Abstracts – 1995

January 1995 – Emmons Lecture

The Crash of Comet Shoemaker-Levy 9 on Jupiter

E.M. Shoemaker
Scientist Emeritus, U.S. Geological Survey; and staff member, Lowell Observatory
C.S. Shoemaker
Lowell Observatory 

Approximately 20 observed nuclei of P/Shoemaker-Levy 9 struck Jupiter between July 16-22, 1994. Comparison of the ejecta plumes with numerical models suggests that most of the brighter nuclei were more or less compact objects ranging from 1 to 3 km in mean diameter. If these estimates are correct, we have been privileged to witness an event that may recur no more frequently than about once per millennium. Fine, dark, soot-like particles made each impact plume visible as a complex dark spot above the sunlit face of Jupiter. The larger dark spots spread to radii greater than Earth’s in times less than 90 minutes. The crash of ’94 is a vivid reminder that the Earth is also subject to the occasional global catastrophe of large-body impact.


February 1995

Mount Rainier:Relationship of Hydrothermal Alteration to Volcanic Hazards at a Decade Volcano

David R. Zimbelman
U.S. Geological Survey 

Mount Rainier, one of about 14 Decade Volcanoes in the world and the highest volcano in the Cascade Range, is an active volcano that has produced huge mudflows (lahars) in its recent past. The largest lahar to emanate from Mount Rainier in the past 10,000 years traveled over 110 km into the Puget Sound Lowlands and covered an area of at least 505 km2. Such lahars form when part of the volcano edifice fails and are not necessarily related to other types of volcanic activity, such as an eruption. Hydrothermal alteration is important to the formation of lahars because it: (1) reduces the structural integrity of the edifice and (2) produces clay minerals which, when incorporated into the lahars, result in lahars that are both larger and which travel farther than lahars that lack clay minerals. As part of the Decade Volcano Demonstration Project, both unaltered and hydro-thermally altered rocks on the edifice of Mount Rainier are being mapped and studied in detail for the first time.


March 1995

3D Solution to a 2D Pitfall: Seismic Detection of Carbonate Buildups, Kiva Field, Paradox Basin, San Juan County, Utah

Cindy Crawley Stewart 

Kiva field produces oil and associated gas from carbonate buildups in the Upper Ismay zone of the Paradox Formation at a depth of 5,800 ft. The stratigraphic trap is formed by a lithologic change within the Upper Ismay zone from a limestone and dolomite reservoir facies to a thick offmound anhydrite facies. The estimated ultimate recovery under waterflood is 3,106 MBO + 3,689 MMcfg. Cumulative production from Kiva field as of January, 1993, is 1,991 MBO + 3,023 MMcfg.

The 1984 discovery of Kiva field was the result of a geologic model that was confirmed by conventional 2D seismic data. The seismic data was carefully processed and took into consideration the cyclic nature of the geology in the Paradox Formation.

After the confirmation well flowed 1,050 BOPD + 750 Mcfgpd from the Upper Ismay, Meridian Oil and BWAB proceeded in January 1985 to acquire a 3D seismic survey over the possible extent of the new field. The results of that survey show that the 3D images the porous part of the algal mound and demonstrates mound morphology. The survey also demonstrates that the conventional 2D strike line images the crest of the mound from out of the plane of the section (sideswipe), and the 3D migration of the data volume is able to place the image of the mound into its proper position.

The ability to detect these prospective algal mounds using high quality seismic data makes these shallow oil reservoirs an excellent exploration target.

In the Tracks of Hayden

Michael Foster
Author and Historian

The first careful government survey in Colorado was carried out by the U.S. Geological Survey from 1873 to 1876. The purpose of the expedition, led by Ferdinand Vandeveer Hayden, was to describe the regional geology, outline the natural history and rescources, and produce maps. This survey published many volumes on its Colorado work, most now classics in their genre. The most noteable was the Atlas of Colorado, published in 1877 and reprinted in 1881.

