Rift Units and Related Sedimentary Facies, Northern Gulf Extensional Province, Baja California

Arturo Martin-Barajas

The late Neogene sedimentary record in the northern Gulf Extensional Province reflects two contrasting depositional settings related to segmentation into rift units. Elongated and relatively wide (10-20 km) fault-bounded basins (e.g. Laguna Salada, Valle San Felipe-Valle Chico) are separated by accommodation zones with pre-and synrift volcanism (e.g. Sierra Las Tinajas-Sierra Pinta, and the Puertecitos volcanic province).

The Puertecitos accommodation zone contains a thin (<100 m). sequence of early Pliocene marine and non marine deposits interbeded with pyroclastic flows (the Puertecitos Formation) that overlies ~6 Ma volcanic rocks. Sedimentary facies are chiefly sandy mudstone and claystone from inter tidal to sub tidal environment, with rapid lateral variations to coarser beach and alluvial deposits close to the range front. The whole sequence is disrupted by a closely spaced array of N-NW to NE normal and strike-slip faults with relatively small (<100 m) offset that controlled the facies distribution. This accommodation zone setting was likely a high-relief volcanic apron.

In contrast, a thicker (>2000 m) sequence of Mio-Pliocene strata that completely lacks volcanic rocks is found in the Laguna Salada basin, a large fault-bounded basin in the rift system. The overall sequence progrades from shelfal to deltaic to alluvial fan deposits and likely represents clastic sedimentation in a rapidly subsiding basin. Depocenters of the alluvial-fan deposits were controlled by the synsedimentary large offsets of the Canada David Detachment and later by the oblique, down to the west, right-lateral Laguna Salada Fault.

We interpret differences in style of sedimentation to reflect higher heat flow and lower subsidence rates in the accommodation-zone setting.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California