ORIGIN OF MARINE-TERRACE DEPOSITS IN THE SANTA CRUZ AREA, CALIFORNIA

The sea floor in the Santa Cruz area, California, is a modern wave-cut platform carved with 2–4 feet of relief into the Miocene Monterey Formation. The platform slopes seaward at an angle of less than 1° and is slightly concave upward within half a mile of the sea cliff. It is either bare or mantled by less than 4 feet of sand whose grain size tends to be less than that of modern beach sand and appears to decrease seaward.

The lowest of five prominent marine terraces in the same area, here called the 100-foot terrace, contains an old wave-cut platform which is similar to the modern platform and probably was developed in the same manner. Overlying this old platform are marine deposits which contrast with the modern platform deposits, but which resemble beach deposits in that their grain size is similar, their thickness is 5–15 feet, and they contain laminated structures that include black sands. Rock-boring mollusk holes in the old platform, now buried beneath the terrace deposits, suggest that these deposits were laid down after the platform was cut. Pyroxene, sphene, and shells have been leached from the terrace deposits to a progressively greater extent toward the old sea cliff; this leaching suggests that the deposits are oldest near the cliff and become progressively younger toward their distal portions. The marine-terrace deposits are interpreted as beach deposits which were prograded seaward across the platform during a period of slow emergence.

The marine-terrace deposits consist of two types of sand: a coarse mica-poor variety similar to beach sand, and a fine mica-rich variety similar to some neritic sediments in Monterey Bay. The coarse sand is interpreted as normal beach sand, derived from streams and redistributed along the prograding beach by longshore drift. The fine sand is interpreted as former neritic material which was reworked during emergence into the prograding beach where it was interlaminated and mixed with the other sand.