January 2004 – Emmons Lecture
“RECONNECTING LAKE ARCTICA TO THE ATLANTIC: GEOLOGICAL CAUSE OF PALEOCENE-EOCENE BOUNDARY EVENTS”
Malcolm C. McKenna
University of Colorado Museum and American Museum of Natural History
When two oceans become separated from each other such that circulation is in one direction only, their sea levels and salinities will become different. Such was the case in the late Paleocene, when increased proto-Icelandic lava outpourings choked the Atlantic marine continuity with the Norwegian/Greenland seas and beyond, and allowed terrestrial interchange directly between Europe and North America. Beringia was simultaneously dry land, blocking Pacific-Arctic circulation.
An isolated Arctic Ocean (about 1% of total oceanic volume) that was fed by rivers of the order of magnitude of those draining into the present Arctic Ocean would soon have exceeded evaporation, freshened, filled the Arctic Basin to its brim, and then would have overflowed southward. Oceanic heat transport to the Arctic Basin would have ceased. At first, overflow would have been through the long Turgai Straits to Tethys in Eurasia.
However, acceleration of sea-floor spreading in the Greenland-Scotland bridge area beginning in geomagnetic chron 24R resulted in a breach in the bridge, allowing rapid release southward of brackish and fresh water. This water would have spread out on top of and into the Atlantic Ocean, severely affecting the marine photic zone and its inhabitants before mixing.
With reconnection and resumption of circulation, oceanic heat transport northward would have resumed, helping to warm the Arctic Basin and its waters and isolating the European terrestrial biota from areas to the west. These changes, in combination with slightly lowered Arctic Basin water level, would have led to extensive methane releases, contributing to what is now thought to have been the root cause of an accelerated episode of global warming that began at 55.5 Ma.
“EVOLUTION OF THE GREAT SAND DUNES, SOUTH-CENTRAL COLORADO.”
U.S. Geological Survey, Denver, CO
Geomorphic and stratigraphic evidence obtained in recent field studies indicate that the sand in the Great Sand Dunes came primarily from a lacustrine source rather than from floodplain deposits of the Rio Grande, as previously supposed. The source of the sand in the Great Sand Dunes has been pondered for more than a century, but since the 1960s, the idea that the flood plain of the Rio Grande was the source has been widely accepted. Closely linked to the issue of source are questions about when and under what conditions the sand was transported. Although most recent publications do not assign dates to the time of dune formation, a few infer that the process began about 12,000 years ago and link it to increased discharge and sedimentation during deglaciation of the San Juan Mountains.
The Great Sand Dunes are a small part (<10%) of an area of windblown sand that blankets the east side of the San Luis Valley for a distance of about 100 km. Stratigraphic evidence and numerical ages show that this area of windblown sand is the product of multiple episodes of eolian transport that occurred intermittently over a time span that probably includes much, if not all, of the Pleistocene.
Accumulation of eolian sand was controlled primarily by climatically driven fluctuations of water-table level. During megadroughts, water table fell and exposed areas of sandy sediment in playas to wind erosion. These areas became primarily by climatically driven fluctuations of water-table level. During megadroughts, water table fell and exposed areas of sandy sediment in playas to wind erosion. These areas became primary sources of new generations of dune and sheet sand. At the same time, drought reduced vegetation on deposits of older eolian sand allowing wind to remobilize parts of them. During wetter times, water table rose and shallow lakes formed, and the sand supply on the basin floor was replenished by inflow from streams that originated in the surrounding mountains and on piedmont slopes.
The distribution of the eolian sand of which the Great Sand Dunes are a part suggests a relationship with the closed basin part of the San Luis Valley rather than with the Rio Grande. This body of eolian sand is nearly coincident with the length of the closed basin, and the parabolic dunes north of the Great Sand Dunes indicate a sand source and transport direction unrelated to the Rio Grande. Also, leeside dune belts, which typically flank riverine sand sources, are conspicuously absent along the leeside of the Rio Grande. Finally, the timing suggested in previous publications for the onset of sand transport and dune formation seems improbable. Archeological, paleontological, and stratigraphic data indicate that at the end of the Pleistocene, lakes, marshes, and vegetation occupied much more of the area between the Rio Grande and the Great Sand Dunes than at present. These conditions would have been unfavorable for widespread eolian erosion and transport of sand.
“DEBEQUE CANYON LANDSLIDE.”
Jonathan L. White,
Colorado Geological Survey
An active landslide has historically impinged upon the Colorado River and transportation corridors on the floor of DeBeque Canyon, located in Mesa County, 21 miles east of Grand Junction, Colorado. Interstate 70 passes through the toe of the landslide on the south wall of the canyon.
DeBeque Canyon was formed by the incision of the Colorado River into gently dipping, Cretaceous Mesa Verde Group strata on the west flank of the Piceance Basin. Exposed on the canyon walls are cliffs of sandstone interbedded with shales and siltstone. The complex landslide extends from the river’s bank to the canyon rim where large and impressive fissures occur.
The large fissures, structural offsets, and tilted blocks indicate landslide morphology characterized by low-angle extensional movements of sandstone strata at the canyon rim and deep-seated deformations within a thick shale stratum below. Below the cliffs of these upper blocks, the main landslide morphology changes to translational down-slope movements of landslide rubble, and at the lower slope to deep-seated rotational failures that periodically impact the highway.
Major reactivation of the slide occurred in April 1998 when the toe of the rotational landslide heaved upward 14 feet and almost closed Interstate 70. An earlier event occurred in February 1958 when the toe heaved 24 feet. Around 1900 much larger movements of the landslide caused the slide toe to enter the Colorado River and alter the river’s course. Portions of the railroad alignment and the work camp of Tunnel on the opposite riverbank were subsequently washed out. This presentation will discuss the results of an investigation of the landslide that was done after the 1998 emergency, specifically the geology, geomorphology, the on-going monitoring work, and the water diversion mitigation project by Colorado Department of Transportation.
“USING ALLUVIAL PALEOSOLS TO INTERPRET CLIMATE CHANGE AT THE PALEOCENE-EOCENE THERMAL MAXIMUM (PETM)”
Mary J. Kraus
University of Colorado at Boulder
The Paleocene-Eocene thermal maximum (PETM) was a short-lived (~100,000 years) spike in global temperature that occurred approximately 55 m.y. ago. The warming event is identified by distinct excursions in oxygen and carbon isotope records as well as floral and faunal extinctions and migrations. This global warming has been attributed to the release of methane hydrates stored in oceanic sediments, which injected methane into the atmosphere.
Although there is broad agreement that temperatures rose during the PETM, the effects of global warming on precipitation and evaporation patterns are less well understood and more contradictory. Data from continental PETM records are important for establishing a better picture of local and regional climatic conditions during the PETM and for thus testing and refining climate models. The analysis of paleosols provides one approach to clarifying precipitation and evaporation patterns during this interval.
Paleosols were examined in the Bighorn Basin, Wyoming, one of the few continental areas where the PETM interval has been convincingly documented based upon isotopic analyses of pedogenic carbonate nodules. Paleosols within the PETM interval are dominated by red soil colors and carbonate nodules compared to slightly older paleosols. These features indicate better drained conditions and suggest that climates became drier during the PETM in the Bighorn Basin and possibly in the region. Paleosols in the upper part of the PETM interval show purple paleosols colors and less abundant to absent carbonate nodules, showing a return to more humid climatic conditions. Other features within the PETM interval (absence of organicrich shales with plant fossils, cut-and-fills with paleosols) are consistent with drier conditions during the PETM.
“DINOVULSION: A NEW STYLE OF AVULSION IN THE UPPER JURASSIC MORRISON FORMATION”
Gus Gustason and Larry Jones
EnCana Energy Resources, Inc.
The low net-to-gross Salt Wash Member of the Morrison Formation, east-central Utah, contains exceptionally well-exposed, low-sinuosity, ribbonshaped sand bodies of two distinct sizes enclosed by variegated mudstones and siltstones. These sand bodies were formed during the Jurassic by avulsion, the relatively abrupt shift of a river to a new channel.
paleontology, and paleoclimate suggest that river channels, and subsequent ribbon sand bodies, were located by a heretofore unrecognized style of avulsion, termed dinovulsion.
Dinosaurs, well-documented inhabitants of the Salt Wash alluvial plain, trampled channel-like trails into the floodplain, creating conduits that served to focus overbank flow during flooding. During major floods, or over a period of more numerous but less intense flooding, flow coalesced to scour a new primary channel while sand clogged the pre-existing channel and nearby small dinosaur pathways. As time passed and the system aggraded, this process repeated, and the present architecture of isolated, very low-sinuosity sand ribbons of two distinct size populations resulted.
