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Reinterpretation of the Cretacous Illes and Williams Fork Formation Contact: Importance of Marine/Nonmarine Transitions, Piceance Basin, Colorado

Thompson, Jesse D.1; Kamola, Diane 1
1 Dept. of Geology, University of Kansas, Lawrence, KS.

The contact between the Iles (Mt Garfield) Fm (marine) and the overlying Williams Fork (Hunter Canyon) Fm (non-marine) is complex and controversial. Published cross sections show a stair-stepping, eastward climbing contact between these two formations east of the Grand Junction area, juxtaposing strata from both formations across a supposed unconformity. This interpretation proposes the upper member of the Iles Fm to consist of multiple, progradationally stacked parasequences comprised solely of marine shoreface successions. It is problematic because these marine strata have no reported up dip, time-equivalent, non-marine component, yet are situated lateral to coal-bearing strata. This interval is re-evaluated via outcrop and subsurface study. The proposed formation contact is re-interpreted as a more simple stratigraphic transition which changes the regional stratigraphic relations in this area.

The Rollins Sandstone Mbr of the Iles Fm directly underlies the Williams Fork (WF) Fm, and consists of two parasequences, separated by a sequence boundary. North and east of Grand Junction, these strata are represented by wave-dominated shoreface deposits. West of Grand Junction, the lower parasequence is eroded and replaced by incised valley fill. The IVF also replaces the upper part of the underlying (Cozzette) member of the Iles Fm. IVF is comprised primarily of tide dominated facies (tidal flats and tidal creeks) as well as bay fill mudstones and bay-head delta facies. Locally, the upper parasequence of this member is eroded and overlain by fluvial deposits: this erosion event may be a second sequence boundary within the member. The high frequency sequences recorded within the Rollins Sandstone Member are consistent with the depositional style of the underlying members of the Iles Formation.

The overlying Cameo-Wheeler coal zone, at the base of the WF Formation, records the change from marine to non-marine sedimentation within the basin, and are reported as younger than the Iles Fm. The Cameo interval consists of thick coals, low energy channelized sandstones and overbank mudstones. Channels are mainly found as isolated sandstone bodies and range from 1 - 5 meters thick. Channel deposits are typically comprised of vfg sandstone and contain tidal indicators locally. Trace fossils (Ophiomorpha and Teredolites) within channel deposits indicate a close proximity to the marine environment during deposition.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009