THE FATE OF AIRFALL VOLCANIC ASH IN LARGE AND SMALL LACUSTRINE SYSTEMS: ASH STRATINOMY OF THE EOCENE GREEN RIVER AND FLORISSANT FORMATIONS

The preservation of airfall volcanic ash beds in lacustrine systems, as primary or reworked layers, is dependent on numerous stratinomic factors (e.g., lake morphometry, water level changes, mixing/stratification, waves, currents, seiches, storms, background sedimentation, burrowing, chemical conditions above/within sediment). Tuff bed stratinomy varies systematically by lake facies and via scaling of processes in large versus small lake systems. Volcanic tuffs in lacustrine facies of the Eocene Green River and Florissant formations (WY, CO) show a broad range of preservation.

Green River lacustrine systems comprise large to medium size lakes, which formed in intermontane basins. Over time, the lakes shifted between underfilled, balance-filled, and over-filled conditions; waters ranged from normal freshwater to hypersaline/evaporative, in profundal to playa-lake to fluvial facies. The Green River lakes were distal from volcanic sources (>200 km), with clastic, carbonate, evaporite, organic-rich, and volcanogenic sediments.

Green River tuffs in oil shale facies (lower LaClede Bed) appear as simple, single-layered, normally graded beds. In playa environments (Wilkins Peak Member) tuffs occur as reworked, complexly multi-layered beds which pinch and swell along outcrop, laterally interfinger or pinch out among background sediments, and may feature playa-lake intraclasts. Basal primary ash fall layers may or may not be preserved. Tuffaceous material in fluvial facies (Sand Butte Bed) is mixed with detrital sediments. No airfall tuffs are known from fully freshwater facies (Luman Tongue).

Lake Florissant was a relatively small, elongate dendritic lake, proximal to volcanic sources (>29 km), formed by lahar damming of a stream valley. Lake waters were commonly meromictic. Significant volcanogenic sediment (via airfall, lahars, and reworked fluvial input) occurs with granitic and metamorphic detritus, and lake-derived diatomaceous sediment.

Volcanogenic sediment in the Lake Florissant system comprises simple, coarse- to fine-grained, normally graded beds, of probable airfall origin; medium to coarse pumiceous beds, of airfall or riverine input; and mixed volcanogenic/detrital sediment of riverine origin. Reworked airfall material is more common in nearshore and fluvial facies.