Algal banks near La Paz, Baja California — Modern analogues of source areas of transported shallow-water fossils in pre-alpine flysch deposits

Abstract

Banks of living red calcareous algae and coral reefs contribute skeletal debris to form algal gravels and calcarenites in the shallow water around La Paz and Isla Espiritu Santo near the southern tip of Baja California, Mexico. A core taken in deep water, on the western slope of the La Paz Basin downslope from the algal banks, displays graded beds containing abundant shallow-water carbonate skeletal debris; downslope transport of this skeletal debris by tidal, storm, and turbidity currents is indicated. The calcareous skeletal debris in these graded beds are tecturally, faunally and florally similar to skeletal carbonate debris described from flysch deposits of Early Tertiary age in Switzerland and surrounding areas. Both present-day southern Baja California with its adjacent basins in the Gulf of California and the flysch cordilleras and basins of Early Tertiary Switzerland share the following characteristics: ( 1) a tropical marine climate prevails around; ( 2) in land areas, including islands chains, of high relief close by deep marine basins and troughs, the topography in large part is controlled by fault block structures; ( 3) the deep marine basins and the land areas are separated by narrow, shallow fringing shelves; ( 4) upon these shelves algal banks and coral reefs grow, contributing skeletal debris to form algal gravels and calcarenites containing coral, algal, molluscan, bryozoan, echinoid and foraminiferal debris which is subject to ( 5) resedimentation by tidal, storm and turbidity currents, thus contributing shallow-water skeletal debris to deeper-water sediments. The La Paz area is of interest to students of Alpine geology because around La Paz one may study the type, scale and intensity of sedimentational processes such as must have been active during the Early Tertiary filling of pre-Alpine flysch basins in Switzerland.