This lecture and slides provide a popular and personal reflection on the division of the Survey that explored the scenic San Juan Mountains in 1874. These reflections are based on the author’s experiences during the summer of 1982 while retracing the route of the Hayden Survey in the southwestern part of the state. Emphasis of the presentation is on Colorado history, mountaineering, alpine fauna and flora, and scenic vistas. The lecture and slides compliment the author’s two books: (1) Summits to Reach, an expanded version of F. Rhode’s field notes on the Survey’s work in 1874 and (2) a biography titled Strange Genius: A Life of Ferdinand Vandeveer Hayden.


April 1995

Metal Cycling in Lakes Resulting in Closure of Open Pit Mines in Nevada

Andrew D. Nicholson, H.K. Kempton and J.M. Sobel
PTI Environmental Services, 2995 Baseline Road, Suite 202, Boulder, Colorado 80301 

Evaluation of waste rock materials and existing pit lakes in Nevada indicates that bulk input of metals and acid is a function of pyrite oxidation in pit walls. However, the biogeochemical cycling pit often will determine the final pit lake water quality. Important processes may include sorption of metals to hydrous ferric oxide (HFO) minerals, settling of HFO in the water column, and the release of metals from lake sediments during diagenesis. Studies of existing Nevada pit lakes in porphyry copper and disseminated gold mines indicate that precipitation of HFO, and subsequent sorption of metals to these minerals as they settle, effectively removes As, Pb, Cu, Cd, and Zn from the water column to levels <50 µg/L and this process can be effectively modeled using electrostatic sorption models. A perturbation study, in which an existing 200,000 L pit-bottom pond was spiked with acid leachate, indicated that HFO settles at a rate of 1.4 x 10-6 m s-1, lowering dissolved metals concentrations, due to sorption and coprecipitation of metals on HFO. Microcosm studies show that the release of arsenic from HFO-rich sediments is closely related to the type and amount of organic matter in the sediments, and the extent of sulfate reduction reactions. Base metals are generally not released from these sediments, due to the formation of insoluble sulfide minerals.

Expansive Soils, Heaving Bedrock and Modern Development: New Legislation for Respecting Geological Hazards in the West Denver Metro Area, Colorado

William (Bill) M. Brown III
U.S. Geological Survey 

The Expansive Soils/Heaving Bedrock Task Force, appointed by the Jefferson County Board of County Commissioners, has been meeting since January 1994 to suggest changes in development regulations for the County. The desires for changes are driven by unique geological conditions, primarily in the Pierre Shale where it tilts steeply against the mountain front of the western Denver metro area. In this region, building practices based on experiences with flatter-lying expansive soils have frequently proven to be ineffective, and new practices were promulgated to mitigate costly damages to new and remedial construction in a special geological hazards overlay district. The multidisciplinary task force has carefully constructed legislation that takes into account the needs of virtually all stakeholders in the overlay zone. We made extraordinary efforts to involve all stakeholders on the task force and in its deliberations_consulting engineers, builders, developers, planners, home-warranty providers, roads and utilities experts, landscape architects, geologists, homeowners’ representatives, real-estate agents, university professors, and local, state, and Federal officials_so that no interests would be neglected. This presentation examines the evolution of policy based heavily on geological considerations, and the respective roles of the U.S. Geological Survey and other participants on the task force in formulating that policy.


May 1995

The Surprising Discovery of Oil and Mesozoic Sediments in the San Luis Basin, Saguache County, Colorado

Tom Watkins, Lexam Explorations (U.S.A.), Inc., Wheat Ridge, Colorado
John Belcher, Peak Energy, Denver, Colorado
Robbie Gries, Priority Oil & Gas, Denver, Colorado
Mark Longacre, MBL, Inc., Denver, Colorado 

The discovery of live oil in mineral exploration drill holes, followed shortly thereafter by the identification of Mesozoic sediments in outcrop and the subsurface, has resulted in a new geologic interpretation for the northeastern San Luis Basin. Oil, sourced from Cretaceous rocks, is present in fractured Precambrian gneiss and Mesozoic sediments along a strike length of more than four miles. Mancos Shale, Dakota Group and Morrison Formation sediments have been observed in 17 shallow drill holes. These are the first occurrences of Mesozoic sediments discovered in the basin. Previous investigations had concluded that the entire Mesozoic section was eroded from the San Luis uplift during the Laramide.

