News and Information: News-October 2006

Faults along the eastern flank of the Colorado Front Range display a diversity of attitudes and juxtaposed age relationships. This paper proposes a modified triangle zone model, one that allows back thrusts within the intercutaneous wedge, to explain the observed fault data. The “evolved” triangle zone model predicts a variety of characteristic fault types including: 1) foreland-dipping roof thrusts, which either show no stratigraphic separation or anomalous younger-over-older relationships; 2) foreland-dipping intercutaneous back thrusts which exhibit both older-over-younger and younger-over-older bedding relationships; and 3) hinterland-dipping floor thrusts that show older-over-younger bedding relationships.

Evolved triangle zones are found at multiple stratigraphic and structural levels along the range. These stacked detachment levels accommodate displacement transfer along the range and give rise to some surprisingly complex but restorable structures. This model may be helpful in understanding apparently anomalous thrust relationships in a variety of tectonic settings.

In 1910, Claude Siebenthal proposed the existence of a Pleistocene lake in the San Luis Basin of southern Colorado based on fossils and sediments exposed in Hansen Bluff, about 10 km southeast of Alamosa. Extensive studies of the bluff exposures in the 1980s by Karel Rogers and others revealed an early to middle Pleistocene section that has abundant plant and animal fossils, two volcanic ashes, and a paleomagnetic record that includes the Brunhes/Matayama boundary. The depositional environments at Hansen Bluff and in nearby cores include shallow to perennial lakes, playas, and meandering streams. However, in the century since Siebenthal’s first work, no one had found morphological expression of the Lake Alamosa or documented its maximum altitude or lateral extent.

During geologic mapping of the Alamosa 1/2° x 1° sheet, I found several new exposures of shallow near-shore and lacustrine deposits. Beach gravels are preserved as bars, spits, and remnant deposits in saddles between bedrock-cored hills. Associated wave-eroded hills (bedrock) suggest that the ancient lake reached a maximum altitude of 2325-2330 m in the San Luis Basin. The most prominent features are well preserved south and east of Alamosa, along the northern margin of the San Luis Hills. Distinct spits at 2310-2330 m wrap around the southwestern side of Sierro del Ojita and Saddleback Mountain and a kilometer-long spit is well preserved on the eastern side of the Rio Grande, just north of the basin’s outlet. None of these or the dozen other constructional lake features in the area is well exposed, but trenching shows they have well developed, meter thick, stage III calcic soils (i.e., 300-500 ka of soil development).

In the middle Pleistocene, the upper Rio Grande had its headwaters in northern New Mexico. At that same time, the uppermost reach of the modern Rio Grande emptied into Lake Alamosa, which probably formed as a result of blockage by voluminous eruption of Servilleta flood basalt in the southern part of the San Luis Basin (SLB) between 3.7 and 4.8 Ma. Servilleta basalt covers most of the Taos Plateau northward to the Colorado/New Mexico border. On the western side of the SLB, basalt is at the surface north to Antonito, Colorado. New USGS aeromagnetic data suggest the subsurface basalt flows continue at least as far north as La Jara at depths of 30-100 m. Servilleta basalt is also preserved on the eastern side of the SLB beneath the Costilla Plain, on San Pedro Mesa, and in the Culebra graben north to Fort Garland, Colorado. Drill-hole data show that basalt extends northward to Blanca in the shallow subsurface. Thus, by middle Pliocene time (ca. 3.5 Ma), Servilleta basalt probably blocked south-flowing drainages in the basin: the resulting closed basin was occupied (episodically) by Lake Alamosa in which the sediments of the Alamosa Formation were deposited. This formation is well known for thick “blue clays” that form confining layers within fluvial aquifers of the SLB.

Alluvial and lacustrine sediment nearly filled the upper SLB prior to the lake’s overflow, sometime around 450 ka as estimated from a preliminary 3He exposure date of ~439±6 ka on a boulder or the spit at Saddleback Mountain. When the lake rose during a middle Pleistocene glacial cycle (perhaps marine OIS 12), it overtopped a hydrologic sill on Oligocene Tertiary rock of the Fairy Hills and cut a deep gorge. The integration of the upper SLB into the upper Rio Grande drainage led to downstream incision of the river, especially north of the Red River and added about 18,000 km2 to the river’s drainage area.

The Colorado Scientific Society invites entries for the Student Presentation Awards. The awards competition will be held in early November, with finalists invited to give a 15 minute talk (format like GSA or AAPG) on the evening of November 16, 2006, at the CSS monthly meeting at the Colorado Mountaineering Center is Golden Colorado. Topics for the talks can be any subject related to earth science. The abstract must include:

Title

Name and University

Abstract (maximum 250 words)

Please abstract, composed in 12pt. Times New Roman font, via email, by the closing date of November 1, 2006, to:

Denver International Petroleum Society meets the second Friday of each month at the Wynkoop Brewing Co., 18th and Wynkoop Streets. Reception begins at 11:30, luncheon at noon, program at 12:30. Make reservations (required) by leaving message at (303) 623-5396. Reservations accepted after 8 a.m. on Friday until 10:30 a.m. on Wednesday prior to the meeting. Cancellations accepted until 11:00 am Wednesday prior to the meeting. Cost: $15 for lunches; talk only is available for $2 (make checks payable to “D.I.P.S.”). Contact Keith Murray at (303) 986-8554 for information.

Denver Region Exploration Geologists’ Society (DREGS) meets in the Mutual Consolidated Water Building, 12700 West 27th Avenue, Lakewood. Social hour 6:00-7:00 p.m. Technical presentation at 7:00 p.m. Meetings are normally scheduled for the first Monday of each month. For information contact Jim Piper, (303) 932-0137, or the website http://www.dregs.org.

Colorado State University, Dept of Geosciences, Rm 320 Natural Resources Bldg, 4:10 pm. Oct 16, Florian Schwander, “Active volcanism: new avenues toward monitoring, hazard assessment, and mitigation”. Oct 30, Robert Jarrett, “Reflections on the Big Thompson Canyon flood, 1976”. 970-491-5661. http://www.cnr.colostate.edu/geo/seminars