Controls on synchronous ephemeral- and perennial-river sedimentation in the middle sandstone member of the Triassic Chinle Formation, northeastern New Mexico, U.S.A.

Abstract

The middle sandstone member of the upper Triassic Chinle Formation in northeastern New Mexico consists of ephemeral-river deposits along the Front Range adjacent to the Ancestral Rocky Mountains and perennial braided-river deposits on the Plains to the east. The two fluvial systems were synchronous; the ephemeral rivers existed as transverse tributaries to a perennial trunk system that flowed from south to north. Proximal ephemeral-stream deposits are characterized by 60–80 m thick sequences of coarse-grained, siliciclastic sediments deposited principally by overbank sheetflooding. Facies associations are dominated by sandstone with horizontal stratification and subordinate sets of tabular-tangential and trough cross-stratification. Incised streamflood deposits are represented by channeled conglomerates and single-story, massive and trough cross-stratified sandstones. Distal ephemeral-stream deposits are distinguished by their lenticular geometries, thicknesses of 20–30 m, and sedimentary structures indicative mainly of lower-flow-regime stream flooding. Channeled conglomerates and trough cross-stratified and parallel-stratified sandstones are capped by trough cross-laminated and subordinate horizontally stratified sandstones. Facies associations in the deposits of the trunk system are comparable to those of the Platte and South Saskatchewan braided rivers. Sets and cosets of tabular-tangential or trough cross-stratified conglomerate and sandstone are separated by variable thicknesses of horizontally stratidied sandstone. These facies dominate 10–30 m thick, sheet-like, siliciclastic bodies that are traceable laterally for tens of kilometers and are separated by comparable or lesser thicknesses of red mudstone. Both extrabasinal (allocyclic) and intrabasinal (autocyclic) controls influenced sandstone body geometries in the middle Chinle Formation. Uplift and subsidence determined the position of base level that, in turn, determined the nature of autocyclic processes. Adjacent to the Ancestral Rocky Mountains, sediment yield was great resulting in accumulation of sheetflood deposits above base level, thereby promoting local incision. Under these conditions and with a continuously rising base level, lenticular, coarse-grained sediment body geometries were produced along the Front Range. On the Plains, sediment yield was moderate and subsidence rates were slow. Bedload sediment accumulated to, but not above base level, thereby promoting avulsion and the development of thin, sheet-like coarse-grained sediment bodies. The contrast between the two coexisting fluvial sedimentation styles is attributed to differences in runoff. Adjacent to the Ancestral Rocky Mountains, seasonal rainfall resulted in ephemeral flow, whereas on the Plains, flow was perennial due to groundwater discharge from the highlands to the west.