AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
3-D Architecture of a Wave-Influenced Parasequence in the Ferron “Notom” Delta, Capital Reef Utah, USA: Implications for Delta Asymmetry Models
(1) BP America Inc., Houston, TX.
(2) Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX.
The recent delta asymmetry models predict sandy shoreface facies on the updrift side and muddier heterolithic lagoon, bay-fill, and barrier/bar facies on the down drift side. These models, however, are largely distilled from modern systems. Studies evaluating their validity in the ancient record are very limited. The Turonian Ferron “Notom” delta in the Capital Reef area of southern Utah, USA, is exposed three dimensionally along a 45 km by 30 km outcrop belt. This provides an ideal opportunity to further testing the asymmetry models.
Thirty four geological sections documented the 3D architecture of parasequence 6 in the fluvial-deltaic complex. The parasequence consists of four facies successions and along depositional strike these show distinct transition from shoreface deposits in the north, into heterolithic, river-dominated delta-front and channel and/or mouth-bar facies south-southeastward, and wave-/storm-reworked delta-front facies farther southeastward. Ichnogenera suites correspondingly show distinct along-strike changes from robust and healthy expressions of the Cruziana and Skolithos Ichnofacies, into suites characterized by horizontal, morphologically simple, facies-crossing structures. Farther southeastward ichnogenera abundance and diversity increase, reflecting the archetypal suites.
The overall facies distribution in the parasequence suggests delta asymmetry. Different from the recent delta asymmetry models, however, significant muddy paralic, lagoon, and bay-fill facies are not identified on the downdrift side. The lack of these facies is attributed to: 1) a negative paleoshoreline trajectory during delta progradation as indicated by the occurrence of sharp-based proximal shoreface successions on the updrift side and the overall downward- and basinward-stepping of the studied parasequence with respect to its previous ones; and 2) subsequent transgressive erosion as shown by the decimeter thick transgressive lag overlying the parasequence.
The observations suggest that, depending on shoreline trajectory and reworking after deposition, facies of asymmetric deltas can be different from those predicted by the asymmetric models. Future studies, thus, need to involve paleoshoreline trajectory and depositional history into these models.