New Mexico Geological Society Annual Spring Meeting — Abstracts

The Upper Cretaceous Point Lookout Sandstone in the Four Corners area of the San Juan Basin, New Mexico and Colorado is composed of complexly stacked sandstones and shales representing an overall regression of strandline and barrier complexes into the Western Interior Seaway.

Close examination of the lithofacies and their geometric relationships indicates that a hierarchy of transgressive- regressive depositional cycles produced the resultant sheet sandstone. Several high-frequency cycles, probably less than 100,000 years duration, are "bundled" within lower-frequency cycles. Transgressive deposits of the higher-frequency cycle are characterized by thin (less than 1 meter) mudrock units. At the lower frequency, an important, recently recognized component of the depositional sequence is the occurence of irregularly stacked transgressive sandstone bodies (total thickness of up to 18 meters) that were deposited in shoreface to inner shelf depths during relative rise in sea level. These transgressive deposits form the "depositional ramp" of Evert, 1987, were deposited contemporaneously with aggrading (transgressive) back-barrier deposits. The time-line associated with the transgressive deposits of these two environments crosses the ravinement surface which is formed by wave action during barrier retreat. These geometric relationships are similar to those described for Holocene transgressive sequences of the Gulf and Atlantic coats of the U. S.