Chemical oceanographic influences on sediment accumulations of a carbonate ramp: Holocene Yucatan Shelf,Mexico     

Eugene C. Rankey  Rodrigo Garza‐Perez  Hassan A. Eltom 《Sedimentology》2021,68(1):324-351

Global controls on the oceanographic influences on the nature of carbonate factories are broadly understood. The details of the influences of changes in temperature and nutrients across individual carbonate shelves are less well constrained, however. This study explores spatial and temporal variations in chemical oceanography along and across the Yucatan Shelf, a modern carbonate ramp, and how these factors relate to variable bottom character, sediment and sediment geochemistry. In‐situ sensors and remote‐sensing data indicate the sporadic presence of cool, upwelled water with low dissolved oxygen and elevated Chlorophyll‐a. This current‐driven, westward flow of upwelled water is most evident in a zone just offshore of the northern peninsular shoreline, but its influence wanes ca 75 km offshore and as the shore turns southward. The impacts of this water mass include a transitional photozoan–heterozoan assemblage with biosiliceous components, relict grains and common thin Holocene sediment accumulations nearshore; further offshore are coralgal reefs and expansive sand plains. Geochemical proxies of bulk sediment, including high δ¹⁸O and elevated HREE/LREE (heavy rare‐earth element/light rare‐earth element) ratios near, and downcurrent of, the upwelling source, are interpreted to represent the signal of nearshore, westward movement of the cool and nutrient‐rich, upwelled water. Collectively, these data emphasize how local processes such as upwelling and longshore transport can variably influence carbonate sediment accumulations and their geochemical signatures, both along and across individual shelves. These data and insights provide an analogue for the influences of spatial variability of water masses in the geological record, and for accurate interpretation of stratigraphic changes of sedimentary and geochemical proxy data in carbonate archives.  相似文献   

Storm deposits and storm-generated coarse carbonate breccias on a pelagic outer shelf (South-East Basin, France)   总被引：2，自引：0，他引：2  

Michel Seguret  Alexis Moussine-Pouchkine  Guilherme Raja Gabaglia  & Frederic Bouchette 《Sedimentology》2001,48(2):231-254

Uppermost Jurassic limestones of the South‐East Basin (France) are organized into four facies associations that were deposited in four distinct zones: (1) peritidal lagoonal limestones; (2) bioclastic and reefal limestones; (3) pelagic lime mudstones; (4) lime mudstones/calcarenites/coarse breccias. Calcarenite deposits of zone 4 exhibit sedimentary structures that are diagnostic of deposition under wave‐induced combined flow. In subzone 4a, both vertical and lateral transitions from lime mudstone/calcarenite to breccia indicate in situ brecciation under wave‐cyclic loading. Breccias were produced by heterogeneous liquefaction of material previously deposited on the sea floor. Deposits in subzone 4a record relatively long periods (>400 kyr) of sedimentation below wave base, alternating with periods of deposition under wave‐induced currents and periods of in situ deformation. In this zone, storm waves were attenuated by wave–sediment interaction, and wave energy was absorbed by the deformation of soft sediment. With reference to present‐day wave attenuation, water depths in this zone ranged between 50 and 80 m. Landwards of the attenuation zone, in zone 3, storm waves were reduced to fair‐weather wave heights. Storm wave base was not horizontal and became shallower landwards. As a consequence, water depth and wave energy were not linearly related. On a small area of the seaward edge of subzone 4a, cobbles were removed by traction currents and redeposited in subzone 4b. There, they formed a 100‐m‐thick wedge, which prograded over 3 km and was built up by the stacking of 5‐ to 20‐m‐thick cross‐stratified sets of coarse breccia. This wedge records the transport and redeposition of cobbles by a high‐velocity unidirectional component of a combined flow. The increase in flow velocity in a restricted area is proposed to result from flow concentration in a channel‐like structure of the downwelling in the gulf formed by the basin. In more distal subzone 4c, the hydrodynamic effect of wave‐induced currents was quasi‐permanent, and brecciation by wave–sediment interaction occurred only episodically. This indicates that, seawards of the attenuation zone, hydrodynamic storm wave base was deeper than mechanical storm wave base. Uppermost Jurassic carbonates were deposited and soft‐sediment deformed on a hurricane‐dominated ramp of very gentle slope and characterized by a zone of storm wave degeneration, located seawards of a zone of sedimentation below wave base.  相似文献   

