Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf off west-central Florida

High-resolution side-scan mosaics, sediment analyses, and physical process data have revealed that the mixed carbonate/siliciclastic, inner shelf of west-central Florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms. The sand ridges, mostly oriented obliquely to the shoreline trend, extend from 2 km to over 25 km offshore. They show many similarities to their well-known counterparts situated along the US Atlantic margin in that both increase in relief with increasing water depth, both are oriented obliquely to the coast, and both respond to modern shelf dynamics. There are significant differences in that the sand ridges on the west-central Florida shelf are smaller in all dimensions, have a relatively high carbonate content, and are separated by exposed rock surfaces. They are also shoreface-detached and are sediment-starved, thus stunting their development. Morphological details are highly distinctive and apparent in side-scan imagery due to the high acoustic contrast. The seafloor is active and not a relict system as indicated by: (1) relatively young AMS ¹⁴C dates (<1600 yr BP) from forams in the shallow subsurface (1.6 meters below seafloor), (2) apparent shifts in sharply distinctive grayscale boundaries seen in time-series side-scan mosaics, (3) maintenance of these sharp acoustic boundaries and development of small bedforms in an area of constant and extensive bioturbation, (4) sediment textural asymmetry indicative of selective transport across bedform topography, (5) morphological asymmetry of sand ridges and 2D dunes, and (6) current-meter data indicating that the critical threshold velocity for sediment transport is frequently exceeded. Although larger sand ridges are found along other portions of the west-central Florida inner shelf, these smaller sand ridges are best developed seaward of a major coastal headland, suggesting some genetic relationship. The headland may focus and accelerate the N–S reversing currents. An elevated rock terrace extending from the headland supports these ridges in a shallower water environment than the surrounding shelf, allowing them to be more easily influenced by currents and surface gravity waves. Tidal currents, storm-generated flows, and seasonally developed flows are shore-parallel and oriented obliquely to the NW–SE trending ridges, indicating that they have developed as described by the Huthnance model. Although inner shelf sand ridges have been extensively examined elsewhere, this study is the first to describe them in a low-energy, sediment-starved, dominantly mixed siliciclastic/carbonate sedimentary environment situated on a former limestone platform.     

Patterns and controls of surface sediment distribution: west-central Florida inner shelf

The west-central Florida inner shelf represents a transition between the quartz-dominated barrier-island system and the carbonate-dominated mid-outer shelf. Surface sediments exhibit a complex distribution pattern that can be attributed to multiple sediment sources and the ineffectiveness of physical processes for large-scale sediment redistribution. The west Florida shelf is the submerged extension of the Florida carbonate platform, consisting of a limestone karst surface veneered with a thin unconsolidated sediment cover. A total of 498 surface sediment samples were collected on the inner shelf and analyzed for texture and composition. Results show that sediment consists of a combination of fine quartz sand and coarse, biogenic carbonate sand and gravel, with variable but subordinate amounts of black, phosphorite-rich sand. The carbonate component consists primarily of molluskan fragments. The distribution is patchy and discontinuous with no discernible pattern, and the transition between sediment types is generally abrupt. Quartz-rich sediment dominates the inner 15 km north of the entrance into Tampa Bay, but south of the Bay is common only along the inner 3 km. Elsewhere, carbonate-rich sediment is the predominate sediment type, except where there is little sediment cover, in which cases black, phosphorite-rich sand dominates. Sediment sources are likely within, or around the periphery of the basin. Fine quartz sand is likely reworked from coastal units deposited during Pleistocene sea-level high stands. Carbonate sand and gravel is produced by marine organisms within the depositional basin. The black, phosphorite-rich sand likely originates from the bioerosion and reworking of the underlying strata that irregularly crop out within the study area. The distribution pattern contains elements of both storm- and tide-dominated siliciclastic shelves, but it is dictated primarily by the sediment source, similar to some carbonate systems. Other systems with similar sediment attributes include cool-water carbonate, sediment-starved, and mixed carbonate/siliciclastic systems. This study suggests a possible genetic link among the three systems.     