April 2004 – Annual Family Night
“SEVEN SUMMITS OF THE SOLAR SYSTEM”
Joe Romig and Glen Porzak
University of Colorado, Boulder
Olympus Mons on Mars is the solar system’s highest mountain, rising 15 miles above the surrounding plain. What would it take to climb such a peak and the highest summits of six other planets? Find out by attending the 2004 Family night presentation by Joe Romig and Glen Porzak. Joe Romig is an astrophysicist associated with the University of Colorado who has been a member of the Voyager space probe science team. Glen Porzak is a climber who has scaled the highest peaks of all the continents on Earth. Romig will provide a guided tour of the solar system and will serve as Porzak’s “consultant” on certain lofty summits in the solar system. Porzak will then describe how these might be scaled by climbers in the 21st century, relying on his own ascent of Mount Everest. The talk will be illustrated with slides and videos in the spectacular Fiske Planetarium at the University of Colorado in Boulder. Be sure to come early to enjoy the planetarium exhibits in the lobby.
“CHANGES IN LOESS AND DUNE DEPOSITION IN RESPONSE TO QUATERNARY CLIMATIC FLUCTUATIONS, COLUMBIA PLATEAU, PACIFIC NORTHWEST”
Colorado State University, Ft. Collins, Colorado
The paired dune-loess eolian system of the Colorado Plateau in Washington state allows the study of dynamic interactions of dune and loess systems. Eolian facies of the region lie in the arid to semi-arid rain shadow of the Cascade volcanic range, and prevailing winds that transport sediments move southwest to the northeast. Eolian sediments have been obtained since at least the late Quaternary and perhaps much earlier from fine-grained slackwater deposits (produced by glacial outburst flooding) exposed in upwind basins. Loess deposition appears to span much of the Quaternary.
Eolian dunes and other sandy eolian deposits lie on the upwind perimeter of the Palouse loess. Three mechanisms appear to control the thickness of loess on the plateau. 1) Topographic traps: Deeply incised valleys effectively separate saltation from suspension processes by sequestering sand that allows the deposition of the suspension load as thick loess on the downwind sides of valleys.
2) Shifts in bioclimate: In the absence of topographic traps, the sand-silt boundary freely transgresses and regresses as a function of climate shifts that control soil moisture and vegetation cover density. Over time, the eolian sand has become interstratified at the margins of deflating basins in response to these climate shifts. The mid- Holocene was dominated by dune activity (lesser vegetative cover and drier surface soils leading to more aggressive saltation processes); the present is dominated by loess deposition. Greater vegetative cover and/or moister soils shift saltation processes to a more arid upwind position, with suspension fall of dust accumulating loess on the flat.
3) Source sediment texture: Source sediment texture controls the balance of dunes versus loess accumulated downwind of specific basins. An “ideal” source sediment is dominated by sand, which limits aggregation and crusting and provides abundant, mobile sand for saltation, while also having significant (20–40%) silt and clay to provide a source of fine dust that is ejected during saltation and forms loess downwind by suspension fall. A source sediment rich in sand but poor in silt results in thin loess.
These three sets of controls appear to have operated separately and in combination to create measured variations in loess thickness. Insight into how saltation and suspension processes interact with each other to control sedimentology and geomorphology of this paired eolian system is key to better understanding the eolian environment of the Columbia Plateau and other eolian systems.
“MARS ROVER RESULTS, LIQUID WATER, AND THE POTENTIAL FOR LIFE ON MARS”
University of Colorado, Boulder, Colorado
The Mars Opportunity rover has returned spectacular results suggesting that there was a substantial body of water at the Meridiani landing site. I’ll discuss these results in the context of the history of liquid water at the surface and in the subsurface, and then I’ll turn to the implications for climate and for the potential that life might have existed or might still exist on Mars. I’ll also discuss the Mars exploration program as it is planned out over the next decade and the implications of President Bush’s recently announced new vision for exploration by NASA.
“A FAULT BOUNDED DEPOSITIONAL BASIN IN CENTRAL COLORADO”
Dr. Paul Myrow
Colorado College, Colorado Springs, Colorado
The Minturn Formation of central Colorado records deposition in an active fault-bounded basin. These strata represent braided rivers, fan deltas, marginal marine settings, and carbonate and siliclastic shallow marine environments. A prominent unit of subaqueously deposited interbedded sandstone and shale has turbidite-like graded beds that contain sole marks such as grooves, prods, and flutes. However, these beds are atypical compared to classic Bouma turbidite sequences. Detailed process-oriented sedimentological analysis reveals internal sedimentary structures that are consistent with deposition under the influence of both excess weight forces and oscillatory flow. Sedimentary structures characteristic of waves include small- and large-scale hummocky cross-stratification and gutter casts. There is also considerable evidence for deposition under combined flows, including ripples with rounded crests and convex-up foresets. Individual beds thus have characteristics of both turbidites and tempestites and were therefore deposited in combned flows of waves and currents driven by excess-weight forces. Abundant plant remains and deep sole marks indicate that the flows were highly charged with plant debris. The paleogeographic context of high topographic relief adjacent to a marine basin suggests that the flows were linked to sediment-charged flood currents that entered the ocean and became hyperpycnal flows (i.e., oceanic floods). These beds are unusual in that they also contain sequences of internal sedimentary structures that record both waning and waxing flow. Such flow is also preserved by reverse-to-normal grading patterns. These patterns may be smooth transitions from fine to coarse to fine, or show a jump in grain size within the bed at the coarsest division. The pattern of waxing and waning flow is interpreted as a record of the hydrographic response to storm events, namely increasing and decreasing discharge. The hyperpycnal flow was dynamically linked to the hydrograph and those beds with reverse-to-normal grading record all stages of the flow, including the waxing stage that in most density-driven flows is not preserved.
“FROM BUTTES TO BOWLS: REPEATED INVERSIONS IN THE LANDSCAPE OF THE COLORADO PIEDMONT”
Vincent Matthews(1), *Matthew L. Morgan(1), Jon P. Thorson(2), Francisco Gutierrez(3) and Matthew T. Grizzell(4)
1 Colorado Geological Survey, Denver, CO, USA
2 Consulting Geologist, Parker, CO, USA
3 University of Zaragoza, Zaragoza, SPAIN
4 BEK/Terranext, Lakewood, CO, USA
Mesas and buttes of the central Colorado Piedmont are composed of at least two distinct rock types, which differ in their cohesiveness and ability to withstand erosion. The lower parts are friable, Early to Middle Paleogene sandstones of the Dawson Formation. The caprock is composed of one or more resistant formations: Castle Rock Conglomerate, Wall Mountain Tuff, and Larkspur Conglomerate — all of late Paleogene age. The three resistant units were originally deposited in topographic lows. The lower slopes of the buttes are armored with colluvium composed of fragments of the capping units and commonly form “talus flatirons” or relict faceted slopes. Once the caprock of a butte or mesa has been removed by erosion, the poorly consolidated Dawson Formation quickly erodes out of the center. This leaves the armored, lower slopes of the former butte as an erosionally-resistant, circular ridge standing as much as 100 meters above the surrounding topography. This process produces a topographic low where the peak of the butte once stood. Some buttes have prominent alluvial fans that record the main phase of butte removal and excavation of the central part of the armored slopes. Soil profiles and height above modern streams suggest the oldest preserved gravel deposit is of middle Pleistocene age; the youngest alluvial fans were deposited during the Holocene.
“TABLE MOUNTAIN SHOSHONITE PORPHYRY LAVA FLOWS AND THEIR VENTS, GOLDEN, COLORADO”
During Early Paleocene time shoshonite porphyry lava was extruded from several plugs about 5 km north of Golden, Colorado, to form lava flows intercalated in the upper part of the Denver Formation. These flows now form the caps of North and South Table Mountains. Detailed field and petrographic studies provide insights into magma development, linkage between vents and flows, and the history of the lava flows. The magna was derived from a deep crustal source, was somewhat turbulent on its way up, paused on its way up in a shallow-level granitehosted chamber, and near the surface followed the steep Golden Fault and the thick, weak, steeplydipping Upper Cretaceous Pierre Shale. At the surface the lava flowed out of several plug and dike vents in a non-explosive manner, at 4 times during a span of about 1 m.y. Potassium-rich material acquired in the shallow-level chamber produced distinctive textures and mineral associations in the igneous rocks. Lava flows 1 (the lowest) and 2 are channel deposits derived from the Southeastern Group of intrusives, and flows 1 lie about 150 feet below the capping flows. Provisionally, an older felty-textured flow, la, is distinguished from younger blockytextured flows, lb. Flow 2, newly recognized in this study, lies immediately beneath the capping flows. Lava flows 3 and 4, more voluminous then the first two, were derived from a plug vent 1-2 km farther north-northwest and flowed south-southeast across a broad alluvial plain. This plug is a composite body; the Rim Phase fed flow #3 and the Core Phase flow 4. During the lapse of time between the effusion of the four flows the composition of the shoshonite porphyry changed subtly, having picked up more alkali. On North Table Mountain lava flows 3 and 4 form an elongate tumulus above an underlying, water-saturated, stream channel. On South Table Mountain a low broad dome on lava flow 3 forced flow 4 into channels now restricted to the west and northeast flanks of that mesa. The mesa-capping lava flows 3 and 4 are broken by many small normal faults and are warped into open synclines, probably in response to local stresses associated with the settling of piedmont deposits into the Denver Basin. Mid-Tertiary deposits are inferred to have covered the upper part of the Denver Formation and its lavas, and to have been instrumental in changing the stream flow direction to the east before the onset of Neogene uplift and consequent canyon cutting across the flows.
