Intrabasinal Faulting in Cretaceous Forearc Basins of Baja California, Mexico

Cathy Busby-Spera, Douglas Smith, William Morris

Cretaceous forearc basins in Baja California show abundant evidence of intrabasinal faulting. This resulted in (1) growth of fan deltas on the margins of intrabasinal horst blocks, (2) development of a submarine canyon along the axis of a half-graben, and (3) rapid, extreme fluctuations in relative sea level due to vertical tectonics. The fill of these tectonically active residual and arc massif basins is much more complex than that of the same basin types, of similar age, to the north in the Great Valley forearc basin of California.

Residual forearc basins lie on oceanic or transitional crust trapped between the arc massif and subduction zone, and ideally show a simple, upward-shallowing fill sequence. In contrast, extensional tectonics dominated the early history of the residual forearc basin of Baja California, causing downdropping of numerous, small grabens to bathyal water depths and uplift of adjacent horst blocks above sea level. On the present-day Vizcaino Peninsula, a single phase of uplift followed by gradual erosion of horst blocks resulted in Albian deposition of an upward-fining deep marine sequence within each graben. Cenomanian to Santonian deep-sea fan deposits then blanketed horst blocks and graben fills during a phase of broad downwarping of the forearc basin. An intrabasinal normal fault remaine active for a longer period of time (Albian to Turonian) at the present-day site of Cedros Island. At that locality, basement-derived breccias were continually shed from the fault margin of a half-graben that also acted as a funnel for arc-derived sandstones and conglomerates (i.e., a structurally controlled submarine canyon). Late Cretaceous arc massif basins of Baja Norte do not show the simple thermal contraction subsidence history documented for the arc side of the Cretaceous Great Valley sequence. Instead, abrupt lateral and vertical alternation of bathyal marine and nonmarine facies indicate syndepositional intrabasinal faulting.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.