Lower Tertiary Paleocurrent Trends, Santa Cruz Island, California: ABSTRACT

The Paleogene sequence of southwestern Santa Cruz Island includes approximately 200 ft of fine-grained, thin-bedded, calcite-cemented, arkosic sandstone of Paleocene (Ynezian(?)-Bulitian) age, approximately 1,400 ft of mainly shale and siltstone of Eocene (Penutian-Narizian) age, and a Narizian unit which consists of a lower coarse-grained sandstone and conglomerate member and an upper shale member. Stratigraphic distribution of cross-bedding, small-scale sedimentary flow structures, imbricated pebbles, and oriented fossils at 36 stations were used to reconstruct the clastic dispersal system and conditions of deposition. The Paleocene sequence contains thin layers of oriented Turritella pachecoensis and Turritella infragranulata. Apical orientations of the larger Turritella specimen have a vector mean of N30°W, whereas apices of the smaller Turritella specimen on the same bedding plane have a vector mean of S70°W. The vector mean of associated cross-beds is S10°E, indicating that in these beds the larger Turritella specimen became oriented with apices pointing upcurrent and the smaller ones acted as rollers and became oriented perpendicular to current-flow direction. Vector means of cross-beds, small-scale flow structures, and imbricated pebbles and shale clasts show a paleoslope of S50°W during Eocene deposition. This is supported by a decrease in thickness and average maximum clast size in the conglomerate member in a southerly direction. Based on Foraminifera, the Eocene mud and silt were deposited in mainly bathyal water, a conclusion also reached by Doerner. Abundant cross-beds, diffuse flat laminations, dish structures, and an absence of graded bedding suggest a nonturbidite flow origin for the sandstone and conglomerate member. Data from this study suggest a northerly source of sediments with the paleoslope changing from S30°E during deposition of the Paleocene deposits to S50°W during deposition of the Eocene. End_of_Article - Last_Page 461------------