THE EFFECTS OF SOURCE LITHOLOGY, TRANSPORT, DEPOSITION AND SAMPLING SCALE ON THE COMPOSITION OF SOUTHERN CALIFORNIA SAND

The Transverse Ranges of southern California represent an uplifted and variably dissected Mesozoic magmatic are, and Mesozoic to Holocene se dimentary and volcanic strata deposited in convergent and transform te ctonic settings. Modern sand within part of the Western Transverse Ran ges represents: first-order sampling scale of the Santa Monica and the San Gabriel Mountains; second-order sampling scale of the Santa Clara River draining both mountain ranges; and third-order sampling scale o f the beach system between the mouth of the Santa Clara River and the eastern Santa Monica Mountains, and turbidite sand of the Hueneme-Mugu submarine fan. Source lithology includes plutonic and metamorphic roc ks of the San Gabriel Mountains, and sedimentary and volcanic rocks of the Santa Monica Mountains. First-order sands have large compositiona l variability. Sand from local coastal drainage of the Santa Monica Mo untains ranges from basaltic feldspatholithic to quartzofeldspathic. S and of the San Gabriel Mountains local drainages has three distinct pe trofacies, ranging from metamorphiclastic feldspatholithic to mixed me tamorphi/plutoniclastic and plutoniclastic quartzofeldspathic. Second- order sand is represented by the main channel of the Santa Clara River ; the sand has an abrupt downstream compositional change, from feldspa thic to quartzofeldspathic. Third-order sand (beaches and deep-sea tur bidite samples) of the Santa Monica Basin is quartzofeldspathic. Beach sand is more quartz-rich than is Santa Clara river sand, whereas turb idite sand is more feldspar-rich than is beach sand. Deep-sea sand has intermediate composition with respect to second-order samples of the Santa Clara River and third-order samples of the beach system, suggest ing that (1) the Santa Clara River is the main source of sediments to the marine environment; and (2) local entry points from canyons locate d near local drainages may generate turbidity currents during exceptio nal flood conditions. Petrologic data of modern sand of the study area are highly variable at first-and second-order scale, whereas third-or der sand is homogenized. The homogenized composition of deep-marine sa nd is similar to the composition of most ancient sandstone derived pri marily from the Mesozoic dissected magmatic are of southern California . This study of the Western Transverse Ranges illustrates the effects of source lithology, transport, depositional environment, and sampling scale on sand composition of a complex system, which provides insight s regarding actualistic petrofacies models.