INTERPRETATION OF NEOVOLCANIC VERSUS PALAEOVOLCANIC SAND GRAINS - AN EXAMPLE FROM MIOCENE DEEP-MARINE SANDSTONE OF THE TOPANGA GROUP (SOUTHERN CALIFORNIA)

Despite abundant data on volcaniclastic sand(stone), the compositional , spatial and temporal distribution of volcanic detritus within the se dimentary record is poorly documented. One of the most intricate tasks in optical analysis of sand(stone) containing volcanic particles is t o distinguish grains derived by erosion of ancient volcanic rocks (i.e . palaeovolcanic, noncoeval grains) from grains generated by active vo lcanism (subaqueous and/or subaerial) during sedimentation (neovolcani c, coeval grains). Deep-marine volcaniclastic sandstones of the Middle Topanga Group of southern California are interstratified with 3000-m- thick volcanic deposits (both subaqueous and subaerial lava and pyrocl astic rocks, ranging from basalt, andesite to dacite). These rocks ove rlie quartzofeldspathic sandstones (petrofacies 1) of the Lower Topang a Group, derived from deep erosion of a Mesozoic magmatic arc. Changes in sandstone composition in the Middle Topanga Group provide an examp le of the influence of coeval volcanism on deep-marine sedimentation. Volcaniclastic strata were deposited in deep-marine portions of a turb idite complex (volcaniclastic apron) built onto a succession of intrab asinal lava flows and on the steep flanks of subaerially emplaced lava flows and pyroclastic rocks. The Middle Topanga Group sandstones are vertically organized into four distinctive petrofacies (2-5). Directly overlying basalt and basaltic-andesite lava flows, petrofacies 2 is a pure volcanolithic sandstone, including vitric, microlitic and lathwo rk volcanic grains, and neovolcanic crystals (plagioclase, pyroxene an d olivine). The abundance of quenched glass (palagonite) fragments sug gests a subaqueous neovolcanic provenance, whereas sandstones includin g andesite and minor basalt grains suggest subaerial neovolcanic prove nance. This petrofacies probably was deposited during syneruptive peri ods, testifying to provenance from both intrabasinal and extrabasinal volcanic events. Deposited during intereruptive periods, impure volcan olithic petrofacies 3 includes both neovolcanic (85%) and older detrit us derived from plutonic, metamorphic and palaeovolcanic rocks. During post-eruptive periods, the overlying quartzofeldspathic petrofacies 4 and 5 testify to progressive decrease of neovolcanic detritus (48-14% ) and increase of plutonic-metamorphic and palaeovolcanic detritus. Th e Upper Topanga Group (Calabasas Formation), conformably overlying the Middle unit, has dominantly plutoniclastic sandstone (petrofacies 6). Neovolcanic detritus is drastically reduced (4%) whereas palaeovolcan ic detritus is similar to percentages of the Lower Topanga Group (petr ofacies 1). In general, the volcaniclastic contribution represents a w ell-defined marker in the sedimentary record. Detailed compositional s tudy of volcaniclastic strata and volcanic particles (including both c ompositional and textural attributes) provides important constraints o n deciphering spatial (extrabasinal vs. intrabasinal) and temporal rel ationships between neovolcanic events (pre-, syn-, inter- and post-eru ptive periods) and older detritus.