Estuarine Facies Within Incised Valley Fill Systems, Mt. Garfield Formation, Book Cliffs, Colorado

Abstract

Incised valley fills (IVF) are complex features which complicate the interpretation of stratal successions. The Upper Cretaceous Mt. Garfield Formation, exposed near Grand Junction, Colorado, consists of shallow marine sandstones truncated by numerous sequence boundaries and contains multiple incised valleys. These valley fills are often nested and show significant lateral variability. When nested, facies successions alone are not distinct enough to distinguish individual valleys. Some IVF are dominantly nonmarine/fluvial with numerous coal beds and paleosols. Other IVF have a strong tidal signature: evidence for tidal influence include multiple reactivation surfaces, double mud drapes, flaser/wavy/lenticular bedding, heterolithic bedding and tidal bundles. Field study of IVF at the facies level yields a detailed sequence stratigraphic analysis at the parasequence scale. Parasequences are traced from canyon to canyon to determine lateral and down dip extent of facies. Valley fills which show tidal dominance contain numerous estuarine facies associations: tidally influenced channel-fill sandstones, estuarine deltas, coals/mires, paleosols and migrating tidal bars/burrowed sandstones interpreted as estuarine floor deposits. Individual facies average 5 to 7 meters in thickness. This is interpreted to reflect accommodation steps during sea level rise of 5 to 7 meters. Some facies have limited lateral extent (coals, tidally influenced channel-fill sandstones) and are aerially restricted within individual valley fill, while other facies are correlated over a distance of kilometers (estuarine floor deposits, estuarine deltas) and seen in multiple canyons. The IVF facies can be partitioned into high energy and low energy/protected depositional settings. Estuarine floor deposits contain meter-scale tidal bars which indicate the velocity of the tidal currents within the estuary were significant. Other estuarine facies, such as the estuarine deltas, were deposited in a more protected part of the estuary. Estuarine deltas are upward coarsening successions and rarely show large scale cross stratification. These heterolithic deposits are thinly bedded, contain flaser/wavy/lenticular bedding and have double mud drapes within finer grained interbeds. These deltas are abundant in the IVF but are not a dominant sub-environment in modern estuaries. Other estuarine floor deposits are mud dominated and highly burrowed, and were deposited in a protected setting.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015