Structural analysis of the Lombard thrust sheet and adjacent areas in the Helena salient,southwest Montana,USA     

《Journal of Structural Geology》2014

The Helena salient is a prominent craton–convex curve in the Cordillera thrust belt of Montana, USA. The Lombard thrust sheet is the primary sheet in the salient. Structural analysis of fold trends, cleavage attitudes, and movement on minor faults is used to better understand both the geometry of the Lombard thrust and the kinematic development of the salient.Early W–E to WNW–ENE shortening directions in the Lombard sheet are indicated by fold trends in the center of the thrust sheet. The same narrow range of shortening directions is inferred from kinematic analysis of movement on minor faults and the orientations of unrotated cleavage planes along the southern lateral ramp boundary of the salient. As the salient developed, the amount and direction of shortening were locally modified as listric detachment faults rotated some tight folds to the NW, and as right-lateral simple shear, caused by lock-up and folding of the Jefferson Canyon fault above the lateral ramp, rotated other folds northeastward. Where the lateral ramp and frontal-oblique ramp intersect, folds were rotated back to the NW. Our interpretation of dominant W–E to WNW–ESE shortening in the Lombard sheet, later altered by local rotations, supports a model of salient formation by primary parallel transport modified by interactions with a lateral ramp.  相似文献   

Physical analog (centrifuge) model investigation of contrasting structural styles in the Salt Range and Potwar Plateau,northern Pakistan     

《Journal of Structural Geology》2015

We use scaled physical analog (centrifuge) modeling to investigate along- and across-strike structural variations in the Salt Range and Potwar Plateau of the Himalayan foreland fold-thrust belt of Pakistan. The models, composed of interlayered plasticine and silicone putty laminae, comprise four mechanical units representing the Neoproterozoic Salt Range Formation (basal detachment), Cambrian–Eocene carapace sequence, and Rawalpindi and Siwalik Groups (Neogene molasse), on a rigid base representing the Indian craton. Pre-cut ramps simulate basement faults with various structural geometries.A pre-existing north-dipping basement normal fault under the model foreland induces a frontal ramp and a prominent fault-bend-fold culmination, simulating the Salt Range. The ramp localizes displacement on a frontal thrust that occurs out-of-sequence with respect to other foreland folds and thrusts. With a frontal basement fault terminating to the east against a right-stepping, east-dipping lateral ramp, deformation propagates further south in the east; strata to the east of the lateral ramp are telescoped in ENE-trending detachment folds, fault-propagation folds and pop-up structures above a thick basal detachment (Salt Range Formation), in contrast to translated but less-deformed strata with E–W-trending Salt-Range structures to the west. The models are consistent with Salt Range–Potwar Plateau structural style contrasts being due to basement fault geometry and variation in detachment thickness.  相似文献   

Facies analysis and palaeoenvironmental interpretation of the Late Oligocene Attard Member (Lower Coralline Limestone Formation), Malta     

