New Mexico Geological Society Annual Spring Meeting — Abstracts

The transgression of the Western Interior seaway into North America is widely accepted as occurring in the Early-Late Cretaceous; however, the precise age of the marine incursion into northern New Mexico is not well understood. As the seaway migrated southwestward, rocks generally termed the Dakota Formation were deposited as its paralic facies throughout the Western Interior. Distal rivers to the west and southwest of the encroaching shoreline backfilled as base level rose, and in north-central New Mexico, stream aggradation in pre-Dakota incised valleys resulted in deposition of the lowest part of the Dakota Formation–the Encinal Canyon Member.

The age of the fluvial Encinal Canyon Member – and thus the shoreline position of the Western Interior seaway during Encinal Canyon deposition – has long been unclear for a lack of age data, though some workers posited a late Early Cretaceous (late Albian) age based on regional stratigraphy and limited palynological data. In northern NM, the Encinal Canyon Member is a discontinuous fluvial sandstone, 0-75 m thick, that has not yielded age-diagnostic fossils and directly overlies a regional unconformity with the Upper Jurassic Jackpile Member of the Morrison Formation. The marine Oak Canyon Member of the Dakota Formation disconformably overlies the Encinal Canyon Member, and was deposited as shallow marine and marginal marine sands when the seaway transgressed over the region. It is mid-Cenomanian in age based on ammonoid biostratigraphy. New U-Pb zircon geochronology resolves the timing of the transition from continental to marine deposition represented by these basal Dakota members associated with the initial transgression of the Western Interior seaway.

U-Pb zircon geochronology of the Encinal Canyon Member at outcrops on the northern flank of the Sandia uplift in Bernalillo County, NM, yields a maximum depositional age of 99.4 ±1.6 Ma (earliest Cenomanian). An ash bed (ash “A”) in the middle of the overlying Oak Canyon Member at the edge of the Albuquerque basin in Sandoval County, NM, provides an igneous age of 97.6 ±1.3 Ma. In addition, the Oak Canyon contains fossils of the mid-Cenomanian Conlinoceras tarrantense ammonoid zone. Both the fossil zone and the ash are laterally continuous throughout the region and greatly improve age resolution of the early marine incursion into northern New Mexico during the Early-Late Cretaceous.

References:

1. Aubrey, W.M., 1989, Mid-Cretaceous alluvial-plain incision related to eustacy, southeastern Colorado Plateau: Geological Society of America Bulletin, v. 101, p. 443-449.
2. Owen, D.E., and Head, C.F., 2001, Summary of the sequence stratigraphy of the Dakota Sandstone and adjacent units, San Juan basin, northwestern NM and southwestern CO, in Broadhead, R., Cather, M., and Brister, B.S., eds., Proceedings for low permeability and underdeveloped natural gas reservoirs of NM, NM Bureau of Mines and Mineral Resources, p. 2-19.