VOLCANIC TUFF BED STRATINOMY IN LACUSTRINE SETTINGS, EOCENE GREEN RIVER FORMATION

Preservation of airfall volcanic tuffs in large-scale lacustrine systems is dependent on many stratinomic factors. These include lake morphometry, water level changes, mixing/stratification, waves, currents, seiches, storms, background sedimentation, burrowing, and chemical conditions above/within the sediment. Tuff bed stratinomy also appears to vary systematically with lake type in the Green River Formation, based on observed tuffs in the underfilled Wilkins Peak Member, balanced-fill lower LaClede Bed (Laney Member), and overfilled upper LaClede Bed.

In the Wilkins Peak Member, tuffs commonly exhibit complex depositional histories. Individual beds often comprise multiple layers, including interbedded tuff and background sediments, which pinch and swell along outcrop. Tuff beds commonly show sharp bases with erosional relief, hummocky cross-stratification, or scour and fill structures. Locally in the Second tuff, internal layering indicates at least 10 separate depositional events. Of note, complex tuff beds commonly feature a basal, thin, relatively tabular bed, which may represent a primary fallout layer, overlain by reworked tuffaceous material. Preservation of these tuffs may have been strongly influenced by wave reworking in relatively shallow water, perhaps by rapid alternations between playa and lacustrine conditions.

Tuffs in the lower LaClede Bed generally occur as thin, analcime-rich layers within finely laminated mudstone, suggesting deposition under relatively quiescent conditions. They often appear normally graded and relatively tabular. Non-tabular bedding, when present, may be related to post-depositional loading, or draping over pre-existing lake floor features (e.g., stromatolites). Some tuffs appear to represent multiple, amalgamated events.

In the upper LaClede to Sand Butte Beds, tuffs are relatively rare, and typically show evidence of subaqueous reworking. The “white marker tuff” for example shows extensive cross-bedding, climbing ripple cross-lamination, and soft-sediment deformation. In addition, pyroclastic material occurs in subaqueous channel-form sandstones, rich in pumice and tuffaceous material (locally in Sand Butte Bed). This suggests that fluvial transport and reworking of volcanic material strongly influenced tuff preservation.