High-Resolution Stratigraphy of an Underfilled Lake Basin: Wilkins Peak Member, Eocene Green River Formation, Wyoming, U.S.A.

Abstract

Abstract Lakes tend to respond noticeably to minor changes in sediment and water balance driven by climatic, tectonic, or geomorphic processes. This unique behavior of lacustrine basins can provide a high-resolution record of geologic processes within the continental setting, far from the globally averaged record of marine strata. The Wilkins Peak Member of the Eocene Green River Formation, in Wyoming, USA, is dominated by aggradation of repetitive sedimentary facies successions recording distinct lacustrine expansions and contractions. These lacustrine consist of up to four successive facies associations: littoral, profundal–sublittoral, palustrine, and salt pan. Because they comprise disparate facies that may never have been simultaneously deposited in the basin, Wilkins Peak Member cycles are non-Waltherian successions that do not readily equate to any established sequence stratigraphic unit. The completeness of the stratigraphic record in the Wilkins Peak Member varies continuously across the basin. At least 126 cycles are present in the ERDA White Mountain #1 core near the basin depocenter, whereas only about one third as many are recognizable 53 km north, nearer to the basin margin. Cycle boundaries terminate northward by gradual amalgamation into palustrine facies, reflecting the interplay between varying lake levels and a south-dipping deposition gradient. Evidence for complete desiccation and hiatuses is commonplace even near the basin center; a truly complete record may not be present anywhere. Based on recent 40Ar/39Ar geochronology of tuffs, the average cycle duration in the White Mountain #1 core is approximately 10,000 years. However, the true average duration is shorter due to the presence of lacunae, and the time required for deposition of the thinnest cycles may have been less than 1000 years. No external driving mechanism is presently known for Eocene cycles of such short duration, raising the possibility that they are instead autogenic cycles related to geomorphic instability of the surrounding landscape. Evaporite deposition corresponded closely in time and space to maximum differential subsidence, suggesting that tectonic influences on drainage patterns and basin accommodation exerted a primary control. Differential accommodation was most pronounced during deposition of the lower Wilkins Peak Member and became progressively less so after that time. Based on structural and stratigraphic observations, a period of increased tectonic uplift commenced concurrent with deposition of the lower Wilkins Peak Member.