COASTAL GEOMORPHOLOGY OF ANDESITE FROM THE CRETACEOUS ALISITOS FORMATION IN BAJA-CALIFORNIA (MEXICO)

Exposed along the Pacific shores of northern Baja California, Mexico, the Alisitos Formation includes a significant amount of andesite which formed during Middle Cretaceous time. In Late Cretaceous time, again in Late Pleistocene time, and now at present, these exhumed igneous reeks were and continue to be the resistant guardians of a rugged shore. The longevity of this rough coastline is at odds with traditional models of shoreline evolution. Features named herein ''keyhole inlets'' are joint-controlled openings in andesite headlands, eroded by intense wave shock on an open coast. They are narrow with a length to width ratio commonly between 3:1 and 7:1. Type I keyholes occur in rocks which are structurally consistent with very few joints or a multitude of chaotically oriented joints with no dominant plane of instability. Maturity of these steep-walled features is associated with the enlargement blowholes. Type II keyholes typically express much lower wall relief, forming in rocks having vertical joint planes with variable frequencies ranging between 4 and 45 joints per meter. Secondary or even tertiary joint sets may be observed at an angle subperpendicular to the first, with the result that two keyholes intersect to form an isolated sea stack. This process of coastal erosion operated on a grand scale during late Cretaceous times, when small andesite islands were separated from the mainland.