Secular variations of the upper crust composition: Implication of geochemical data on the Upper Precambrian shales from the Southern Urals western flank and Uchur-Maya region

In the mid-1980s, it was concluded based on geochemical study that Th, Sc, La concentrations and ratios Th/Sc, La/Sc and Eu/Eu* did not wary significantly in the post-Archean time. It was impossible to judge about compositional variations of upper crust during the Riphean and Vendian, because data of that time characterized a limited number of samples from the post-Archean basins of Australia, New Zealand, and Antarctic. Considered in this work are variations of Eu/Eu*, LREE/HREE, Th/Sc, and La/Sc ratios in Upper Precambrian fine-grained siliciclastic rock of the Southern Urals western flank (Bashkirian meganticlinorium) and Uchur-Maya region (Uchur-Maya plate and Yudoma-Maya belt). As is established, only the Eu anomaly in the studied siliciclastic rocks is practically identical to this parameter of the average post-Archean shale. Three other parameters plot on the Riphean-Vendian variation curves with positive and negative excursions of diverse magnitude, which do not coincide always in time. It is assumed that these excursions likely mark stages of local geodynamic activity, destruction of pre-Riphean cratons, and progressing recycling of sedimentary material during the Riphean.     

龙门山地区泥盆纪碳酸盐与硅质碎屑的混积相与混积机理

碳酸盐与陆源碎屑的混合沉积研究至今尚未引起足够重视。文献屈指可数。扬子西经缘龙门山地区盆纪发育了了这类良好的混合沉积。作者根据实际材料，首次从这一思路出发，识别该区泥盆纪Ｅｍｓｉａｎ期到Ｇｉｖｅｔｉａｎ早期的混积相四种，混积亚相约１４种，阐明了其优势混积作用；根据混积作用特征、沉积背景关系，划分出两大混积类型，即泥质背景下的混积１型和砂灰质背景下的混积Ⅱ型；对混积作用机理进行了探讨；并指出不同级次     

Complex hydraulic interactions between compartmentalized carbonate aquifers and heterogeneous siliciclastic successions: a case study in southern Italy

The sedimentary sequences containing lithologic units with low permeability represent hydrogeologic systems, which, as of now, have been little studied despite their diffusion worldwide. A hydrogeologic study, aimed to assess the main factors controlling the groundwater flow dynamics in such systems and their hydraulic interactions with nearby carbonate aquifers, has been carried out in Longano (Isernia, Southern Italy). The analysis of the hydraulic heads, combined with the regimes of the springs and the electric conductivity of the groundwater, mainly reflect vertical and lateral heterogeneities of the media in terms of hydraulic properties. In particular, the flow system is controlled by lateral heterogeneities, which characterize a surficial horizon made up of clayey colluviums and talus deposits, separated from the deeper saturated, fissured bedrock. One‐to‐ten relationships in hydraulic heads, monitored in piezometers crossing the fissured media, further uphold the crucial role played by the lateral contrasts of permeability in controlling the flow dynamics. On the whole, significant interactions with the nearby carbonate hydrostructure take place. Nevertheless, the heterogeneities of the siliciclastic succession and surficial horizon, coupled with the compartmentalization of the carbonate system, lead to a complex hydrogeological scenario. In a wider perspective, this study gives information of utmost importance in order to improve the implementation of mathematical models and configuration of tapping works within these heterogeneous and complex settings. Copyright © 2015 John Wiley & Sons, Ltd.     

