New Mexico Geological Society Annual Spring Meeting — Abstracts

The Abo and Lower Veso Formations in the southeastern Nacimiento and southwestern Jemez Mountains constitute an alluvial plain redbed sequence. The mudstones, feldspathic sandstones, and intraformational conglomerates represent the deposits of an evolving fluvial system within the distal reaches of a large clastic wedge. Three major lithofacies associations, two within the Abo Formation and one within the overlying Yeso Formation, delineate distinct fluvial settings.

The lowermost division (Unit A) is mudstone dominant with isolated fine-to-medium grained composite-channel-sandstone sheets. These sandstones commonly show a progressive upward decrease in the of internal bedforms wi th a lower scour surface containing intraformational conglomerate. Associated overbank mudstones contain fully developed, laterally extensive caliche horizons attesting to prolonged periods of subaerial exposure.

The middle division (Unit B) is characterized by thick multilateral channel sandstones and thinner mudstones. The sandstones are coarse-grained and contain intraformational conglomerate.

The uppermost division (Unit C) (Lower Yeso Fm.) is dominated by ripple laminated and horizontally bedded, very fine-grained sandstone sheets and is gradational with overlying Yesa eolian deposits.

The fluvial sequence from Unit A to Unit 8 represents the transition from a mixed-load to a bed-load fluvial system. The transition to Unit C reflects a change to sheetflood, ephemeral-stream controlled sedimentation. Tectonism and climate serve as first-order controls on these fluvial transitions.