RASBURY, E. T.¹ and W. B. WARD^2
1Queens College CUNY, Flushing, NY
²Earthworks, P.O. Box 297, West Redding, CT

Abstract: U-Pb Age of Former Marine Botryoidal Aragonite from U-Pb Ratios in Brown Replacement Calcite, Laborcita Formation (Lowermost Permian), Sacramento Mountains, New Mexico

Phylloid algal mounds of the Laborcita Formation (Wolfcampian), have abundant brown calcite that replaced cavity-filling botryoidal aragonite. The brown calcite hosts microscopic patches of preserved aragonite. Petrographic relationships (Ward 1988) indicate botyroidal aragonite precipitated from marine porewaters before initial burial of the algal mounds by overlying sediments and that the main episode of aragonite replacement by brown calcite was after initial burial. Major and Ward (1993) interpret the C and 0 isotope ratios of the brown calcite to indicate aragonite replacement occurred when the mounds were covered by as much as 500 m of sediment. Patches of white and clear calcite replaced the brown calcite much later.

We sampled brown calcite for U-Pb dating. Five aliquots from one Laborcita botryoid give a very precise and concordant age of 303+/-2 Ma (MSWD =0.8). The Carboniferous-Permian boundary lies just below the mounds. The age from the brown calcite is within the uncertainty of the 301+/-2 Ma age estimated for the Carboniferous Permian boundary based on U-Pb dating of paleosols by Rasbury et al. (1998). These results suggest that, despite the complex diagenetic history, the U-Pb ratio from the aragonite is preserved in the brown calcite and aragonite inclusions and that the age is approximately the time of sedimentation.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas