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Stratigraphy and Facies of the Pliocene Mayrán Lacustrine Basin System, Northeast México

Amezcua Torres, Natalia

[Thesis]. Manchester, UK: The University of Manchester; 2012.

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Abstract

Lake basins contain depositional systems whose stratigraphy and sedimentology are highly influenced by allogenic and authigenic processes. Identifying the relative roles of these controls is complicated in settings where both tectonic and climate influences control sediment inputs and accommodation availability. In order to identify how these controls may interact in carbonate lake basins the lacustrine Mayrán Formation, northeast México was investigated. The Mayrán Formation is well exposed and the strata geometries are well preserved. The necessary data needed to unravel these complexities was obtained using field based techniques (geological and structural mapping, sedimentary logging and sampling), combined with optical and electron optical microscopy, mineralogical (XRD), and geochemical (oxygen and carbon stable isotopes, total organic carbon [TOC], organic carbon isotopes, and U-Pb carbonate dating) techniques, to characterise the lake basins.These data reveal that the Mayrán Formation was deposited in four, northerly, down-stepping, broadly coeval, hydrologically linked, carbonate-dominated lake subbasins (the Mayrán Basin system), that formed at different elevations spanning ~500 m. Subbasin geomorphology was controlled by the inherited topography of the exhumed and eroded Parras foreland fold-and-thrust belt, as a result, the subbasins are compartmentalized by prominent sandstone ridges that form sills between subbasins. Sedimentary infill of the subbasins consists of a fluvio-lacustrine succession (carbonate and clastic) that overly an angular unconformity truncating the folded and faulted Cretaceous bedrock. Clastic sediments were preferentially trapped in the most proximal subbasin in the south, adjacent to the mountain front. In more distal settings to the north, carbonate production resulted in deposition of horizontally bedded lacustrine limestones with subordinated siliciclastics and evaporite minerals. Carbonate deposition occurred during overfilled to balance filled conditions. Prominent aggradational to progradational tufa clinoforms form at the spillover points between the subbasins, and interfinger up- and down-dip with the horizontally bedded lacustrine limestones. The tufas consist of steeply dipping clinoforms that prograde northwards. They formed by inorganic and biogenically mediated calcite precipitation as waters from the topographically upper lake overflowed the sill. U-Pb dating of speleothem cements formed in the tufas yield a Pliocene age (3.06 ± 0.2 Ma). The horizontally bedded limestones that predominantly fill the subbasins are composed of nine lithofacies. These are: ostracode mudstone-wackestone, clotted boundstone, gastropod wackestone-packstone, oncoid wackestone-packstone, stromatolite, charophyte wackestone-packstone, marlstones, crystalline carbonate and evaporite minerals. These lithofacies contain variable proportions of production-derived components, with subordinated detrital and diagenetic components. They overall contain inorganic and organic calcite (average 91%), organic matter (average 12.6 % TOC) and subordinated detrital quartz, feldspar and clay. Stable isotope conditions from stromatolite laminations (δ13C -6.3‰ and δ18O -8.0‰), and micritic matrix from samples in all the subbasins (δ13C -5.7‰ and δ18O -8.4 ‰), indicate that overall calcite deposition occurred in freshwater lakes, under predominantly oxic conditions. Carbonate particles deposited mainly by in situ disaggregation and by settling. Significant preservation of the organic matter likely occurred because of the fine grained carbonate matrix, and because of moderate rates of both carbonate sediment and organic matter production. Stacking patterns in the lake successions show an overall shallowing upward cyclicity. It is considered, that the effects of inherited basin geomorphology and climate, ultimately controlled facies variability and strata geometries in these subbasins.