A low-angle, detachment-style fault, related to development of the Rio Grande rift, forms the northeastern margin of the San Luis Basin. Immediately adjacent to the mountains, the detachment separates Tertiary, basin-fill sediments of the Santa Fe Formation from Precambrian basement. Southwest of the mountain front, Mesozoic sediments comprise the hanging wall of the detachment. Gravity, magnetic and seismic data place the east margin of the Baca graben three miles southwest of the mountains, creating an intermediate fault block between the Sangre de Cristo Range and the basin.

Structurally, the Grant Canyon, Bacon Flat and Blackburn fields in Nevada are excellent analogs for the northeastern San Luis Basin. A thick section of Mesozoic sediments, which represent favorable source and reservoir rocks, is interpreted to be present in the hanging wall of the detachment fault. Seismic data suggest that the sediments are cut by a series of listric normal faults which merge with the detachment.

Active Volcanoes in the Goma Region, Zaire And Their Role in the Rwandan Refugee Crisis

Thomas J. Casadevall
U.S. Geological Survey 

In July, 1994, ethnic violence and civil unrest in Rwanda prompted a flood of more than one million Rwandan refugees into eastern Zaire. These refugees settled in camps on the slopes of Nyamuragira and Nyiragongo volcanoes, both of which were in eruption at the time. Concern that eruptive activity would affect the air relief effort as well as refugees settled on the volcanoes, prompted an international response by volcanologists from the USA, Zaire, France, and Japan to evaluate the eruptive activity, to assess the volcanic hazards facing the refugees and air relief operations, and to make recommendations to United Nations officials managing the relief effort.

In this presentation, I’ll discuss the volcanological field studies which were carried out in the Goma area, and how those studies were integrated with other considerations of the refugee crisis. This talk weaves together the roles of ethnic differences, civil unrest, and the two active volcanoes to explain the current refugee crisis.


September 1995

Geology of Maya Mountains Archeological Project Sites, Belize

William E. Brooks, U.S. Geological Survey
Peter S. Dunham and Keith M. Prufer, Cleveland State University 

Geologic mapping of Cenozoic to Paleozoic plutonic, volcanic, and sedimentary rocks and comparative study of building materials at several recently discovered Mayan ruins in the Maya Mountains, southern Belize, were carried out under informal agreement between the USGS and the Maya Mountains Archeological Project (MMAP), a multidisciplinary research project funded by the National Geographic Society.

Triassic Cockscomb Granite is the source of ruin material at two sites in the Cockscomb Basin. Euhedral 2-3-cm alkali feldspar megacrysts give both the source granite and ruin material a distinctive porphyritic texture. In the Cockscomb Granite, intersection of N-S and E-W joints with exfoliation planes permits easy quarrying of 20 – 60 cm blocks. A meter-sized stream-rounded granite boulder was used as plaza decoration at one site, and stream-rounded, centimeter-sized cobbles of Paleozoic phyllite, quartzite, and vein quartz provided other construction fill. Sparse outcrops of ferricrete along drainages provided a nearby source of iron oxides for pigment. A regionally distributed, pink-tan, micritic Cretaceous limestone is the source of most building material at the Esperanza-Snake Creek ruins. Intersection of vertical joints with thick (>1 m) to thin (20-30 cm) limestone beds provided easily quarried building material from nearby outcrops. Sparse, 2-3-cm chert lenses in the limestone may have provided material for cutting tools. Paleozoic phyllite mapped nearby and in float is a likely source for millimeter- to centimeter-sized pyrite used for dental inlay or other decoration.

Ruin material at two sites near the Bladen Branch of the Monkey River was also quarried mainly from nearby outcrops of the jointed, thick- to thin-bedded Cretaceous limestone. At one site, however, limestone and stream-rounded centimeter-sized cobbles, chiefly of volcanic origin, were used in about equal proportions. The NE-trending Bladen fault extends from the Esperanza-Snake Creek area into the Bladen Branch study area and downdrops Cretaceous limestone against silicified Triassic rhyolite. On the basis of composition and proximity to Bladen Branch ruins, this rhyolite is a possible source for obsidian blades found at the ruins; however, obsidian of Triassic age is likely to have devitrified, making the Bladen rhyolite an unlikely source for obsidian at Bladen Branch ruins. It is more likely that obsidian at Bladen Branch ruins was obtained from more distant sources with Cenozoic volcanic terrane.

Pleistocene Fossils and Origin of Porcupine Cave, Central Colorado

Don Rasmussen
RMGD, 410 17th St., Denver, CO 80202 

Porcupine Cave is located in Ordovician carbonates in a remote part of South Park at an elevation of 9,500 ft (2,900 m). There is evidence that the origin of the cave is very ancient and formed prior to the Laramide Orogeny which tilted the Paleozoic rocks in the area. Porcupine Cave is the highest Pleistocene vertebrate site in North America and contains the richest and most diverse Irvingtonian (medial Pleistocene) vertebrate fauna in North America. During the Irvingtonian, Porcupine Cave had numerous entrances and all were receiving debris which was washed deep down into the cave. Some of the larger entrances were used by carnivores for dens, some of the smaller openings were used by marmots and rabbits, and all openings were used by packrats who built extensive middens and nests deep within the cave. Carnivores left the chewed bones of their prey and even their own bones in areas close to the surface; marmots, rabbits, and other small animals left their skeletons when they fell into the cave and were unable to get back out; but most of the bones and teeth, many of which were gnawed, were hauled in by the packrats. The packrats even carried in carnivore scat and raptor pellets which were loaded with small jaws, bones and teeth. In July a partial skeleton of a large camel was found indicating that at least one entrance trapped large mammals. At about 350,000 years ago, all of the entrances became completely plugged and Porcupine Cave was isolated from the outside, even from the packrats. Porcupine Cave remained sealed like a sacred tomb until the late 1800’s when it was inadvertently opened by prospectors. Pleistocene fossils were not recognized in the cave until almost a century later.


October 1995

The Dinosaur Origin of Birds

Dr. John Ostrom
Professor Emeritus, Peabody Museum-Yale University 

Abstract not available.


November 1995

Origin of Devils Tower, Wyoming

Charles S. Robinson
Mineral Systems, Inc.

Devils Tower is a conspicuous physiographic feature on the western flank of the Black Hills of Wyoming. Because of its striking appearance, it was established as the first National Monument in 1906. Its most striking feature is the columnar jointing, which has been used by most geologists to interpret the origin of the Tower.

The Tower is about 600 feet high, about 300 feet in maximum diameter at the top, and 800 feet in diameter at its base.

Devils Tower is composed of phonolite porphyry, and is one of a series of igneous intrusions of similar composition that intruded the sedimentary rocks of the western flank of the Black Hills 55 to 33 Ma. Exposed at the base of Devils Tower is the Sundance Formation of Jurassic age and in the vicinity of the Tower, are the Gypsum Spring Formation, of Middle Jurassic age, and the Spearfish Formation of Triassic age.