MARCO BRANDANO  VIRGILIO FREZZA  LAURA TOMASSETTI  MARTYN PEDLEY†  RUGGERO MATTEUCCI 《Sedimentology》2009,56(4):1138-1158

The Oligocene represents a key interval during which coralline algae became dominant on carbonate ramps and luxuriant coral reefs emerged on a global scale. So far, few studies have considered the impact that these early reefs had on ramp development. Consequently, this study aimed at presenting a high‐resolution analysis of the Attard Member of the Lower Coralline Limestone Formation (Late Oligocene, Malta) in order to decipher the internal and external factors controlling the architecture of a typical Late Oligocene platform. Excellent exposures of the Lower Coralline Limestone Formation occurring along continuous outcrops adjacent to the Victoria Lines Fault reveal in detail the three‐dimensional distribution of the reef‐associated facies. A total of four sedimentary facies have been recognized and are grouped into two depositional environments that correspond to the inner and middle carbonate ramp. The inner ramp was characterized by a very high‐energy, shallow‐water setting, influenced by tide and wave processes. This setting passed downslope into an inner‐ramp depositional environment which was colonized by seagrass and interfingered with adjacent areas containing scattered corals. The middle ramp lithofacies were deposited in the oligophotic zone, the sediments being generated from combined in situ production and sediments swept from the shallower inner ramp by currents. Compositional characteristics and facies distributions of the Attard ramp are more similar to the Miocene ramps than to those of the Eocene. An important factor controlling this similarity may be the expansion of the seagrass colonization within the euphotic zone. This expansion may have commenced in the Late Oligocene and was associated with a concomitant reduction in the aerial extent of the larger benthonic foraminifera facies. Stacking‐pattern analysis shows that the depositional units (parasequences) at the study section are arranged into transgressive–regressive facies cycles. This cyclicity is superimposed on the overall regressive phase recorded by the Attard succession. Furthermore, a minor highstand (correlated with the Ru4/Ch1 sequence) and subsequent minor lowstand (Ch2 sequence) have been recognized. The biota assemblages of the Attard Member suggest that carbonate sedimentation took place in subtropical waters and oligotrophic to slightly mesotrophic conditions. The apparent low capacity of corals to form wave‐resistant reef structures is considered to have been a significant factor affecting substrate stability at this time. The resulting lack of resistant mid‐ramp reef frameworks left this zone exposed to wave and storm activity, thereby encouraging the widespread development of coralline algal associations dominated by rhodoliths.  相似文献   

Biostratigraphy and Sequence Stratigraphy of the Tarbur Formation (Maastrichtian) from Iranian Zagros Foreland Basin, Southwest of Iran     

Hossein GHANBARLOO  Amrollah SAFARI  Hossein VAZIRI-MOGHADDAM 《《地质学报》英文版》2022,96(1):147-166

Shallow carbonate deposits(Tarbur Formation)were formed in the Zagros foreland basin with dynamic tectonics during the Maastrichtian age.From the viewpoint of reconstruction of depositional conditions in these deposits,studies of biostratigraphy,microfacies,microtaphofacies,and sequence stratigraphy were performed in a single area at Tang-e Shabi Khoon,northwest of Zagros.Based on the identification of two assemblage zones consisting of benthic foraminifera in these strata,the formation was deposited during the middle to late Maastrichtian.The number of cycles in test size and type of coiling in Loftusia decreased from the study area toward the northwest of the Neotethys basin.The input of clastic sediments affected the distribution of Loftusia and rudists in the study area.Nine microfacies,six microtaphofacies,and one terrigenous facies(shale)were identified based on the sedimentary features.These deposits of the middle-late Maastrichtian were deposited on a homoclinal carbonate ramp.The platform can be divided into restricted and semi-restricted lagoon,shoal,and open marine environments.In the study area,the deposits of the Tarbur Formation were deposited during four third-order depositional sequences.Local fault activities affected the formation of depositional sequences in the study area.  相似文献   

Are carbonate barrier islands mobile? Insights from a mid to late-Holocene system,Al Ruwais,northern Qatar     

John Rivers  Max Engel  Robert Dalrymple  Ruqaiya Yousif  Christian J. Strohmenger  Ismail Al-Shaikh 《Sedimentology》2020,67(1):534-558