“SCIENCE AND SOCIETY: CULTURAL AWARENESS IN MANAGING NATURAL DISASTERS”
United States Geological Survey, Denver
Natural hazards include a wide range of earth processes that are often perceived as risky or dangerous such as earthquakes, floods, fires, landslides, and volcanoes. In the absence of people and property, these natural events may go unnoticed. As the global population grows, more and more people and our supporting infrastructure are being built “in harms way”. Natural disasters often bring out the best behaviors in the global community to assist with disaster relief efforts and post-disaster recovery. Additionally, mitigating the threat of disasters often brings together scientists and managers to assist with pre-disaster planning and development activities. Working effectively in these challenging situations requires that we be actively aware of the social and cultural environments in which we work. This presentation will draw on examples from one type of natural hazard—volcano hazards—and show how these have been managed and mitigated in a variety of countries including the United States, Indonesia, the Congo.
December 2004 – Presidental Address
“CENOZOIC HISTORY OF THE LARAMIE MOUNTAINS IN WYOMING AND ITS RELATION TO THE PHYSIOGRAPHIC DEVELOPMENT OF THE COLORADO FRONT RANGE”
University of Colorado, Boulder
The physiographic development of the Colorado Front Range is difficult to unravel. Major Late Cenozoic uplift especially affected the west flank and southern margin of the range. After late Cenozoic erosion, Tertiary rocks in the Front Range are relatively few, and scattered sedimentary rocks (mostly conglomerates) are poorly dated. Glaciers have modified the uplands in the Front Range such that little remains there of the original pre- Quaternary topography. Nevertheless, the general consensus is that there are an older rolling topography of Cenozoic age along the eastern flanks of the range, broad valleys below the rolling topography, and very deep canyons cut by modern streams into the older topography. Unfortunately, these physiographic features in the Front Range itself do not indicate their time of formation. The Laramie Mountains in southeast Wyoming is a northern extension of the Front Range that is still covered by Cenozoic sedimentary rocks. The highest peak in the Laramie Mountains (Laramie Peak at an elevation of 3,130 m) is below the limit of Pleistocene glacial ice, so no glaciation occurred in the range. Tertiary sedimentary rocks cover much of the range. Finally, the flanks of the range contain physiographic features similar to those in the Front Range. Unlike the Front Range, these landforms can be related to Cenozoic deposits and can therefore be dated. The broad valleys within the Laramie Mountains are filled with the fine-grained ash deposits of the latest Eocene and early Oligocene White River Formation. The White River Formation filled valleys with high relief (maximum relief = 1,170 m). From the highest crest in the northern part of the range, White River drainages flowed into adjacent basins. The broad rolling surface above these White River paleovalleys is associated with broad sheet conglomerates of Oligocene and Miocene age (Arikaree and Ogalalla Formations). Locally these conglomerates extend far into the Precambrian core of the range and represent gravels deposited on pediments cut during the long tectonic quiescence of the middle Cenozoic. Modern streams have cut deep canyons in an unusual pattern across much of the Laramie Mountains. In the northern part of the range, almost all of the drainages that flow west and southwest from the topographic crest of the range are barbed and flow east or northeast across the entire width of the range. In the southern part of the range, where the low-level Sherman Surface is developed, all of the drainages flow east across the range from the western margin of the range. Therefore, the modern drainage divide is at the margin or even within the adjacent Laramie and Shirley basins. This anomalous drainage pattern reflects tilting of the range to the east and northeast during the late Cenozoic, after deposition of the Ogallala Formation. Extending these landforms southward to the Front Range, the broad valleys below the gently rolling topography may reflect the late Eocene surface; the rolling topography may be as young as Miocene in age; and the deep canyons reflect late Cenozoic uplift of the Front Range and adjacent Great Plains. This talk is dedicated to Donald L. Blackstone, Jr. (1909–2004), whose first publication was on the development of wind gaps in the Laramie Mountains, and whose last studies also included the structure and Cenozoic history of the range.