earth element patterns for the carbonate samples have positive La and Eu anomalies, suprachondritic Y/Ho ratios, and slight heavy rare earth element enrichment in most samples. Basin lithofacies are characterized by heavy rare earth element enrichment and positive Eu anomalies. One sample had a significant negative Ce anomaly. This data probably indicates restricted circulation in the supra and intertidal areas and possibly the development of spatially limited areas where oxygen production could move the redox boundary out into the water column. This article is protected by copyright. All rights reserved.

Description: The continental settings of Central Asia witnessed increased desertification during the Cenozoic as a result of mountain uplift and the Paratethys retreat. The interaction of these tectonic-scale processes with orbitally-forced climate change and their influence on Asia's atmospheric moisture distribution are poorly constrained. A Miocene succession of continental mudflat deposits, exposed in the Aktau Mountains (Ili Basin, southeast Kazakhstan), has great potential as a terrestrial palaeoclimate archive. About 90 m of the 1700 m thick succession comprise alluvial mudflat deposits and appear as cyclic alternation of coarse sheet floods, mudflat fines and semi-arid hydromorphic soils. In this study, bulk-sediment mineralogy and geochemistry, magnetic susceptibility, sediment colour and palynology are used to reconstruct environmental conditions by determining changes and forcing mechanisms in the intensity of sediment discharge, weathering and pedogenesis. The results presented here indicate four major periods of arid soil formation and one palustrine interval characterized by higher evaporation rates under highly alkaline/saline conditions. A positive correlation between weathering indices and the Mg/Al ratio suggest that these horizons correspond to maximum rates of evapotranspiration and aridity. The formation of mudflat fines is, instead, interpreted to represent higher detrital sediment production by more intense alluvial fan activity during times of higher precipitation. Time-series analysis of weathering indices, colour and magnetic susceptibility data yields cycle-to-frequency ratios with the potential to represent Milankovitch cyclicity with short and long eccentricity as dominant periodicities. Periods of pronounced aridity, paced by long eccentricity forcing, reflect changes in moisture availability. On longer tectonic timescales, the persistent appearance of gypsum indicates a shift towards more arid conditions. This trend in climate is considered to result from the closure of the eastern gateway of the Mediterranean to the Indian Ocean that restricted circulation and enhanced salinity within the Eastern Paratethys. This article is protected by copyright. All rights reserved.

Description: Disentangling shallow-water bulk carbonate carbon isotope archives into primary and diagenetic components is a notoriously difficult task and even diagenetically screened records often provide chemostratigraphic patterns that significantly differ from global signals. This is mainly caused by the polygenetic nature of shallow-water carbonate substrates, local carbon cycle processes causing considerable neritic-pelagic isotope gradients and the presence of hiatal surfaces resulting in extremely low carbonate preservation rates. Provided here is an in-depth petrographic and geochemical evaluation of different carbonate phases of a mid-Cretaceous (Barremian-Aptian) shallow-water limestone succession (Jabal Madar section) deposited on the tropical Arabian carbonate platform in Oman. The superposition of stable isotope signatures of identified carbonate phases causes a complex and often noisy bulk carbon isotope pattern. Blocky sparite cements filling intergranular pores and bioclastic voids evidence intermediate to (arguably) deep burial diagenetic conditions during their formation, owing to different timing or differential faulting promoting the circulation of fluids from variable sources. In contrast, sparite cements filling sub-vertical veins reveal a rock buffered diagenetic fluid composition with an intriguing moderate enrichment in 13 C, probably due to fractionation during pressure release in the context of the Miocene exhumation of the carbonate platform under study. The presence of abundant replacive dedolomite in mud-supported limestone samples forced negative carbon and oxygen isotope changes that are either associated with the thermal breakdown of organic matter in the deep burial realm or the expulsion of buried meteoric water in the intermediate burial realm. Notwithstanding the documented stratigraphically variable and often facies related impact of different diagenetic fluids on the bulk rock stable isotope signature, the identification of diagenetic endmembers defined δ 13 C and δ 18 O threshold values that allowed the most reliable ‘primary’ bulk carbon isotope signatures to be extracted. Most importantly, this approach exemplifies how to place regional shallow-water stable isotope patterns with evidence for a complex multi-stage diagenetic history into a supraregional or even global context. This article is protected by copyright. All rights reserved.

Description: The stratigraphic record of microbially-induced sedimentary structures spans most of the depositional record. Today, microbes continue to generate, bind and modify sediments in a vast range of depositional environments. One of the most cited of these settings is the coastal microbial mat system of the Persian/Arabian Gulf. In this setting, an extensive zone of microbial mat polygons has previously been interpreted as resulting from desiccation-related contraction during episodic drying. This study employs 15 years of field-based monitoring of the interaction between environmental factors and the development and evolution of polygon morphologies to test the desiccation model in this setting. On the basis of these observations, a new model is proposed that accounts for the genesis and development of microbial polygons without the need for desiccation-induced shrinkage. Conversely, the formation, development and erosion of microbial polygons is a direct result of the production of large amounts of organic matter in a healthy, yet spatially-limited, microbial community. The recognition of microbial polygons has previously been applied as a diagnostic tool for the reconstruction of ancient depositional environments. The present study calls these interpretations into doubt. It is inferred that preservation of the microbial polygons as a recognisable form would be rare. Biological degradation and compaction will reduce polygons to produce the ‘wispy’ laminae that are a common feature of ancient sabkha lithofacies. This article is protected by copyright. All rights reserved.