Lacustrian to Fluvial Transition in the Tertiary Wasatch Formation, Three Canyons, Desolation Canyon, Utah

Ford, Grace L.¹, Piret Plink-Borklund², Marieke Dechesne³ (1) EOG Resources, Denver, CO (2) Colorado School of Mines, Golden, (3) Denver Museum of Nature & Science, Denver, CO

Sands of the Tertiary, Wasatch Formation in Three Canyons of Desolation Canyon, Utah display well behaved, but non-documented, lacustrian to fluvial base-level cycles. Detailed analysis of bounding surface architecture and genetically related sedimentary structure show a progressive evolution from lacustrian mouth bars to distributary channels.

The mouth bars are characterized by upper-flow regime hydrodynamics with accretion towards the north-northwest, 305°. Basal bounding surfaces are inherently flat with minimal erosion; where present, it is typically less than 0.5 meter. Horizontal planner laminations and low-angle laminations dominate these fine grained sands. Thicknesses range from 2 to 9 meters. Lateral to the depositional axis they grade into thin bedded, very fine grained, ripple laminated sands and moderately bioturbated silts and muds. These thinner beds display a downlap geometry onto the sands of the mouth bars in the accretionary direction.

In contrast, channel forms display multiple cut-fill cycles, typically cannibalizing their own mouth bars. They are characterized by undulating, erosional basal surfaces with up to 5.5 meters of scouring. They exhibit a marked grain size jump, (course-to-medium), and clay rip-ups along their basal surfaces. Sedimentary structures include trough cross-bedding, low-angle laminations, soft sediment deformation and ripple laminated caps. Paleo-current indicators show an overall north-northwest, 320°, flow. These are typically overlain by a moderately bioturbated, silty-muds.

Characterization of this lacustrian to fluvial transition demonstrates the complexity of what have been assumed to be fluvial dominated sands. Understanding the variability of these systems is key to better reservoir management in analog petroleum reservoirs.