Subtropical to temperate facies from a transition zone, mixed carbonate–siliciclastic system, Palaeogene, North Carolina, USA

Palaeogene passive margin sediments on the US mid‐Atlantic coastal plain provide valuable insight into facies interaction and distribution on mixed carbonate–siliciclastic shelves. This study utilizes well cuttings, outcrop, core, and seismic data to document temporal and spatial variations in admixed bryozoan‐rich skeletal carbonates and sandy siliciclastic units that were deposited on a humid passive margin located in the vicinity of a major marine transition zone. This zone was situated between north‐flowing, warm waters of the ancestral Gulf Stream (carbonate dominated settings) and south‐flowing, cold waters of the ancestral Labrador Current (siliciclastic dominated settings). Some degree of mixing of carbonates and siliciclastics occurs in all facies; however, siliciclastic‐prone sediments predominate in nearshore settings, while carbonate‐prone sediments are more common in more open marine settings of the inner shelf break and deep shelf. A distinctive dual‐break shelf depositional profile originated following a major Late Cretaceous to Palaeocene transgression that drowned the earlier shallow platform. This profile was characterized by prominent mid‐shelf break dividing the shallow shelf from the deep shelf and a major continental shelf/slope break. Incomplete filling of available accommodation space during successive buildup of the shallow shelf preserved the topographic break on this passive margin. Storm wave base also contributed to the preservation of the dual‐break shelf geometry by beveling shallow shelf sediments and transporting them onto and seaward of the mid‐shelf break. Sediment fines in deep shelf facies were produced in place, transported downdip from the shallow shelf by storm ebb currents and boundary currents, and reworked from adjacent areas of the deep shelf by strike‐parallel boundary currents. Regional climate and boundary currents controlled whether carbonate or siliciclastic material was deposited on the shelf, with warmer waters and more humid climates favouring carbonate deposition and cooler, more arid conditions favouring glaucony and siliciclastic dominated deposition. Continuous wave and current sweeping of the shallow shelf favoured deposition of mud‐lean facies across much of the shallow shelf. Skeletal components in much of the carbonate‐rich strata formed in warm, nutrient‐rich subtropical waters, as indicated by widespread occurrences of larger benthic foraminifera and molluscan assemblages. These indicators of warm water deposition within the bryozoan‐mollusk‐rich carbonate assemblage on this shelf provide an example of a warm water bryomol assemblage; such facies generally are associated with cooler water depositional settings.     

Heterogeneous aquitard properties in sedimentary successions in the Apennine chain: case studies in southern Italy

The aim of this study was to analyse the hydrogeologic role of siliciclastic deposits and marly‐clayey‐calcareous successions within the carbonate Apennine chain (southern Italy). The study was carried out along the northern part of the Matese carbonate massif through (1) the hydraulic characterization of siliciclastic rocks in a test site, by means of Lugeon tests, and (2) the identification of the groundwater flow system discharging at an important spring located within a marly‐clayey‐calcareous succession in a second test site, by means of isotopic investigations. The results showed that the investigated siliciclastic deposits and marly‐clayey‐calcareous successions may allow significant groundwater discharge from carbonate aquifers. Thus, they do not everywhere behave as aquitard, contrary to the previous model. Instead, groundwater flows through the upper part of these successions, where stress release fracturing enhanced rock permeability in the near‐surface bedrock. Thus, these successions may locally be a new groundwater source within the southern Apennine chain. Copyright © 2009 John Wiley & Sons, Ltd.     

Coeval versus reciprocal mixed carbonate–siliciclastic deposition, Middle Devonian Baltic Basin, Eastern Europe: implications from the regional tectonic development

