Abstract

The Miocene Vinchina Formation is made up of more than 5100 meters of siliciclastic sediments deposited mostly in fluvial environments in a broken foreland basin without any connection with the sea during a period of arid to semiarid climatic conditions. Repetitive changes in fluvial facies allow the subdivision of the formation into seven cyclothems, each floored by a subaerial unconformity. Within-sequence changes in fluvial systems determined by the proportion of channel vs floodplain, multi-story vs single-story channels, and channel connectedness were controlled by changes in accommodation. Proximal (i.e., northern and younger) sequences show transitions from braided to low-sinuosity (wandering) or high-sinuosity (meandering) fluvial systems, whereas more distal sequences (southern and older) show changes from sand-bed dominated to anastomosed systems. Bounding unconformities and facies stacking patterns are used to define six third-order depositional sequences.

Depositional sequences in the Vinchina Formation are either asymmetrical showing a fining-upward trend or nearly symmetrical (sand clock type). Each cyclothem is bounded by an incision surface (sequence boundary) developed during a base-level (i.e., fluvial equilibrium profile) fall and initial base-level rise floored by basal coarse-grained deposits representing the low-accommodation systems tract (LAST). The overlying sediments fine upward and contain both isolated and multistory sandstone bodies within extensive flood-plain deposits corresponding to the early high-accommodation systems tract (HAST). This interval is sometimes capped by sheets of amalgamated sandstones corresponding to the late HAST and developed during a decline in the rate of base-level rise. The development and/or preservation of the capping amalgamated sandstones and therefore the symmetrical shape of the sequences is favored in high-accommodation settings. Vertical and lateral facies changes recorded in the Vinchina Formation indicate that both the fold-and-thrust belt located to the west and an uplifted basement block to the north played important roles controlling subsidence, source areas, and drainage patterns. The stratigraphic record of this broken foreland basin differ from the existing models for ¨simple ¨ foreland basins and can be compared to those of the Laramide sedimentary basins in the United States.