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Facies Associations (facy + association)
Selected AbstractsLower Carboniferous peritidal carbonates and associated evaporites adjacent to the Leinster Massif, southeast Irish MidlandsGEOLOGICAL JOURNAL, Issue 2 2005Zsolt R. Nagy Abstract Analysis of a 275,m-thick section in the Milford Borehole, GSI-91-25, from County Carlow, Ireland, has revealed an unusual sequence of shallow subtidal, peritidal and sabkha facies in rocks of mid?-late Chadian to late Holkerian (Viséan, Lower Carboniferous) age. Sedimentation occurred on an inner ramp setting, adjacent to the Leinster Massif. The lower part of the sequence (late Chadian age) above the basal subtidal bioclastic unit is dominated by oolite sand facies associations. These include a lower regressive dolomitized, oolitic peloidal mobile shoal, and an upper, probably transgressive, backshoal oolite sand. A 68,m-thick, well-developed peritidal sequence is present between the oolitic intervals. These rocks consist of alternating stromatolitic fenestral mudstone, dolomite and organic shale, with evaporite pseudomorphs and subaerial exposure horizons containing pedogenic features. In the succeeding Arundian,Holkerian strata, transgressive,regressive carbonate units are recognized. These comprise high-energy, backshoal subtidal cycles of argillaceous skeletal packstones, bioclastic grainstones with minor oolites and algal wackestones to grainstones and infrequent algal stromatolite horizons. The study recognizes for the first time the peritidal and sabkha deposits in Chadian rocks adjacent to the Leinster Massif in the eastern Irish Midlands. These strata appear to be coeval with similar evaporite-bearing rocks in County Wexford that are developed on the southern margin of this landmass, and similar depositional facies exist further to the east in the South Wales Platform, south of St. George's Land, and in Belgium, south of the Brabant Massif. The presence of evaporites in the peritidal facies suggests that dense brines may have formed adjacent to the Leinster Massif. These fluids may have been involved in regional dolomitization of Chadian and possibly underlying Courceyan strata. They may also have been a source of high salinity fluids associated with nearby base-metal sulphide deposits. Copyright © 2005 John Wiley & Sons, Ltd. [source] Late Cenozoic structural and stratigraphic evolution of the northern Chinese Tian Shan forelandBASIN RESEARCH, Issue 3 2010Honghua Lu ABSTRACT Three successive zones of fault-related folds disrupt the proximal part of the northern Tian Shan foreland in NW China. A new magnetostratigraphy of the Taxi He section on the north limb of the Tugulu anticline in the middle deformed zone clarifies the chronology of both tectonic deformation and depositional evolution of this collisional mountain belt. Our ,1200-m-thick section encompasses the upper Cenozoic terrigenous sequence within which ,300 sampling horizons yield an age span of ,8,2 Ma. Although the basal age in the Taxi He section of the Xiyu conglomerate (often cited as an indicator of initial deformation) is ,2.1 Ma, much earlier growth of the Tugulu anticline is inferred from growth strata dated at ,6.0 Ma. Folding of Neogene strata and angular unconformities in anticlines in the more proximal and distal deformed zones indicate deformation during Miocene and Early Pleistocene times, respectively. In the Taxi He area, sediment-accumulation rates significantly accelerate at ,4 Ma, apparently in response to encroaching thrust loads. Together, growth strata, angular unconformities, and sediment-accumulation rates document the northward migration of tectonic deformation into the northern Tian Shan foreland basin during the late Cenozoic. A progradational alluvial,lacustrine system associated with this northward progression is subdivided into two facies associations at Tugulu: a shallow lacustrine environment before ,5.9 Ma and an alluvial fan environment subsequently. The lithofacies progradation encompasses the time-transgressive Xiyu conglomerate deposits, which should only be recognized as a lithostratigraphic unit. Along the length of the foreland, the locus of maximum shortening shifts between the medial and proximal zones of folding, whereas the total shortening across the foreland remains quite homogeneous along strike, suggesting spatially steady tectonic forcing since late Miocene times. [source] Clinoform nucleation and growth in coarse-grained deltas, Loreto basin, Baja California Sur, Mexico: a response to episodic accelerations in fault displacementBASIN RESEARCH, Issue 3 2005Estelle Mortimer We investigate the controls on the architecture of coarse-grained delta progradational units (PUs) in the Pliocene Loreto basin (Baja California Sur, Mexico), a half-graben located on the western margin of the Gulf of California. Dorsey et al. (1997b) argued that delta progradation and transgression cycles in the basin were driven by episodic fault-controlled subsidence along the basin-bounding Loreto fault. Here we test this hypothesis by a detailed analysis of the sedimentary architecture of 11 exceptionally well-exposed, vertically arranged fluvio-deltaic PUs, each of which shows lateral facies transition from proximal alluvial facies palaeo-seaward into distal pro-delta facies. Of these 11 PUs, seven exhibit a lateral transition from a shoal water to Gilbert-delta facies associations as they are traced palaeo-seaward. This transition is characterised by down-transport development of foresets, which grow in height up to 35 m. Foreset units thicken in a basinward direction, with initially an oblique topset,foreset geometry that becomes increasingly sigmoidal. Each delta is capped by a shell bed that records drowning of the delta top. This systematic transition in delta architecture records increasing water depth through time during individual episodes of progradation. A mechanism that explains this transition is an accelerating rate of fault-controlled subsidence during each PU. During episodes of low slip rate, shoal-water deltas prograde across the submerged topography of the underlying delta unit. As displacement rate accelerates, increasing bathymetry at the delta front leads to steepening of foresets and initiation of Gilbert deltas. Subsequent delta drowning results from sediment starvation at the shoreline at high slip rates because of sediment trapping upstream. The observed delta architecture suggests that the long-term (>100 kyr) history of slip on the Loreto fault was characterised by repetitive episodes of accelerating displacement accumulation. Such episodic fault behaviour is most likely to be because of variations in temporal and spatial strain partitioning between the Loreto fault and other faults in the Gulf of California. A physical explanation for the acceleration phenomenon involves evolving frictional properties on the episodically active Loreto fault. [source] Late Carboniferous-Early Permian Sequence Stratigraphy and Depositional Evolution in the Northeast Ordos Basin, North ChinaACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 5 2010YANG Minghui Abstract: Sequence stratigraphical analysis was applied to the Upper Carboniferous-Lower Permian sedimentary succession of the northeastern Ordos Basin, north China based on data acquired from ten entire logging curves and eight outcrops. The facies framework of the lithostratigraphical unit, the Taiyuan Formation comprises seven facies in two facies associations, varying from fluvio-delta to shelf-barrier islands. The facies are presented within a chronostratigraphical framework, linked by systems tract, which in turn are limited by flooding surfaces and sequence boundaries. Six third-order depositional sequences are recognised, bounded by six type 2 unconformities. An upwards-shallowing epicontinental sea sedimentary model is created, which consists of a sandstone, coal seam and carbonate succession. [source] | ||||||||||