HANSON, A. D., C. N. THOMSON, D. L. LOGUE, and P. L. ABBOTT, San Diego State University, San Diego, CA

ABSTRACT: Unusual Paleocene Limestone at Mesa la Sepultura, Baja California, Mexico

At Mesa la Sepultura, a resistant Paleocene limestone disconformably overlies the lower Maastrichtian Rosario Formation of marine upper slope and shelf deposits. The Rosario Formation is dark gray, finely laminated mudstone with thin planar-tabular beds of very fine sandstone, which increase in frequency and coarsen upsection. The uppermost Rosario Is a planar-based plutonic-volcanic conglomerate consistent with derivation from the Peninsular Ranges to the east. Paleocene limestone caps Mesa la Sepultura and is unusual in that correlative Paleocene rocks to the north and south are terrigenous clastics. Mesa la Sepultura is the type area for the Sepultura Formation, which has a 23-m thick glauconitic-clastic lower member and a 32-m thick limestone upper member. Kaolinite-rich paleosols locally underlie the base of the Sepultura Formation. The basal beds contain Turritella peninsularis of late-early Paleocene age. Therefore the K-T boundary is not preserved at this site. The dominant lithology of the lower member is calcite-cemented, glauconitic, fossiliferous arkose or lithic arkose of apparent Peninsular Ranges provenance. It contains red algal nodules, storm deposits, nested channels, and iron-stained corrosion surfaces. The upper member is a foraminiferal, red algal biosparrudite lacking terrigenous sediments; this indicates separation of the basin from the crystalline rock terrane. Red algal genera include Archeolithothamnion (a cool water genus) and Lithothamnion (a warm water genus), suggesting a middle latitude, normal marine salinity environment. The morphology of the red algae changes from encrusting spheroids in the lower limestone to branching forms near the top of the section reflecting increasing water depths, possibly from 40 to 100 m. Micropaleontological data suggest a middle to outer shelf environment and an early late Paleocene age for the uppermost Sepultura beds.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.