Dill, H. G., Khishigsuren, S., Melcher, F., Bulgamaa, J., Bolorma, K., Botz, R. and Schwarz-Schampera, U. (2005) Facies-related diagenetic alteration in lacustrine-deltaic red beds of the Paleogene Ergeliin Zoo Formation (Erdene Sum area, S. Gobi, Mongolia). Sedimentary Geology, 181 (1-2). pp. 1-24. DOI 10.1016/j.sedgeo.2005.06.007.

Abstract

The Late Eocene Ergeliin Zoo Formation, Mongolia, was subdivided into four facies associations/stratigraphic units which are characterized by various carbonate minerals: (1) mud-sand flat (low-Fe dolomite, high-Mg calcite), (11) delta front (high-Mn dolomite, low-Mg calcite), (111) delta plain (high-Mn calcite, low-Mg and moderate-Fe calcite), (IV) calcretes (high-Mg calcite). Besides carbonate minerals, prevailing among the cement minerals, some sheet silicates (Ca smectite, palygorskite, illite), apatite and Fe oxide-hydroxides occur in the siliciclastics of these lacustrine-deltaic red beds. The prodelta/mud-sand flat deposits (unit 1) were dolomitized and cemented by high-Mg calcite during evaporative pumping at times of low lake-stand under redox conditions greater than 0 and intrastratal solutions of strong alkalinity. Manganoan dolomite is fairly widespread in the delta front sediments (unit 11), the Mn content of which is likely to have been derived from decomposition of vertebrate remains. The Mn/Fe ratio in the carbonate minerals was controlled by the redox conditions and the shallow burial depth. In the porous delta front sediments Eh values around zero and pH values slightly above 7 occurred as a result of basinward fluid movements (high-Mn dolomite, high-Fe dolomite, low-Mg calcite). Fluid movement was locally impeded by finer-grained delta plain deposits (unit III), intertonguing with arenaceous aquifers (low Mg-moderate Fe calcite, high-Mn calcite). Higher up on the delta plain with deposits laid down in an environment transitional between distal alluvial and deltaic (unit IV), closed lake basin conditions reappeared as fluids emerged from the distal alluvial-fluvial deposits or were driven by capillary force to ascend and form calcretes abundant in Mg-bearing calcite. The climatic conditions in the Erdene Sum area, Mongolia, are likely to have been semi- and transitional into arid. (c) 2005 Elsevier B.V All rights reserved.

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