Barrier islands are important landforms in many coastal systems around the globe. Studies of modern barrier island systems are mostly limited to those of siliciclastic realms, where the islands are recognized as mobile features that form on transgressive coastlines and migrate landward as sea-level rises. Barrier islands of the ‘Great Pearl Bank’ along the United Arab Emirates coast are the best-known carbonate examples. These Holocene islands, however, are interpreted to be anchored by older deposits and immobile. The mid-Holocene to late-Holocene depositional system at Al Ruwais, northern Qatar, provides an example of a mobile carbonate barrier island system, perhaps more similar to siliciclastic equivalents. Sedimentological and petrographic analyses, as well as ¹⁴C-dating of shells and biogenic remains from vibracored sediments and surface deposits, show that after 7000 years ago a barrier system with a narrow back-barrier lagoon formed along what is now an exposed coastal zone, while, contemporaneously, a laterally-extensive coral reef was forming immediately offshore. After 1400 years ago the barrier system was forced to step ca 3 km seaward in response to a sea-level fall of less than 2 m, where it re-established itself directly on the mid-Holocene reef. Since that time, the barrier has retreated landward as much as 1000 m to its current position, exposing previously-deposited back-barrier lagoonal sediment at the open-coast shoreline. In modern neritic warm-water carbonate settings mobile barrier island systems are rare. Their construction and migration may be inhibited by reef formation, early cementation, and the relative inefficiency of sourcing beach sediments from open carbonate shelves. Carbonate barrier island systems likely formed more commonly during geological periods when ramps and unrimmed shelves predominated and in calcite seas, when meteoric cementation was minimized as a result of initial calcitic allochem mineralogy. As with their siliciclastic analogues, however, recognition of the influence of these transient landforms in the rock record is challenging.  相似文献   

山东半岛风电爬坡事件的识别与天气分析研究     

郭燕玲  赵晶  周林  张文煜  郭振海 《气候与环境研究》2017,22(1):97-107

根据山东省24个风电场2013年全年每15 min的风电输出功率实测数据以及相应的WRF(Weather Research and Forecasting)气象预测数据和FNL(Final Operational Global Analysis)再分析资料,分析了区域大规模风电爬坡现象与大尺度天气系统演变的联系。结果显示,大尺度天气系统,特别是阻塞高压系统,是诱发山东地区大规模风电爬坡的重要因素;爬坡事件的预测应当考虑天气演变的因素。进一步结合旋转门(The Swinging Door,TSD)算法重新讨论了爬坡事件的定义与识别,并在此基础上分析了爬坡事件的特征及其可预报性;并指出风电功率上报、考核制度应当重点考虑爬坡时间段的预测水平,以提高电网运行的稳定性。  相似文献   

森林冠层和森林边界层大涡模拟   总被引：3，自引：0，他引：3       下载免费PDF全文

苗世光  蒋维楣 《地球物理学报》2004,47(4):597-603

在采用各向异性湍流动能闭合方案和3阶Runge Kutta时间积分方案的大涡模式中,引入由森林冠层粗糙元造成的动量拖曳项、热量输入项和TKE耗散项,以模拟森林冠层和森林边界层的气象场. 通过中性和不稳定层结条件下不同叶面积指数算例的模拟分析及与已有观测结果的比较表明,本文所建大涡模式对森林冠层和森林边界层有较好的模拟效果. 进一步研究表明：不稳定层结条件下较稠密的森林冠层中特有的Kinking & Pairing湍涡结构与森林边界层中湍流的大涡运动相互作用,形成了森林冠层附近的温度斜坡型结构.  相似文献   

云南罗平中三叠统关岭组二段碳酸盐岩微相分析   总被引：5，自引：0，他引：5       下载免费PDF全文

白建科  张启跃  尹福光  周长勇  吕涛  冯静 《沉积与特提斯地质》2009,29(3):15-21

通过详细的野外观察和室内镜下薄片分析,于中三叠统关岭组二段中初步识别出8种微相类型。结合宏观沉积构造及罗平生物群保存特征,认为云南罗平关岭组二段为碳酸盐岩缓坡沉积,可划分出深水盆地、深缓坡、深缓坡远端变陡、浅缓坡和后缓坡5个沉积相。沉积相在垂向上是向上变浅(海退)的沉积序列。罗平生物群的沉积环境为深水、低能、安静的海洋环境,突发的风暴事件导致了生物群大量死亡,并被迅速埋藏。  相似文献