Late Pleistocene (Last Interglacial) terrace deposits, Bahia Coyote, Baja California Sur, Mexico

摘要

Late Pleistocene age terrace deposits are exposed in the narrow cliffed coastal plain of Bahia Coyote, Baja California Sur, resting unconformably on the lagoonal-shallow water volcaniclastics of the early Miocene Cerro Colorado Member of the El Cien Formation. The terrace is dissected by widely spaced arroyos and partically covered by alluvial fans in the inner and central areas. The marine deposits vary in thickness from 0.5 to 10 m and were laid down in pre-existing erosional channels and depressions in the Pleistocene landscape. The sequence begins with a cobble conglomerate with oyster shells, overlain by poorly bedded molluscan-rich bioclastic sands and coral rubble, beds of massive Porites in growth position and coral-rhodolith sands and marls. Beach sands and gravels and coastal dunes cap the sequence. Samples of Porites panamensis selected for U/Th dating are well-preserved aragonite ( > 95%). Preliminary results yield U/Th ages of 109-209 ka but the corals have initial δ ~(234)U values in excess of modern seawater values. This indicates open-system behavior and uncertainty associated with the ages. A corrected age for the top of the massive Porites unit suggests that the corals grew during the last interglacial, marine isotope stage (MIS) 5e sea level high stand. Assuming global sea level during MIS 5e was ca. 4-5 m above present-day sea level (McCulloch and Esat, 2000) and the growth position of the corals was 1-5 m below sea level, the terraces have been uplifted between 12 and 25m (12-15 cm/kyr). This is consistent with other terrace-based uplift rates for the central Baja California peninsula, north of the La Paz fault.