ABSTRACT: The Transgressive Depositional Record at Parasequence Scale: A Model from the Upper Cretaceous Point Lookout Sandstone, New Mexico and Colorado

WRIGHT DUNBAR, ROBYN, Rice University, Houston, TX, PAUL E. DEVINE, B.T.A. Producers, Denver, CO, and DANNY KATZMAN, Shell Western E & P, Houston, TX

Stratal patterns and facies relationships indicate that transgressive deposits are a locally significant and predictable component of progradational parasequences. Research in northwestern New Mexico, and subsequently in southwestern Colorado, established that maximum development of back-barrier deposits coincided with transgressive, rather than regressive, depositional phases. Incipient base level rise in the back barrier setting is indicated by superposition of lagoonal and tidal deposits over either (1) regressive marine strandplain lithosomes, or (2) fluvial/tidal channels scoured into contemporaneous regressive strandplain deposits. Continuing base level rise is recorded vertically by stabilization and aggradation of the retrograde barrier, and by deposition of channeled estuarin and coastal plain deposits landward of the barrier.

The correlative marine transgressive disconformity is marked by a marine flooding surface with: (1) a discontinuous lag, (2) a thin (0-4 m), fining-upward inner shelf mudrock, or (3) a thick (0-20 m), sandstone-dominated inner shelf shoal. Transgressive shelf deposits become stratigraphically intercalated within the regressive strandplain record, and differentiation from associated regressive phase units may be subtle. Unlike regressive sandstones, shelf shoal bodies are composed of stacked discontinuous sandstones and display better sorting and higher percentage of authigenic minerals. Transgressive mudrocks fine upward to a clay-rich maximum flooding surface, which distinguishes them from downlapping regressive inner shelf or shoreface deposits.

The different expression of the shelf transgressive record may reflect a hierarchy of parasequence scales, variation in shelf sediment supply during transgression, and/or differences in rate or magnitude of the barrier shoreline retrogradation.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)