Siliciclastic-Carbonate Interactions in Laborcita Formation, Sacramento Mountains, New Mexico: ABSTRACT

Outcrop exposures of the Laborcita Formation (Wolfcampian) reveal an active depositional environment with abrupt lateral and vertical changes between fan-deltaic and carbonate sediments. The siliciclastic source area was to the east, in the Pedernal uplift. Fan-delta lobe shifting was important in producing the cyclic nature of deposition. Away from the area affected by the fan-deltas, deposits are increasingly calcareous. Shales are the predominant lithology in the Laborcita Formation, due to the abundance of carbonate-inhibiting terrigenous matter, especially in restricted areas. A few digitate stromatolites and Archeolithophyllum sp. mounds indicate subaerial exposure. Toward the edge of the narrow shelf, large phylloid-algal buildups (20 m or 65 ft thick) occur. An exposure in Coyote Canyon, near the northern end of the Laborcita exposure, shows an onlapping sequence of several mounds. This mound zone was terminated when muds and fine to medium-grained terrigenous sands migrated in and inhibited growth of the carbonate-producing organisms. The terrigenous sediments were in turn overlain by grainstones exhibiting lon , low-angle (15°) cross-bedding, which dips landward (southeast). Individual grainstone beds are thin (0.5-1.5 m or 1.6-5 ft), extend along strike laterally for about 2.5 mi (4 km), and are composed largely of bioclastic carbonate grains (not oolitically coated) with 5% quartz grains. Direction of migration was southwest to northeast. Gradual emergence is recorded by the Laborcita Formation. With continual progradation of terrigenous deposits, interrupted by marine incursions resulting in deposition of shallow-water carbonated deposits, the transitional Laborcita Formation was ultimately overlain by the terrigenous Abo Formation. End_of_Article - Last_Page 256------------