3-D Sequence Stratigraphic Framework for the Middle Jurassic Ellis Group, Wyoming and Montana through Correlation of Outcrop Descriptions, Gamma Ray Profiles, and Well Logs

Parcell, William C. ¹, Jaclyn C. Venhaus¹ (1) Wichita State University, Wichita, KS

A sequence stratigraphic framework for the Middle Jurassic Ellis Group and equivalent units is defined based on correlation of section measurements and outcrop gamma ray profiles to subsurface wireline logs and well cuttings across the northern Wyoming and southern Montana. For decades, regional correlation of the Middle Jurassic in the northern Rocky Mountains of the United States has been complicated by the lack of comparable datasets between outcrops and wells.

Measurement of 19 sections of the Sawtooth, Piper, Gypsum Spring, Rierdon, Swift, and Sundance formations in combination with outcrop gamma ray profiling provide key references to relate outcrop descriptions to well log data. Gamma ray signatures from shales show the greatest response, argillaceous carbonates and siliciclastics are intermediate, whereas the fossiliferous, oolitic limestones and gypsum/anhydrite are lowest. Five major stratigraphic sequences are recognized in the Ellis Group. Typically, sequences have a gamma ray signature increasing across transgressive systems tracts (TST) characterized by carbonates and evaporates. Highstand systems tracts (HST) dominated by shales or argillaceous lithologies generally show high gamma ray responses. Field descriptions, wireline logs, and outcrop gamma response allow documentation of the regional trends, 3-D geometries, and sequence stratigraphic setting of these mixed clastic-chemical deposits in a retro-arc foreland basin.

This study provides a refined sequence stratigraphy for reconstructing the depositional history of the Middle Jurassic of Wyoming and Montana. Furthermore, it examines what gamma ray signatures and profiles to anticipate in mixed clastic-chemical sedimentary deposits.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana