DEPOSITIONAL PALEOENVIRONMENTS AND STRATIGRAPHY OF LATE PLEISTOCENE DEPOSITS AT RANCHO LAS ANIMAS, BAJA CALIFORNIA SUR, MEXICO

At Las Animas, located in Baja California Sur (Mexico), are a series of Late Pleistocene terrace deposits that extend about 24 km along the coastline of Bahía del Coyote and 3-km inland. These deposits are believed to be the remains of a large paleo-embayment which covered the area during the last major sea level high stand (MIS 5e), approximately 125 kyr BP. Discussion of coral dating and dates are reported separately (RD, et al). Terraces are 15 to 28 m above present day sea level with Late Pleistocene deposits making up the top 0.5 – 10 m. General stratigraphy of the terraces begins with a basal conglomerate of volcanic cobbles laying uncomformably on top of the San Isidro Formation (Late Miocene). This is overlain by a molluscan-rich sand facies. These sands are fine to coarse with common occurrences of Ostrea palmula,Chione californiensis,Isognomom chemnitizianus,and Pinctada mazatlantica.An in situcoral facies up to 6 m follows, consisting of large branching Porites californicus,with common Pinctada mazatlantica,Isognomon chemnitzian,and Diadema mexicanumspines. Within the coral facies are beds of coral rubble layers and storm deposit layers. Overlaying the coral patches is a coral-rhodolith sand facies, consisting of pencil size corals, rhodoliths common molluscans Chione californiensis,Serpulorbis sp.,Strombus sp.,and Oliva spicata.Overlaying the coral-rhodolith facies is a beach gravel facies consisting of rounded bajada gravel and coarse sands. Capping this sequence is a coastal sand dune. Preliminary grain size analyses indicate a textural gradient in the sediment matrix. This trend implies changes in sediment sources with sample location and time. Further analyses will yield possible sediment sources. The stratigraphy and paleoecology define a transgressive-regressive sequence on top of topography of arroyo and bajada much like the present. Coral facies was developed during the MIS 5e highstand with the maximum at 123 – 125 kyr BP and waters deep enough to support coral patches of 10 m in height. As Gulf waters retreated the patch reefs were covered with coral and rhodolith marl and sands, then beach gravel, and finally dune deposits.