High resolution stratigraphy and facies differentiation of the shallow marine Annot Sandstones, south-east France |
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| Authors: | H D SINCLAIR |
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| Institution: | Department of Geological Sciences, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, UK |
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| Abstract: | High resolution stratigraphical analysis divides a rock succession into the basic genetic units of stratigraphy which are here termed small scale stratigraphical cycles. Each cycle records the sedimentological response to an episode of shallowing and deepening. Assuming that these changes in water depth reflect changes in the shoreline position, they can be considered as regressive/transgressive episodes. Each cycle comprises a regressive and transgressive facies tract which will be variably proportioned; in some examples a facies tract may only be represented by a hiatal surface of no deposition, erosion and/or bypass. In the Annot Sandstones of south-east France, variations in facies types, proportions and associations can be demonstrated both laterally and vertically through the succession. First, it is demonstrated that facies variations occur within regressive or transgressive facies tracts as a function of the stratigraphical stacking pattern of the cycles (i.e. landward, vertical or seaward stacked); this is termed ‘vertical facies differentiation’. Second, the proportions of facies tracts and their constituent facies types within an individual cycle vary between more landward and more seaward palaeogeographical locations; this is termed ‘lateral facies differentiation'. The upper Eocene/lower Oligocene Annot sandstones outcrop in the Maritime Alps of south-east France, within the thin skinned outer fold and thrust belt of the Alpine arc. The sandstones are well exposed in the area of the Col de la Cayolle on the north-west margin of the Argentera Massif, where lithostratigraphical correlations are possible over 3·5 km in a NNW/SSE direction, perpendicular to the edge of the depositional basin. Traditionally, these outcrops have been interpreted as deep marine turbidite lobe sediments; this study reflects a significant reinterpretation of this succession as having been deposited in a shallow marine environment. Seven sedimentary sections were measured through the succession, which is divided into 10 small scale stratigraphical cycles. These cycles are described in terms of eight facies which are separated into their transgressive or regressive facies tracts. In eight of the 10 cycles, the regressive facies tracts reflect the progradation of storm influenced braid deltas over shelf muds and silts. In two of the 10 cycles, the regressive facies tracts reflect barrier inlet and wash-over sands interfingering with back barrier deposits. These latter two cycles are located within landward stepping cycle sets; this is an example of vertical facies differentiation. Transgressive facies tracts locally reworked the upper surface of the regressive facies tract and also comprise barrier and back barrier deposits. The facies succession within each cycle varies according to its position with respect to the palaeoshoreline. The more landward portion of an individual cycle comprises a deltaic shoaling upward succession, culminating in coarse distributary channel conglomerates, overlain by a transgressive barrier/inlet system with extensive back barrier deposits. Beyond the delta front, the more seaward equivalent of individual cycles comprises an erosive base, with aggradational massive pebbly sandstones sitting directly upon offshore heterolithics; these sandstones are interpreted as hyperconcentrated fluvial efflux into the nearshore environment. This grades upward into offshore heterolithics and graded storm deposits representing the products of ravinement, which are then overlain by shelf mudstones. In summary, the more landward portions of cycles preserve predominantly regressive facies tracts, whereas the more seaward portions preserve aggradational to retrogradational strata of the transgressive facies tract; this is an example of lateral facies differentiation. |
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