Abstract: Basin Analysis of Lower Middle Miocene Strata, Santa Rosa and Santa Cruz Islands, California

D. G. Howell, H. McLean

On Santa Cruz Island (SCI) at least 700 m of lower Miocene breccia deposits grade westward to less than 300 m of sandstone and mudstone. The breccia can be divided into two facies. The lower facies is composed principally of clasts of hornblende diorite, greenschist, and silicified volcanic rocks. The upper breccia facies comprises clasts principally of greenschist, blueschist, garnet amphibolite, quartz-schist, and silicified volcanic rocks.

At least three tectonic models can be postulated to explain the stratigraphic position of the two facies: (1) a block-fault model in which unroofing of the basement terrane on the south part of SCI is followed by unroofing of a blueschist-greenschist-amphibolite terrane north of the SCI fault; (2) a strike-slip model that palinspastically moves the south part of SCI toward the southeast; (3) an alternative strike-slip model that moves the south part of SCI to the southeast by palinspastic adjustment, resulting in a juxtaposition of a possible basement terrane associated with a southern extension of the Coast Range thrust. The unroofing of a partial ophiolite sequence that structurally overlies metamorphic Franciscan rocks, would result in stratigraphic relations comparable to that of he two breccia mentioned previously.

The two breccia units are overlain conformably by at least 650 m of middle Miocene conglomeratic-volcaniclastic beds that also become thinner and finer toward the southwest and west. The composition of volcanic clasts in these beds is more siliceous and may be younger than more andesitic rocks of the volcanic terrane on the north side of SCI which tends to support the strike-slip models.

On Santa Rosa Island (SRI), lower Miocene rocks consist of nearshore to shelf-deposited sandstone and mudstone that overlie Oligocene(?) nonmarine sandstone. The shallow-marine deposits become finer grained northward and sedimentary structures also indicate a northward paleotransport direction. Petrographically the sandstone resembles nearby Paleogene rocks and is inferred to be derived from uplifted Paleogene strata. On SRI middle Miocene volcaniclastic rocks, including massive boulder-conglomerate beds, are interbedded with thin-bedded sandstone that grades westward into diatomaceous shales. These beds are distal-lateral equivalents to the coeval volcaniclastic strata on SCI.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC