The Southwestern Section of the Association of Engineering Geologists and
The Southern Nevada Chapter of the Geological Society of Nevada


Field Trip to the Alamo Impact Crater/Alamo Breccia, Alamo, Nevada
Saturday, May 26, 2001- (information)

Led by:
Brian Ackman
Geologist, Edge Petroleum Company

Field Trip Coordinators:
Dr. Marvin (Nick) Saines (702) 896-4049;
Max Blanchard (702) 363-9171;

The early history of the Alamo Breccia: (Brian Ackman-EDGE Petroleum-Houston, TX)

The Devonian Guilmette Formation of southeastern Nevada contains a megabreccia deposit (Alamo Breccia) representing the effects of a single catastrophic event. This catastrophic event represents the resulting deposit from a bolide impact (basinward of the study area) of the lower Upper Devonian within the Frasinian Stage at ~367 Ma and ~3.5 m.y. prior to the Frasinian/Famennian extinctions. Several workers have speculated on the bolide composition, size, and trajectory from indirect evidence (volume of sediment effected, oldest unit effected, aerial distribution, etc.). Until the actual impact site has been documented and the internal features of the crater studied the composition of the bolide and it's trajectory are speculative. The Alamo Breccia deposit exhibits several attributes attributable to an impact origin, shocked quartz, elevated iridium levels, and sphericals/lapilli. Yet, several other attributes remain undocumented, crater diameter, depth, shape, shattercones, internal peak, dewatering/degassing pipes. Early in our research it became apparent that the undocumented tectonic overprint at outcrop produced a major obstacle in reconstructing the original facies distribution of the Alamo Breccia. The detailed stratigraphic information gathered to date has produced a jumbled facies juxtaposition reflecting the tectonic framework of the area that has yet to be adequately documented. The state of the current information concerning the Alamo Breccia is much like a jigsaw puzzle without the frame pieces. Although more than a decade has past since the Alamo Breccia was identified as a catastrophic event many questions remain unanswered.

In 1990 Colorado School of Mines was contacted by Unocal to conduct a stratigraphic research project of the Devonian Guilmette Formation of southeastern Nevada. At the time Unocal held several thousands of acres under lease and required surface work to aid their evaluation of the acreage. Reconniassance work began in the spring of 1990; during this work the interpretations of previous workers concerning the breccia unit (Alamo Breccia) came under question. These previous workers effectively described the Alamo Breccia but their resulting interpretations failed to adequately conform to the surrounding strata (i.e. Wilson, facies models both lateral and vertical). The general interpretation was that the breccia represented a reef front depositional setting. The problem with the reef front depositional environment interpretation was that the breccia directly overlays shallow-water subtidal to supratidal carbonates generally not associated with a deeper water reef front depositional setting. Also the basal contact is extremely sharp with no reefal build-ups at the stratgraphically equivalent horizon land ward. Therefore the interpretation was questioned and the search for an alternative interpretation was the focus of our work. This inspired us to first regionally describe and document variations in, the basal contact, internal composition and structure; upper contact, and the bounding units. It became obvious quite quickly that we were dealing with a catastrophic single event but the physical evidence for the cause eluded us until the fall of 1990. During thin-section evaluation of samples I obtained from the Worthington Mountains locality I discovered a quartz grain containing numerous sets of parallel lineations, which crosscut each other. Further SEM work confirmed the grain a quartz and further workers have confirmed that the sets of lineations are in fact shock lamella characteristic of those produced from impact. This evidence pointed to a single catastrophic impact event and aid us in defining our search for accessory attributes associated with impact deposits.

At this point we wrote a paper and submitted it to the Geological Society of America for their publication Geology. Upon review it was rejected with comments from the editors implying that we had clearly missed the boat in interpretation. In response we conducted a field workshop with the purpose of getting those individuals recognized throughout the geologic community as the leaders in the research of the Devonian, deep-water megabreccias, reef front depositional environments, and impacts (including the editors which rejected our paper). This field trip was conducted in the spring of 1991 resulting in three-days of on outcrop discussions and debates resulting in a unanimous vote of confidence for our single catastrophic event interpretation, resulting in creation of accommodation space on the shallow shelf and subsequent deposition of the Alamo Breccia. At the end of the 1991 spring semester I defended and entered the petroleum industry. The first paper published concerning the Alamo Breccia was co-authored by John Warme, Brian Ackman, Yarmanto, and Alan Chamberlain in the 1993 Nevada Petroleum Society Field Conference Guidebook. Since this introductory paper many Alamo Breccia publications by various authors have been published, yet many questions remain unanswered.

For the past ten years I have been cheering from the sidelines at each new discovery. Last semester I decided to engage the quest again and began my Ph.D. at the University of Houston.