EXAMINING THE IMPORTANCE OF PROCESS-BASED FIELD STUDIES ON INTERPRETING MUDROCK LITHOFACIES ON CONTINENTAL SHELVES

Mudrocks dominate convergent continental margin strata, and heterogenous mudrock lithofacies reflect the relative influence of supply, dispersal, and diagenetic processes. Decades of process-based field studies in these settings have identified how modern muddy continental shelf and slope lithofacies form. However, these observations have been made during a period of high eustatic sea level, a warming climate, and human modification of sediment sources. We can examine the temporal evolution of these facies throughout the late Pleistocene to explore how large-scale changes in climate and eustacy impact supply, dispersal, and accumulation of continental margin mudrock. Here we examine continental margin muddy facies spanning the late Holocene to the mid-late Pleistocene to document the ability to interpret mudrock lithofacies using modern field studies as an interpretive guide. Examples are from muddy continental shelves in Alaska and South Island New Zealand. Both areas are supplied with glacigenic sediment via rivers, creating thick shelf mud depocenters. The relative influence of fluvial processes dominates highstand/regressive lithofacies in both settings and is recognized in physical properties, texture/grain size distributions, and sedimentary structures. Identical characteristics are commonly observed in modern shelf settings. The depositional character of the fluvial facies varies as a function of climate in Alaska and relative sea-level in New Zealand. Temporal changes in lithofacies in both settings fit conceptual models of the evolution of continental shelf strata as a function of evolving sediment supply and relative sea level, supporting the use of observations from modern field studies in the development of such models.