VOLCANIC ASH PRESERVATION/STRATINOMY IN MARINE AND LACUSTRINE SETTINGS: BIASES OF PROCESSES AND SCALE

The preservation of primary volcanic ash layers/bentonites in the sedimentary record is highly dependent on a range of stratinomic (physical, biological and chemical) processes. These include, but are not restricted to, water level changes, waves, tides, currents, storms, floods, turbidity currents, water column stratification, seiches, biological burrowing, and chemical conditions above and within the sediment (e.g., O2 concentration, salinity, Eh, pH). Other key factors include the character and influx rate of ash and non-ash sediments, and climate effects. In terrestrial/fluvial environments, preservation of ash layers is generally the exception. Below base level, however, deposition predominates. Biasing factors involved in the preservation, mixing, or destruction of primary ash layers may be similar or vary, however, between marine and lacustrine systems.

Some biasing processes are largely specific to marine or lacustrine settings (e.g., marine tides; stratification and seiche effects, more significant in lakes). This includes a lower degree of infaunal burrowing/bioturbation in lacustrine settings (largely small insect larva, unionid clams). Other factors, while acting in both settings, have more impact in one (e.g., greater climate effects in lakes).

Some processes occur in both settings, but are subject to the effects of scale. The scale-related intensity/magnitude of some processes (e.g., waves, currents) is obvious, when comparing ocean basins, epicontinental seas (modern North Sea, Devonian Appalachian Basin), large lakes (modern Great Lakes, Eocene Green River Fm.) and small lakes (modern small lakes and ponds, Eocene Florissant Fm.). Some processes are more active and effective at larger scales (e.g., fetch-dependent intensity of waves and wind-driven currents). Others are perhaps more effective at smaller scales (e.g., sedimentation events in small lakes). Within lacustrine settings, non-scalar biases may also affect preservation potential of volcanic ash layers (e.g., lotic/flow-through lakes, with fluvial-like currents and processes, versus lentic/still-stand lakes, with only wind-driven currents).

Overall, the effects of stratinomic processes and other factors should lead better preservation of volcanic ash layers in lacustrine rather than marine settings.