--> Abstract: Calcrete Overprinting of Palustrine Carbonate Facies: The Role of Phreatic Water Movement, by Erik L. Gulbranson and Timothy Demko; #90039 (2005)

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Calcrete Overprinting of Palustrine Carbonate Facies: The Role of Phreatic Water Movement

Erik L. Gulbranson and Timothy Demko
University of Minnesota Duluth, Duluth, MN

Calcrete modification in paleosols and palustrine carbonates has important ramifications in the study of continental basin evolution and paleoclimates. This study examines the different phases of calcrete development from two rock units, the Early Cretaceous Cedar Mountain Formation (CMF) and the Owl Rock Member (ORM) of the Late Triassic Chinle Formation, and their relation to the paleoclimatic setting in which they formed. Calcrete development in these strata was strongly affected by groundwater movements. These changes due to groundwater modification are observed to have similar characteristics of phreatic calcretes overprinted onto pedogenic calcretes and their host medium. Phreatic modification typically is seen in climates with heavy seasonal precipitation in an overall low precipitation/evaporation setting. In the ORM, phreatic modification is distinct and overprints most calcrete phases, while pedogenic processes dominate the CMF calcretes. Therefore, the hydrologic conditions during ORM deposition are consistent with a sub-humid approaching semi-arid regional climatic setting, while hydrologic conditions during CMF deposition were consistent with a dominantly sub-humid setting. In sub-humid to semi-arid climates, phreatic groundwater processes are often coupled with pedogenic processes in calcrete formation. This coupling creates a readily identifiable phase in calcrete growth where mottling pervades most of the calcrete and coarse circumgranular cements are well established. However, where this phase is absent, it is likely that phreatic water movement was stable for the duration of calcrete growth whereby precipitation rates could have increased and/or evaporation rates decreased.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005