The Rocks Sandstone, an Eocene Sand-rich Deep-sea Fan Deposit Northern Santa Lucia Range, California

Abstract

ABSTRACT The Rocks Sandstone of Thorup (1941), which crops out in the northern Santa Lucia Range, was deposited as a deep-sea fan in a restricted Eocene continental borderland basin. It is as thick as 640 m and consists primarily of medium-grained sandstone with interbedded mudstone and conglomerate. The average grain size and bed thickness increase toward the east, where The Rocks Sandstone is overlain by the nonmarine to shallow-marine Berry Formation and underlain by the deep-marine Lucia Mudstone of Dickinson (1965). To the west, The Rocks Sandstone thins and is overlain by the deep-marine Church Creek Formation and is underlain by the deep-marine Lucia Mudstone. Foraminifers from underlying and overlying units indicate a late early Eocene (Ulatisian) to middle Eocene (Narizian) age for Th Rocks Sandstone and deposition in a moderately deep basin connected to the open ocean. Thick channelized sandstone sequences typical of a middle-fan facies association characterize The Rocks Sandstone. Beds consist primarily of facies B and locally facies A of Mutti and Ricci Lucchi (1972, 1975) and contain graded beds, sole marks, dish structures, and other sedimentary structures indicative of deposition by sediment gravity flows. Amalgamated sequences of thick-bedded sandstone form fining- and thinning-upward channel deposits. These sequences appear to form anastomosing bodies of sandstone that directly overlie basin-plain deposits of the Lucia Mudstone. The middle-fan channel sequences are commonly superposed upon one another, forming thick sections that have very high sandstone-to-shale ratios. Interchannel deposits are relatively rare within these sandstone sequenc s, but where present consist of repetitive, thin, alternating beds of fine-grained sandstone, siltstone, and mudstone dominantly of Multi and Ricci Lucchi facies D and E beds. The minimal development of the outer-fan facies association and difficulties in separating the middle- and inner-fan facies associations characterize this sand-rich borderland-type deep-sea fan deposit. Deposition of large amounts of sand abruptly at the base of slope apparently resulted in a fan characterized chiefly by the middle-fan facies association. Without sufficient clay-sized sediment, turbidity currents that travel long distances and deposit outer-fan and basin-plain turbidites apparently did not develop. The petrographic data indicate derivation mainly from felsic plutonic rocks, with a minor contribution from volcanic rocks. Paleocurrent, thickness, and stratigraphic data indicate primarily northward and westward sediment transport. A granitic source terrane, located to the east and southeast, and probably comprising parts of the Salinian block and possibly part of the present Mojave Desert area, is suggested by the petrography and paleocurrents.