BIOSTRATINOMY/TAPHONOMY OF CHARCOAL (WOOD) CLASTS AND A RECORD OF MIOCENE WILDFIRES IN TURBIDITE SEQUENCES OF THE MILL CREEK/POTATO SANDSTONE, SOUTHERN CALIFORNIA

Modern paleofire research relies almost wholly on analysis of charcoal retrieved from lacustrine sediments younger than 50 ka. A thick section of about 11 Ma, upper Miocene nonmarine siliciclastic turbidites are commonly referred to either the Potato Sandstone or Mill Creek formations and are exposed on the western flank of the San Bernardino Mountains in southern California. The turbidite caps are commonly enriched with apparent charcoal wood clasts and very few small noncarbonized woody fragments. Morphological and size analyses of experimentally produced firewood charcoal provide a model and an effective “control sample” for the interpretation charcoal clasts in the Mill Creek turbidites.

Charcoal is extremely porous—volatiles are removed by rapid combustion, leaving a network of carbonized cellulose and vascular systems. Charcoal clasts are transported and deposited differently than other sediments because of their extreme low density compared to typical siliciclastics. Size and shape of charcoal clasts are strongly correlated in both Mill Creek turbidites and the “control sample”. Both clast populations possess decreased length-width ratios with decreasing size. Density differentiation between siliciclastics and charcoal during transport effectively eliminate charcoal-rounding by abrasion and impact. Rounding of charcoal is dominantly fire-produced, and common only in larger clasts. As wood burns, inherent woody texture/structure imposes a minimum size limit to charcoal fragmentation, which in larger specimens is typically prismatic, parallel to wood grain and at ring growth boundaries. Based on both samples, the minimum size limit for charcoal fragmentation, which produces almost equant clasts, appears be about 3mm. Our studies indicate that morphological analysis of charcoal can provide a much greater understanding the history of wildfire, especially for interpreting modern climate change.