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NEW FOR 2018!    RMS-SEPM Brown Bag Luncheon Series

Our Next Luncheon Talk

(11:30 Reception; 11:45 Lunch; 12:15 Talk)


The first talk of the 2018 RMS-SEPM Luncheon Lecture Series will be held on Tuesday, January 30th, 2018.

Subcrop reinterpretation of the Ordovician Bighorn Dolomite and Devonian Darby Formation: Implications for upwarping across the Transcontinental Arch, Southwest Wyoming

Donna Anderson

Affiliate Faculty, Geology and Geological Engineering, Colorado School of Mines


11:30 Reception, 11:45 Lunch, 12:15 Talk
at the
Wynkoop Brewing Company
1634 18th St., Denver, CO

The cost is $25.00 for current members and $35.00 for non-members ($10 of which pays for an annual RMS-SEPM membership). Unemployed individuals may sign up for lunch for just $10.00. Persons who do not wish to have lunch are welcome for a $10.00 fee. Walk-ins may purchase a lunch for the standard fees ($25.00 or $35.00) although quantities are limited. Walk-ins without a lunch are charged a $10.00 fee.

Please submit reservations by 10:00 a.m., the Friday before the talk.

After the reservation deadline, you may attend the talk for a $10.00 "walk-in" fee. Reservations may be secured online, by e-mail at
or by calling Jason Eleson at 281-382-1839


A new interpretation of the subcrop geometries of the Ordovician Big Horn Dolomite and the overlying Devonian Darby Formation across southwestern Wyoming arises from revising the stratigraphy in a core from the Mountain Fuel Supply UPRR-11-19-104-4 well drilled on the crest of the Rock Springs Uplift in 1962. One of only a few wells to penetrate all or part of the Lower Paleozoic succession in the subsurface of southwestern Wyoming, the well was almost continuously cored through the Devonian-Cambrian succession. From a reinterpretation of the stratigraphy in the core, 22 feet of Bighorn Dolomite is recognized based on the characteristic Thalassinoides-bioturbation fabric in skeletal dolowackestone typical of Late Ordovician subtidal carbonate facies ranging from Nevada to Greenland along the western margin of the Great American Carbonate Bank. This lithology is in complete contrast with the alternating dolomitic flat-pebble conglomerate and dolomudstone of the underlying Cambrian Gallatin Limestone and the cyclical units of brecciated anhydritic dolomudstone and quartzose sandstone of the overlying Lower Jefferson member of the Darby Formation.

Stratigraphic reinterpretation yields insights regarding Ordovician-Devonian stratal geometries across the southwestern Wyoming. More widespread than previously portrayed, the Bighorn Dolomite pinches out on the eastern flank of the Rock Springs Uplift. Similar to past interpretations, the Darby Formation pinches out east of the Rock Springs Uplift at Brady Field. A true-geometry multi-datumed stratigraphic cross section yields insights not obtainable by mapping. Regionally top-truncation of stratigraphic units below the base-Madison unconformity normally progresses stratigraphically deeper eastward. However, in southwestern Wyoming, the Darby Formation overlaps the older Bighorn Dolomite by marked onlap across the Rock Springs Uplift and then pinches out by top-truncation/onlap near Brady Field, forming an "abnormal" overlap relationship along the northern margin of the Transcontinental Arch. The underlying Bighorn Dolomite shows little to no onlap onto the underlying Cambrian section, but is markedly top-truncated below the Darby Formation.

Comparing proportions of onlap versus top-truncation for the two formations constrains the timing of two successive upwarping episodes along the northern margin of the Transcontinental Arch across southwestern Wyoming. The first is arguably Middle Devonian, and the second spans the Devonian-Mississippian boundary. Two subtle angular unconformities created by these two episodes initiate at successively southeastern positions. They imply a southeastward migrating fold or tilt axis that sequentially was reactivated along the northern margin of the Transcontinental Arch in southwestern Wyoming.


Speaker Biography:

Donna Anderson is currently an Affiliate Faculty member in the Geology Dept at Colorado School of Mines. For the past 40+ years she has worked in the oil and gas industry (recently retired from EOG Resources), academia and consulting. She has been active in the RMS SEPM, RMAG, RMS AAPG and AAPG as a committee member and officer. A graduate of three universities, CSU Fullerton, UCLA and CSM, she claims to have never met a rock she didn’t like, except for those of the Spearfish Formation of the Williston Basin.



Speaker List

Click on the talk title for a PDF version of the abstract.


Sep 26

Ali Jaffri, Applied Stratigraphix

Our Belief in Benches; Will it Ever Go Away? A Sequence Stratigrapher's Perspective on Layer-Cake Stratigraphy

Oct 31

Steve Cumella, Whiting Consultant

Fun Things to Do with Digital Logs in Your Spare Time

Nov 28

Sven Egenhoff, CSU

What is so Special about Shales? Concepts, Questions, and Problems (and, a few Answers)

Dec   Break

Jan 30

Donna Anderson

Subcrop reinterpretation of the Ordovician Bighorn Dolomite and Devonian Darby Formation, Southwest Wyoming

Feb 27

Larry Rasmussen

Early Triassic Moenkopi Petroleum System of Central Utah
Mar 27

Michael Gardner

Tectonics and Sedimentation of Permian Deepwater Systems of the Delaware Basin
Apr 24

John Curtis

Time-Lapse Geochemistry of Source-Rock Reservoirs: One Approach & Initial Observations
May 29 TBD  


In order to keep our Luncheon Program solvent and operating on schedule,
the RMS-SEPM Board has adopted the following guidelines for reservations and seating at the Wynkoop:

  • Reservations for this Luncheon Talk will be closed at 10:00am on Friday, before the talk. (No reservations are needed for walk-ins.)