Lakes in Tropical Western Pangaea: Siliciclastic- and Carbonate-Dominated Lacustrine Deposits of the Upper Triassic Chinle Formation, Colorado Plateau, U.S.A.

Timothy Demko, J. J. Beer, and E. L. Gulbranson
University of Minnesota Duluth, Duluth, MN

Siliciclastic- and carbonate-dominated lacustrine facies in the Upper Triassic Chinle Formation record deposition in overfilled and balanced to underfilled lake basin types under tropical megamonsoonal conditions. Siliciclastic-dominated lacustrine facies are present in the Monitor Butte Member in the lower part of the Chinle Formation in northern Arizona and south central Utah. These facies are characterized by steep, muddy clinoforms and rippled sandstone delta fronts and distributary channel/stream-mouth bars. Distal delta front and prodelta mudstones contain abundant detrital terrestrial organic material, in some cases preserved as allochthonous peat layers. These facies represent deposition in a delta shoreline system that was rapidly aggrading in a confined setting within an incised valley. Carbonate-dominated lacustrine and palustrine facies in the Owl Rock Member in the upper part of the formation are characterized by micritic and brecciated phases that represent deposition and early diagenesis in shallow littoral and fringing wetland settings in a broad, low gradient, closed basin. Pedogenic modification and early diagenetic alteration of these carbonates suggests that periodic lake level fluctuations resulted in subaerial exposure of subaqueous deposits, colonization by plants and animals, subsequent transgression, and resumption of lacustrine deposition. Pangaean megamonsoonal conditions provided the Chinle lakes with abundant meteoric water input during rainy seasons. However, long dry seasons and the tropical continental setting also increased the potential for low precipitation/evaporation ratios. The facies characteristics of these two lacustrine intervals depended both on the tectonic and topographic nature of the depostional basin and the climatically controlled variables of sediment and water fluxes.

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