The Middle Devonian Narva succession in the Baltic Basin represents a significant turnaround in the history of the basin. The detailed study of core and outcrop sections and the three‐dimensional correlations across the Baltic Basin reveal a carbonate‐dominated, mixed retrogressive succession, overlain by a siliciclastic‐dominated, progradational succession. The palaeogeographic reconstructions show how the shallow, tide‐influenced basin expanded from south‐west to north‐east and, later during the transgression, also to the north, south and east. The transgressive portion of the basin fill is dominated by carbonate‐rich sabkha and supratidal to intertidal deposits on the basin margins, and subtidal carbonates in the basin centre. Siliciclastic material was derived by tidal currents and storm waves from the south‐west through a tidal inlet and flood‐tidal delta complex. This initial transgressive phase is characterized by the lack of subsidence or even episodic uplifts in the northern/north‐western part of the basin margin, shown by convergence of timelines and the thin (30 m) transgressive succession. In contrast, on the southern margin, the facies associations stack vertically into a 70 to 80 m thick succession, indicating significantly higher subsidence rates. The upper part of the transgressive phase indicates subsidence across the whole basin. The upper, progradational portion of the basin fill is dominated by coarse, siliciclastic, tide‐influenced deltaic deposits that rapidly prograded from north‐west to south‐east. This detailed study on the Narva succession shows that siliciclastic and carbonate deposition was coeval and that mixing occurred at different temporal and spatial scales. The mixing was controlled by grain‐size, volume and location of siliciclastic input rather than relative sea‐level changes as suggested in widely used reciprocal mixing models. It is suggested that the forebulge of the Scandinavian Caledonian fold‐and‐thrust belt migrated to the north‐western margin of the Baltic Basin during the earliest Eifelian, as indicated by the lack of subsidence and probable uplift in the northern/north‐western margin during the early transgressive phase. The forebulge migration ceased although the forebulge had already started to subside during the later stages of the transgressive phase. The deltaic progradation is interpreted to be associated with the orogenic collapse and uplift in the Scandinavian Caledonides that caused the erosion of the foreland basin fill and the coarse sediment transport into the Baltic Basin.     

层序地层学研究进展:国际层序地层学研讨会综述

结合"国际层序地层学研讨会"的相关内容,阐述了层序地层学理论和技术方法研究的进展;内容包括碎屑岩层序地层学(涉及海相、湖相、河流相等)、碳酸盐岩层序地层学、煤和其他沉积物层序地层学、层序地层标准化等领域的重要研究成果及其在常规油气与非常规油气勘探中的应用。最后指出了层序地层学未来的发展趋势,主要为研究方法标准化及应用地区特殊化,尤其要加强深水层序、碳酸盐岩层序、层序模拟等方面的研究。     

Depositional processes of the Zhushadong and Mantou formations (Early to Middle Cambrian), Shandong Province,China: roles of archipelago and mixed carbonate–siliciclastic sedimentation on cycle genesis during initial flooding of the North China Platform

To understand the depositional processes and environmental changes during the initial flooding of the North China Platform, this study focuses on the Lower to Middle Cambrian Zhushadong and Mantou formations in Shandong Province, China. The succession in the Jinan and Laiwu areas comprises mixed carbonate and siliciclastic deposits composed of limestone, dolostone, stromatolite, thrombolite, purple and grey mudstone, and sandstone. A detailed sedimentary facies analysis of seven well‐exposed sections suggests that five facies associations are the result of an intercalation of carbonate and siliciclastic depositional environments, including local alluvial fans, shallowing‐upward carbonate–siliciclastic peritidal cycles, oolite dominant shoals, shoreface and lagoonal environments. These facies associations successively show a transition from an initially inundated tide‐dominated carbonate platform to a wave‐dominated shallow marine environment. In particular, the peritidal sediments were deposited during a large number of depositional cycles. These sediments consist of lime mudstone, dolomite, stromatolite and purple and grey mudstones. These shallowing‐upward cycles generally resulted from carbonate production in response to an increase of accommodation during rising sea‐level. The carbonate production was, however, interrupted by frequent siliciclastic input from the adjacent emergent archipelago. The depositional cycles thus formed under the influence of both autogenetic changes, including sediment supply from the archipelago, and allogenic control of relative sea‐level rise in the carbonate factory. A low‐relief archipelago with an active tidal regime allowed the development of tide‐dominated siliciclastic and carbonate environments on the vast platform. Siliciclastic input to these tidal environments terminated when most of the archipelago became submerged due to a rapid rise in sea‐level. This study provides insights on how a vast Cambrian carbonate platform maintained synchronous sedimentation under a tidal regime, forming distinct cycles of mixed carbonates and siliciclastics as the system kept up with rising relative sea‐level during the early stage of basin development in the North China Platform.