--> Abstract: Tidal Channel to Bayhead Delta Couplets: Key to 4th Order Proximal Sequence Identification and Correlation, Upper Iles Formati; #90063 (2007)

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Tidal Channel to Bayhead Delta Couplets: Key to 4th Order Proximal Sequence Identification and Correlation, Upper Iles Formation, Colorado

 

Gomez, Carolina A.1, Ron Steel2 (1) University of Texas at Austin, Austin, TX (2) The University of Texas at Austin, Austin, TX

 

Fourth-order stratigraphic sequences have been identified as the fundamental building blocks in the Upper Iles Formation, using stacking patterns and facies associations from outcrops and well log data. These sequences have been correlated from proximal to distal reaches, along a ca. 150 km regional transect extending from southwest Wyoming to northwest Colorado.

 

The fourth-order cycles are some 20-35 m thick and in their proximal-medial reaches are often composed of a transgressive tidal channel association (T1) an intervening transgressive muddy zone (T2), and overlying regressive bayhead-delta facies association (R1).

 

Facies association T1 is recognized in the well logs by upward-fining, serrated or blocky gamma ray patterns, passing upwards into muddier facies (T2). In the outcrop T1 is 7-13 m thick, composed of up to 3 m-thick stacked fine-grained sandstone channels. These channels were filled with 2-D dunes that exhibit signs of tidal influence. The base of such channels cuts down into bayhead delta deposits, sometimes cannibalizing them completely. Facies T2 is usually 7-10 m thick and not well preserved.

 

Facies Association R1 shows a coarsening -upward, serrated log pattern. In outcrop it is composed of 8-10 m thick, medium-grained sandstones, showing large-scale, downstream-accreting surfaces and some bi-directional dunes. Each of the sand beds is few tens of centimeters thick and interbedded with organic-rich, millimeter-thick mudstone layers.

 

Our ability to recognize these fourth-order sequences in well logs, understand their internal configuration/architecture in the field, and correlate them along sub-regional transects should aid our understanding of the variability of these as reservoirs.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California