DUNCAN, ED, BP Exploration (Statoil Alliance), Stavanger, Norway

ABSTRACT: Sequence Stratigraphic Framework and Evolution of Delta Types: Norias Delta System, Oligocene (Frio Formation), South Texas

The Norias delta system was a major depositional component of a second-order sequence infilling the Rio Grande Embayment of South Texas during the Oligocene. The Norias delta system was comprised of third and higher order cycles that effectively filled the embayment with expansive, sandy deltaic sediments by the end of the Oligocene. Initiation of deltaic progradation may have been associated with the onset of the Chattian sea level fall between 30 Ma. and 32 Ma. Deltaic sedimentation continued through the subsequent sea level rise and was punctuated by a regional flood event at approximately 24 Ma.

Incorporated within this complex depositional sequence, a well defined evolution of delta styles can be documented and tied through time toaccepted global sea level curves. Earliest Norias, fluvially dominated high constructive lobate deltas dominated the highstand systems tracts of the sea level cycle. The early Norias phase of sedimentation was punctuated by extensive Type 1 and 2 unconformities. Middle Norias deltas appear to havebeen generally high constructive lobate to wave modified arcuate within shelf margin and prograding lowstand wedge systems tracts. Early and middle Norias sequences likely include lowstand fan systems tracts though in most areas this section is undrilled. Late Norias deltas evolved toward high destuctive wave dominated elongate systems within transgressive systems tracts. These deltas developed as relative sea level rose rapidly forcing the delta system as a whole to be perched well back on the underlying early and middle Norias platform.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.