Abstracts – 1996

January 1996 – Emmons Lecture

HAWAIIAN GLACIAL AGES

Stephen C. Porter
Quaternary Research Center, University of Washington, Seattle 

The Pacific Ocean covers approximately a third of the Earth’s surface area, with small isolated mid-oceanic islands constituting only an insignificant percentage of this total region. Several of the high volcanoes of the Hawaiian Islands, located in the midst of this vast expanse of ocean, reach altitudes of more than 4000m and preserve a unique record of terrestrial environmental changes in a mid-ocean setting. For this reason, the paleoclimatic record of Hawaii is especially significant, for it constitutes a single source of terrestrial data for a large segment of the Earth’s surface. The island group lies within a belt dominated by the tradewinds, but is relatively close to the southern limit of westerly air flow; it therefore may be an important source of information regarding possible changes in mid-Pacific atmospheric circulation during successive Pleistocene glacial/interglacial cycles. Furthermore, the Hawaiian Islands possess the only record of mountain glaciation inside the limits of the deep Pacific Ocean basin.

Kohala and Mauna Kea, the northeasternmost of six volcanoes comprising the island of Hawaii, are typical Hawaiian volcanoes in which the exposed post-shield-stage rocks are divisible into two lithologic groups. In each volcano, the bulk of the older group consists of basaltic lava flows and associated cinder cones and tephra layers distributed across mountain slopes and concentrated along major rift zones. These rocks are overlain by an alkalic suite, mainly consisting of thick aa flows (largely hawaiite and mugearite) and associated large tephra cones and pyroclastic layers. Sediments interstratified with lavas on the upper slopes of Mauna Kea provide evidence of repeated glaciations during the middle and late Pleistocene. This alpine glacial record is one of the best dated of the last 200,000 years. Three glacial drifts are exposed on the surface and in deep gulches, each overlain and underlain by lava flows. Associated with the middle and upper drifts is unmistakable evidence of subglacial eruptions, including hyaloclastites, lava flows marked by steep and embayed ice-contact margins, pillows, spiracles, and chilled glassy flow surfaces.

 


February 1996

Restoration of Large Rivers: The Case for the Colorado River in Grand Canyon

G.R. Marzolf
(U.S.G.S.,3215 Marine Street, Boulder, CO 80303; email: rmarzolf@usgs.gov)

Glen Canyon Dam, completed in 1963, is the most important component of the Colorado River Storage Project and one of the principal mechanisms for regulating flow under the Colorado River Compact of 1922. However, the presence and operation of the dam, has changed the downstream riverine environment in Grand Canyon. Lake Powell, impounded by the dam, retains all of the 60 x 10E6 Mg of annual sediment load that previously moved through the canyon. The dam has reduced the 2-yr flood magnitude by greater than 60 percent. The water temperature, that previously fluctuated 0-27°C seasonally, is now a nearly constant 9°C. The presence and (or) operation of the dam has led to aggradation of many of the debris fans that control rapids, depletion of sand stored in alluvial bars, establishment of thickets of native and non-native riparian vegetation along the previously barren channel banks, and, with introductions and invasion by non-native organisms, the altered river has a new biotic assemblage. The native humpback chub (Gila cypha) is an endangered species unable to reproduce at the low temperature of most of the river, but trout now thrive in the cold, clear water. Another endangered species, the southwestern willow flycatcher, nests in non-native tamarisk trees in post-dam riparian thickets, and a common habitat for subadult chub is associated with the densely vegetated shorelines.

Several laws affect management of the Colorado River through Grand Canyon, including: the Endangered Species Act, the Grand Canyon Protection Act, and the organic act that established Grand Canyon National Park in 1919. Alteration of dam operations may be justified under any or all of these acts if it would enable restoration of the riverine environment of Grand Canyon, but changes may be costly because the dam’s power plant generates about 75% of the hydropower in the upper Colorado River basin. The pattern of daily releases through Glen Canyon Dam could be modified from that of a load-following power plant to that of a base-load facility, but a change in discharge within power plant capacity alone cannot restore pre- dam conditions in Grand Canyon. Occasional releases significantly higher than power plant capacity could be used to erode debris fans, rebuild many sand bars in recirculation zones and along channel margins, reestablish competitive conditions for endangered native fish, and reduce near-shore riparian vegetation; however, the habitat of the endangered flycatcher may be at risk under this scenario.

Selective withdrawal from the lake could be used to control water temperatures in the Colorado River to enable reproduction of humpback chub, but this action would be accompanied by more favorable conditions for non-native fish that compete with chub. Sediment might be added to the river, which would permit rebuilding sand bars and provide cover (increased turbidity) for native fish species, but the advantage to the trout will be compromised by this action.

Clearly, restoration of large rivers involves many choices with few clear-cut options, and full restoration is not feasible. The river must be managed for a most-valued primary resource in a manner to reduce negative effects on other resources.

 


March 1996

Risk in the Rockies: The Mt. Zirkel Wilderness Area Story

John Turk
(U.S.G.S., Denver Federal Center, Denver, CO) 

John Turk will discuss why the most polluted snow in the Rocky Mountains, Sierra Nevada, and Cascade Mountains occurs in and just south of the Mt. Zirkel Wilderness Area, Colorado. Lakes in the Wilderness are among the most sensitive to acid effects in the Rocky Mountains. Although larger lakes have not become acidic, episodic damage to breeding pools and headwater streams may occur during early snowmelt.

The Tertiary Dinosaurs of the San Juan Basin?

Jim Fassett
(U.S.G.S., Denver Federal Center, Denver, CO) 

(No abstract)

 


April 1996

Sequence Stratigraphy: So What’s the Big Deal?

Jeffrey A. May, Ph.D.
(Geological & Geophysical Consultant, Littleton, CO) 

Sequence stratigraphy is the study of genetically related strata within a chronostratigraphic framework. This approach emphasizes (1) recognition of key surfaces of erosion and/or sediment starvation, (2) description of sediment packages relative to their time-stratigraphic associations, and (3) genetic interpretation of sedimentary cycles. Sequence stratigraphy provides a guide for observation and prediction.

On The Edge of a Rift–Late Tertiary Evolution of the Southern Sangre de Cristo Mountains, Colorado

Alan R. Wallace
(U.S. Geological Survey, Denver, Colorado) 

The Rio Grande rift in southern Colorado is composed of two principal elements: the rift-axis San Luis basin and the flanking horst of the Sangre de Cristo Mountain. The San Luis basin asymmetrically deepens to the east, with major depocenters at the Great Sand Dunes and south of Fort Garland. South of Blanca Peak, the rift margin is displaced 25 km to the east to form the Culebra reentrant along the western flank of the Culebra Range, the southern segment of the Sangre de Cristos. The development of the rift along the western margin of the Sangre de Cristo Mountains and Culebra Range involved orogenic sedimentation and high-angle normal faulting and block rotation. Prior to rifting, the rift in southern Colorado was covered by a partially eroded, intermediate-composition, 28-30 Ma volcanic field. Early rifting began about 26 Ma with subsidence of the broad San Luis basin which was then filled with sediments of the late Oligocene and Miocene Santa Fe Group. The main source for the sediments was to the east. The basal rift sediments contain abundant volcanic clasts derived from the Spanish Peaks and La Veta Pass areas, east of the modern range crest. Continued downcutting into the rising source highland produced increasingly Paleozoic- and Proterozoic-rich sediments. Basalt flows of the Hinsdale Basalt (~18 Ma) and Servilleta Formation (~4 Ma) locally were erupted locally during sedimentation. Hydrothermal activity at about 24 Ma produced the large San Luis gold deposit, indicating mineralizing activity during early rift development; fluorspar deposits elsewhere along the Rio Grande rift formed during very late rifting.

High-angle normal faulting began in mid- to late-Miocene time and produced a complex mosaic of mutually offsetting faults. Limited evidence suggests that fault activity migrated westward into the developing San Luis basin, and that faulting of the Santa Fe Group in the main depocenter began only at the very end of sedimentation, causing major block rotation of the Santa Fe and all older units. The Culebra Range is an east-tilted homocline, and rift-related faults on the western flank of the range extend to nearly the modern range crest. In contrast, the central part of the Sangre de Cristos, including the Blanca Peak massif, is bounded on the east, west, and south sides by high-angle normal faults. East of Blanca Peak, faults related to uplift of Blanca Peak truncate faults related to uplift of the Culebra Range, indicating that the northern horst formed somewhat later than the southern block. Significant tilting of 4 Ma basalt flows indicate uplift-related deformation into the Pliocene, and offset of Pinedale-age glacial outwash deposits indicates recent uplift. The overall tectonic configuration suggests that the San Luis Valley basin is composed of two east-facing half-grabens rather than a single graben as previously believed. The older half-graben, centered on the Fort Garland depocenter, is west of the Culebra Range and south of Blanca Peak. The younger half-graben, centered on the Great Sand Dunes depocenter, is west of the Sangre de Cristo Mountains north from Blanca Peak.

 


May 1996

GIS-based Geoenvironmental Assessments of Colorado Watersheds Affected by Metal-mining Activity

Geoffrey S. Plumlee1, Kathleen S. Smith1, William R. Miller1, Alan R. Wallace2, Randall K. Streufert3, Dana J. Bove2, Gregory K. Lee1, and Steven M. Smith1
1U.S. Geological Survey, ms 973, Federal Center, Denver, CO 80225
2U.S. Geological Survey, ms 905, Federal Center, Denver, CO 80225
3Colorado Geological Survey, 1313 Sherman St., Denver, CO, 80203

The environmental effects of metal-mining activity are receiving ever increasing attention from the public, regulators, land managers, and the mining industry. There are several issues currently of primary interest: (1) assessment and remediation of environmental problems at abandoned metal-mine sites; (2) prediction and mitigation (or prevention) of potential environmental effects from future mining activity; (3) determination of baseline conditions that existed in mineralized areas prior to mining (conditions which in many mineralized areas were naturally degraded) and; (4) development of effective, scientifically-realistic regulations to govern past, present, and future mining activity. This talk will summarize a GIS-based approach using geologic and geochemical data layers to help estimate potential environmental impacts of metal-mining activities on surrounding watersheds. The approach is based on the fact that mineral-deposit geology as well as geochemical processes exert fundamental and predictable controls on the environmental conditions in mineralized areas prior to mining, and environmental conditions that result from mining and mineral processing. Similarly, the geologic and geochemical characteristics of the rock units in the watersheds surrounding mining districts and mineralized areas can strongly influence the spatial extent and magnitude of environmental impacts on the surrounding watersheds.

Results of prototype work in the state of Colorado will be presented, with a focus on environmental effects on surface-water quality; however, the techniques are generally applicable, with appropriate changes in complexity of scale, for areas ranging in size from a single watershed to a large state and for other environmental effects such as smelter emissions and windblown solid contaminants. A second prototype for the state of Montana is currently under development.

A Critical Review of Determining Paleoelevations from Fossil Floras

Dr. Emmett Evanoff
University of Colorado Museum, Boulder 

Recent paleoclimatic interpretations of the late Eocene Florissant flora have concluded that temperatures were cool in the uplands of Colorado and that elevations were as high or higher than at present. These interpretations have important implications concerning the Cenozoic uplift history of the Southern Rockies. Are the current elevations a result of Laramide or late Cenozoic uplift? This talk will review the methods and assumptions behind the paleobotanical method of determining paleoelevations. Are the assumptions valid and can the paleoelevations be accepted? Does the method work for other ancient floras?

 


September 1996

Recent Earthquake-Induced Catastrophic Landslides in the Andes of Ecuador and Colombia

Robert L. Schuster
Scientist Emeritus, U.S. Geological Survey, Denver, CO 

On March 5, 1987, two strong earthquakes (M=6.1 and 6.9) struck the eastern slopes of the Andes Mountains near Reventador Volcano in northeastern Ecuador. On June 6, 1994, a M=6.4 quake shook the Paez River valley and surrounding mountains near Nevado de Huila (Colombia’s highest mountain/volcano) in the Andes of southwestern Colombia. In both cases, the seismic events occurred after several weeks of heavy rainfall, which had saturated the residual soil cover. On steep slopes the seismic shaking resulted in hundreds of this slides in these soils. The sliding masses almost instantaneously liquified, turning into debris flows in the gullies and tributary streams, from which they flowed into the main rivers as large, destructive debris flows/floods. In Ecuador, and estimated 1,000 people were killed by these flows/floods; economic losses due to all types of landslides were estimated at US$1 billion. In Colombia, there were 271 reported deaths with about 1,700 people missing; more than 32,000 people were evacuated from the area.

Rediscovering Lost Worlds: Climate and Ecosystem Changes in Alaska and Yukon Territory During the Past 20 Million Years

Dr. Thomas Ager
U.S. Geological Survey, Denver, CO 

An interdisciplinary research project focused upon reconstructing past climates and ecosystems in Alaska and Yukon Territory began in 1990, and is currently finalizing results. The research team includes scientists from U.S. Geological Survey, Geological Survey of Canada, and several academic institutions in the U.S. and Canada. Field seasons in 1990, 1991, 1992, and 1993 involved team investigations of natural exposures of Neogene and Quaternary deposits in Alaska and Yukon Territory. In 1994 the team conducted a drilling project at Fort Yukon, located a few kilometers north of the Arctic Circle in northeastern interior Alaska. The borehole reached a depth of 390 meters and resulted in the recovery of a sediment core that preserves much of the history of climate change and ecosystem development in interior Alaska during the past 16 million years. The results of the investigations demonstrate that past global warming events had profound influence on northern ecosystems.

A major global warming 18 to 15 million years ago (during the Miocene Epoch) resulted in a dramatic northern movement of temperate deciduous forests north of the Arctic Circle in Alaska and northern Canada. Whereas today the forests of interior Alaska consist of a few hardy boreal trees and shrubs such as spruce, birch, poplar, alder, and willow, the forests that developed during the middle Miocene global warming were rather similar to forests now growing in northern Virginia, Pennsylvania, southern New York State, and adjacent areas. Alaskan forests included tree types such as hickory, ash, oak, maple, holly, chestnut, elm, sweetgum, basswood, walnut, wingnut, redwood, dawn redwood, and many others. Global cooling after 15 million years ago eliminated most of the temperate deciduous tree types, replacing them with a relatively rich assemblage of conifers (e.g., hemlock, fir, Douglas fir, dawn redwood, spruce, pine, larch) as well as birch, alder, and willow. During the past 3 million years, major cooling events eliminated many of the conifers , leaving only hardy taxa such a spruce and larch.

 


October 1996 – Student Night

Seismic Stratigraphy and Seafloor Morphology of the Kangerlussuaq Region, East Greenland: Evidence for Glaciations to the Continental Shelf Break During the Late Weichselian Age and Earlier

Andrew B. Stein
University of Colorado at Boulder – INSTAAR 

Seismic stratigraphic analysis of the Kangerlussuaq Trough on the East Greenland continental margin reveals a regional unconformity which separates pre-unconformity bedrock, marine onlap, and fluviodeltaic seismo-stratigraphic units from post-unconformity glacial and interglacial seismo-stratigraphic units. Cruises conducted by WHOI, IML, and BIO in 1988, 1991, and 1993 collected air gun, sleeve gun, and high resolution Huntec DTS seismic profiles, sediment samples, side-scan sonograms, and bottom photographs from the continental margin. Seismic data provide a qualitative estimate of the extent and timing of seven glaciations of the continental shelf. Radiocarbon dates from gravity cores, piston cores, and grab samples suggest that the last glaciation of the shelf lasted from approximately 28,000 to 15,000 yrs BP and that the ice margin had retreated to the middle continental shelf by ca. 15,000 yrs BP. These dates combined with the seismic stratigraphy and the seafloor survey indicate an ice margin reached the continental shelf break during the Flakkerhuk stade, the Last Glacial Maximum (LGM).

Above the regional unconformity, glacial sequences on the continental shelf and slope contain depositional features from ice margins extending to the shelf break several times. The Late Weichselian seismo-stratigraphic units on the continental shelf contain mounded, chaotic reflections of glacial ice-contact sediments 1 to 94 m thick capped by parallel, continuous reflections of deglacial ice-proximal, ice-distal, and post-glacial sediments 1 to 24 m thick. Ice-contact sediments on the continental slope are capped by thin deposits of winnowed and compressed deglacial sediments 1 to 5 m thick. In the inner Kangerlussuaq Trough, the Flakkerhuk glacial and deglacial sediments are relatively undisturbed. Push moraines and end moraines display evidence of a stillstand after the LGM. Side-scan sonograms of the shelf display extensive iceberg furrows generated by icebergs calved directly into the Kangerlussuaq Trough by the retreating ice margin. Increased furrow density on western bathymetric barriers indicates the southwesterly flowing East Greenland Current transported icebergs down the Denmark Strait and across the shelf. On the continental slope, seismic data show that winnowed post-glacial sediments form boundaries between five debris flow seismo-stratigraphic units generated by the shedding of sediments from shelf break ice margins. On the continental rise, debris flows thin considerably and sediments are reworked into contourite deposits.

Geochemical and Petrographic Evidence for Fluid Flow and Dissolution in Some Low-Angle Normal Faults, Utah and Nevada

Sharon F. Diehl
Colorado School of Mines 

Geochemical and petrographic studies of fault gouge, breccia, and veins from four low-angle normal faults (LANFs or detachment faults) show that fluid flow and dissolution are closely associated with fault movement. The LANFs studied are: (1) the Castle Cliff fault, southwestern Washington Co., UT; (2) the Tule Springs and (3) Petroglyph faults in the Mormon Mountains, southern Lincoln Co., NV; and (4) a newly discovered fault near Salina, Sevier Co., UT. Each fault has abundant evidence of brittle fracture and brecciation that could have resulted from seismogenic failure.

The Castle Cliff, Tule Springs, and Petroglyph LANFs are characterized by attenuated carbonate stratigraphic sections. The fault zones are typically composed of layers of solution boudins, bounded by calcite-cemented breccia. In both the upper and lower plates, pressure solution seams and the surrounding characteristically altered grayish pink host rock have significant negative shifts in d18O values (to -14.50‰ PDB). We interpret the depleted d18O values in zones of solution boudins as evidence of the passage of hydrothermal fluids, with accompanying dissolution of the host rock. Oxygen isotopes are less depleted (a 5-7‰ shift) at distances less than a meter from the solution boudins. At scales ranging from <1 mm to tens of meters, steeply dipping normal faults in upper-plate strata terminate at or merge with dissolution surfaces. These dissolution surfaces probably record an open fluid-flow system that operated along the LANF and accommodated upper-plate faulting and brecciation. If so, the prospect that these LANFs and their upper-plate structures produced large earthquakes is diminished.

At Salina, the LANF separates folded and faulted lower-plate Jurassic Arapien Shale from faulted and tilted upper-plate Tertiary sedimentary and volcanic rocks. The LANF is marked by a conspicuous red alteration zone 1-30 m thick in lower-plate rocks. As much as 2 km of stratigraphic section is missing at the LANF. Brecciation and shear fabrics are poorly developed and are restricted to a zone <2 m thick. This LANF, with its unspectacular minor structures and fabrics, has many attributes of detachment faults, but its direct proximity to the western margin of the Colorado Plateau province makes large-magnitude simple shear an unviable process to explain the missing strata. Although dissolution has occurred at this LANF, it does not appear to be the dominant process.

Redesignation of the Upper Lithofacies in the Hensel Formation on the Southwestern Flank of the Llano Uplift, Texas: A New Interpretation of Hensel Geology and Depositional Environments

Aaron J. Kullman
Colorado School of Mines 

The Hensel Formation is the terrigenous component of the last limestone-clastic couplet in the upper Trinity Group. Most previous studies involving the Hensel have been in depocenters to the southeast of the Llano Uplift. Analysis of the Hensel to the southwest of the uplift has been cursory. Exposures of the Hensel Formation along the upper Llano River drainage basin around Junction, Texas, exhibit three distinct lithofacies. A basal conglomerate consists of clast supported, high energy fluvial channel lag lenses exhibiting poor sorting and crude lateral accretionary crossbedding. The middle facies is extensively calcretized alluvial sandstones and mudstones with paleosol horizons and well developed rhizoconcretion zones. An upper marine facies is composed of calcareous siltstone intercalated with thin limestone beds. Both siltstone and limestone beds are fossiliferous. Provenance of clasts in the lower and middle facies, and the siltstone of the upper facies are derived from Pre-Mesozoic limestone and crystalline rocks from the Llano Uplift area. The Hensel was deposited as a series of coalescing fan deltas that propagated from the positive Llano Uplift.

Traditional interpretation of the depositional setting of the upper facies siltstones and mudstones assume the Llano Uplift to be the sediment source and that they are a continuation of terrestrial fan deposition. However, detailed analysis of fossil ostracodes and foraminifers in the facies indicate that they are Cretaceous age and marine in origin and lithofacies analysis supports making the facies a thin westward extension of the down dip laterally equivalent Glen Rose Limestone.

 


November 1996 – Family Night

An Evening in Our Incredible Universe

Dr. Joe Romig
University of Colorado

Dr. Romig will give a non-technical talk on the earth, moon, and planetary system. Dr. Romig is a member of the Voyager science team and is involved with research on the outer planets. Slides from NASA’s manned and unmanned missions together with slides from Earth-based and near-Earth telescopes will be used to give the audience a “Grand Tour of the Solar System”. The origin and evolution of the solar system will be described together with the birth and death of other stars and stellar systems. Black holes, neutron stars, and white dwarf suns will be touched on. Finally, the origin and evolution of the universe itself will be described. No “final answers” are advanced for the origin of the universe, rather members of the audience are left to draw their own conclusions.

 


December 1996 – Presidential Address

Black Blizzards and the Next Great Drought: Historic and Prehistoric Records of Climatic Change on the Great Plains

Richard F. Madole
U.S. Geological Survey 

During historic time, inhabitants of the Great Plains have experienced a succession of climatically driven cycles of economic boom and bust as intervals of above average annual precipitation alternated with intervals of drought. Geological data and tree-ring studies show that a similar pattern of climatic oscillation has prevailed for many centuries, if not many millennia. Thus, there is reason to believe that this pattern will continue and that droughts lasting as long as 3-10 years or more will recur on the Great Plains in the 21st Century. The instrumental record, which in most of the region spans 100-150 years, combined with estimates of annual precipitation based on tree-ring studies indicate that severe droughts have occurred here 3-5 times per century for much of the past millennium. For at least the past few centuries, droughts equal to or greater in severity than that of the 1930s have occurred about twice per century. Wind erosion in the western Great Plains is more extensive and severe than anywhere else on the continent. Massive wind erosion of soil during the “dirty thirties”, the 1930s drought, gave rise to the term Dust Bowl for a region that encompasses more than 100 million acres in eastern Colorado, western Kansas, easternmost New Mexico, and the panhandles of Texas and Oklahoma. As might be expected, deposits of windblown sediment are as widespread in the Dust Bowl as wind-eroded landscapes. For example, 60% of eastern Colorado is mantled by windblown sediment, of which about 30% is sand and 70% is loess. Several different ages of windblown deposits are present. The deposits span more than 10,000 years and show that episodic mobilization of sediment by wind is not a recent phenomenon in this region. However, the most recent episode of regional mobilization of sand and the formation of dune fields occurred within the present millennium, sometime after 1150 A.D. (800 yr BP) but before establishment of permanent settlements in the mid-19th Century. It should be noted that none of the historic droughts were severe or prolonged enough to cause widespread formation of dune fields, which is to say that droughts more severe than any in the instrumental record are possible under the present climate.