The origin of Devils Tower has been attributed to an igneous intrusion, or plug, that intruded the sedimentary rocks but did not reach the surface, to the remnant of a laccolith, and to the neck of an extinct volcano. The volcanic-neck origin is based primarily on a comparison of the columnar jointing of the Tower with the columnar jointing in volcanic necks exposed in the Mount Taylor volcanic field of New Mexico. The columnar jointing is the only similarity between the volcanic necks and Devils Tower. The volcanic necks are composed of basic rocks, basalt, basalt breccia, or andesite, showing considerable variation in mineralogy, texture, and structure within the same neck. They were vents for magma that repeatedly poured out to form many square miles of primarily basalt flows. In contrast, Devils Tower consists of phonolite porphyry, an intermediate to silicic rock, which, in general is homogeneous in mineralogy, texture, and structure. There is no evidence of extrusive igneous rock in the vicinity of Devils Tower, and at the time of the intrusion of Devils Tower (ca. 50 Ma), the total stratigraphic section above the present level of the top of Devils Tower was approximately 7,000 feet. The White River Formation, younger than Devils Tower, does not include pyroclastic material.

Based on the regional and local geology, and the mineralogy, texture, and structure of Devils Tower, it is concluded that Devils Tower is the remnant of an igneous intrusion that did not breach the surface. The intrusive rock reached the level of the Lower Cretaceous shale where it expanded to an inverted teardrop shape, where it cooled and the columnar joints formed. The present Tower shape is the result of removal by erosion of the intruded, relatively soft, sedimentary rocks, and the weathering and erosion of the relatively hard, columnar jointed igneous rock of the Tower.

Avalanche Science: The Basics for Awareness and Forecasting

Knox Williams
Director of the Colorado Avalanche Information Center 

Avalanche hazard forecasting is equal parts science and art. It involves a synthesis of terrain, weather, and snowpack factors. These are called the contributory factors to avalanche formation. I will focus on how a forecaster, or a back-country traveler, would gather and evaluate data to arrive at a forecast that would give a likelihood of avalanche release. Personal avalanche safety is directly related to probability of avalanche release, and the prudent back-country traveler possesses the knowledge, skills, and tools to eliminate or minimize his or her exposure to risk. This presentation will include a brief talk, followed by the viewing of a video on personal avalanche safety.


December 1995 – Presidential Address

The Midcontinent Rift System Revisited

Susan M. Landon 

My involvement in the Colorado Scientific Society was initiated by a talk on the Midcontinent Rift System presented to the Society roughly five years ago. It seems appropriate that I would return to this topic for my Presidential Address as this year draws to a close. I will not repeat the earlier talk, but will instead update the current status of this play and use it to outline the changes that have defined the domestic oil and natural gas industry within the past decade. I have, as an independent petroleum geologist, continued to pursue this frontier play that began ten years ago during my tenure with Amoco Production Company. During this time, the original geological model has been systematically refined with the addition of an extensive volume of geophysical data and several exploration wells. As a result, interest continues in the hydrocarbon potential of this play. Pursuit of the Midcontinent Rift play by an independent has been made possible by several dramatic changes in the petroleum industry. First, the strategic decision of most major companies to refocus their efforts overseas in the mid 1980’s reduced competition in the United States and created access to large volumes of data including seismic. An innovative arrangement allows for examination, reprocessing, and interpretation of many hundreds of miles of 1984 seismic data in the Rift. Second, advances in data processing techniques have resulted in seismic displays that are significantly improved over those generated at the time the records were first acquired and processed. This new processing has allowed sequence stratigraphic interpretation in contrast to the original displays that only provided a rough structural picture of the rift. Third, the revolution created by personal computers and market-driven software development allow even the smallest organization access to leading edge technology. With a PC-based software package, we are able to examine individual seismic attributes (amplitude, frequency, and phase) to assist in our interpretation of the sediment fill in the post-rift sag basin of the Rift. Affordable software also allows for low cost, sophisticated basin modeling, which provides information on timing and volumes of hydrocarbon generation. A decade ago technology like this was only available to organizations with the resources to maintain mainframe computers and proprietary software. The Midcontinent Rift System continues to be a viable frontier exploration play and is even included in the 1995 USGS Resource Assessment. Interesting geology, reduced risk and costs, access to data, and a level technological playing field make this play both geologically exciting and an economically realistic exploration target for the 90’s.