WAVE-MODIFIED TURBIDITES: HYPERPYCNAL FLOW DEPOSITS IN THE MINTURN FORMATION, CENTRAL COLORADO

The Minturn Formation of central Colorado records deposition in an active fault-bounded basin. These strata represent braided rivers, fan deltas, marginal marine settings, and carbonate and siliclastic shallow marine environments. A prominent unit of subaqueously deposited interbedded sandstone and shale has turbidite-like graded beds that contain sole marks such as grooves, prods, and flutes. However, these beds are atypical compared to classic Bouma turbidite sequences. Detailed process-oriented sedimentological analysis reveals internal sedimentary structures that are consistent with deposition under the influence of both excess weight forces and oscillatory flow. Sedimentary structures characteristic of waves include small- and large-scale hummocky cross-stratification and gutter casts. There is also considerable evidence for deposition under combined flows, including ripples with rounded crests and convex-up foresets. Individual beds thus have characteristics of both turbidites and tempestites and were therefore deposited in combined flows of waves and currents driven by excess-weight forces. Abundant plant remains and deep sole marks indicate that the flows were highly charged with plant debris. The paleogeographic context of high topographic relief adjacent to a marine basin suggests that the flows were linked to sediment-charged flood currents that entered the ocean and became hyperpycnal flows (i.e., oceanic floods). These beds are unusual in that they also contain sequences of internal sedimentary structures that record both waning and waxing flow. Such flow is also preserved by reverse-to-normal grading patterns. These patterns may be smooth transitions from fine to coarse to fine, or show a jump in grain size within the bed at the coarsest division. The pattern of waxing and waning flow is interpreted as a record of the hydrographic response to storm events, namely increasing and decreasing discharge. The hyperpycnal flow was dynamically linked to the hydrograph and those beds with reverse-to-normal grading record all stages of the flow, including the waxing stage that in most density-driven flows is not preserved.