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Analysis
and Modeling of Geologic
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For more than forty years the area between the towns of Forney and Terrell in northeast Texas has been relatively stable geologically although the tract is underlain by argillaceous strata (marl and shale) that form thick expansive soils. Recently, small-scale landslides have remodeled the land surface, collapsed some wooden structures, severed sewer and water pipes, snapped telephone cables, and resulted in leaning power poles and fence posts. Currently, an investigation is being conducted to determine probable causes of downslope movement in this area. Preliminary data suggest that the mass wasting is related to locally heavy rains during the last few years. From 1990-1992, rainfall in the region ranged from 25 percent to 60 percent above normal. The excessive precipitation caused the local water table to rise near ground level and collapse several low hills. Because of increased weight and pore pressure as well as decreased friction, the saturated argillaceous mass became unstable and moved downslope in response to gravitational forces.
The study of the distribution of fossil organisms in space and time (paleobiogeography)
is uniquely suited to test or provide data for paleogeography or the reconstruction of
Earth's surface. Paleobiogeography can fulfill such a role because the distribution of
fossil marine invertebrates were controlled, for the most part, by chemical, thermal and
physical barriers which were in turn strongly correlated with the position of landmasses.
The presence or absence of oceans, the proximity and continuity of continental shelves,
presence of juxtaposed and geographically unrelated terranes, the general paleolatitudes
of landmasses, and uniquely, the general paleolongitude (magnitude of separation) of
landmasses are all easily within the predictive and illuminative powers of the science.
In any paleobiogeographic study, the attainable degree of resolution is dependent on the
sensitivities to both major and minor barriers of the group being studied. My work has
concentrated on nautiloid cephalopods because they were particularly sensitive to typical
biogeographic barriers. Unlike most marine invertebrates that achieve dispersal during the
period when the larval stage is borne about by oceanic currents, the ontogeny of
nautiloids did not incorporate a larval stage and young were released from a stationary
egg as juveniles. For this reason, nautiloid distributions were sharply defined by common
barriers such as distance, water depth and latitude. The funded research has further
concentrated on the distribution of nautiloids within the Silurian and Devonian periods of
Earth time. These time periods contain many of the events which ultimately led to the
assembly of the supercontinent Pangea some 250 millions years ago, the disassembly of
which produced our present configuration of ocean basins and continents. A reduction
tectonic actively during these periods created rock units which are not particularly well
suited to paleomagnetic studies of the type normally used in the repositioning and
reassembly of paleogeography. Marine strata of this age contain a wealth of marine
diversity as marine shelf areas were extensive and many landmasses lay between subtropical
latitudes. With funding from the National Science Foundation, I began in 1988 a sequence
of field studies of Silurian and Devonian nautiloid cephalopods from regions of southern
France, Spain, Portugal, northern Africa and western Australia. Attention was focused on
these regions as they are thought to have constituted the northern margin of the
supercontinent Gondwana. This margin would have faced the southern margin of most of the
remaining landmasses which today makeup North America, northern Europe and Siberia.
Supplemented with existing museum collections, this material provides one of the bet
documented biogeographic database of any major marine invertebrate group.
While the field collections from northern Africa and western Australia are still being
evaluated, the data have already been successful in distinguishing between competing
models for reconstructing the position of landmasses for Silurian time. Using a measure of
faunal similarity based on Monte Carlo simulation, statistical probabilities can be
assigned to the number of genera or species observed in common among sampling localities
as opposed to those expected in common. Such a measure of similarity imparts rigor to
faunal comparisons which is generally missing from similar studies.
Supported by the National Science Foundation.
Study of deep-sea sediments presents several unique problems including small amounts of
material available for analysis (usually less than 5 grams), the low concentrations of
many diagnostic sediment components, and the large number of samples typically analyzed.
Reflectance spectrophotometry in the near ultraviolet, visible, and near infrared
wavelength ranges addresses many of these problems. Analysis of samples by reflectance
spectrophotometry utilizes small samples, less than 0.2 grams, that are analyzed in about
two minutes. A wide variety of minerals and sediment components can be distinguished by
reflectance spectrophotometry including hematite, goethite, carbonate, opal, chlorite, and
organic matter.
This technique is now being applied to several marine geological problems. Hematite is a
common component in wind-blown dust but is present in marine sediments in such low
concentrations, less than 0.2% by weight, that it is not easily analyzed. With reflectance
spectrophotometry hematite can be determined at concentrations as low as 0.03%. This
technique is currently being used to monitor temporal changes in winds that blow across
the Atlantic Ocean from the Sahara. Goethite, unlike hematite, is formed in warm humid
conditions and is transported by rivers draining tropical regions. Like hematite, goethite
is present in low concentrations in marine sediments that are easily determined by
reflectance spectrophotometry. By analyzing goethite in a series of cores of the coast of
South America, changes in Amazon River outflow are being deciphered. Finally, reflectance
spectra are sensitive to changes in organic content. Because sediments underlying
upwelling zones are characterized by a high organic content, reflectance spectrophotometry
is also useful for studying changes in upwelling. Rapid determination of organic content
may have application beyond marine geology. In conjunction with a scientist at Texas
Wesleyan University, the technique has been used to identify producing zones in oil wells.
A long-standing problem in plate tectonics of Central America has been the nature of
the earthquake faults that traverse the isthmus. In 1978 it was determined that there was
a horizontal offset of 130 kilometers across the Guatemalan plate boundary fault. More
recently, the relationships between the major transform faults and extensional structures
in Central America have been determined, and it has been shown that segmentation of the
active volcanic chain of Central America is related to movement of microplates between
these major faults.
It has also been shown that rivers and drainage divides of prehistoric times have been
offset across one of the plate boundary faults and that these can be reconstructed.
Reconstruction of ancient drainage systems offset across a major transform fault has never
before been accomplished and should open a new area of geologic study which can be
referred to as Tectonic Geomorphology.
Using the theory of plate tectonics as an explanation for the movement of
parts of the Earth's crust has resulted in the reinterpretation of much of the geologic
data gathered about the western part of North America, and it has stimulated intense study
of rocks and possible processes which formed the western margin of North America. These
studies have led to the interpretation that much of this western margin, from Baja
California to Alaska, is made of parts of old ocean floor, seamounts island arcs, and
possible microcontinents that have been added in the past as so-called accreted terranes.
Microfossils found in these rock formations are frequently the most valuable and often the
only clues to their ages. Recent studies of microfossils in the Grindstone terrane of
central Oregon and the Baker terrane of eastern Oregon suggest that the rocks of the
Grindstone terrane probably formed relatively close to the western margin of
"paleo-North America," while those of the Baker terrane formed farther out in
the "paleo-Pacific" much closer to eastern Asia.
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White areas in center are Permian age limestones southeast of Baker, Oregon. This limestone contains a colonial coral previously reported only in Mongolia.
Plate Tectonic Modeling & Geodynamics. A major research goal has been the
construction of an interactive computer graphic model describing the evolution of the
continents and ocean basins during the last 800 million years. Plate tectonic models and
computer animations of plate motions, which are produced using UNIX workstations, provide
a framework for research in the areas of paleoclimatology, paleobiogeography,
paleoceanography, and paleogeography. This kinematic model of plate motions also allows us
to better understand the tempo and mode of plate evolution, and to gain insights into the
driving mechanisms of plate tectonics.
Global Paleogeographic Mapping. An Atlas of 30 paleogeographic maps has been produced and
will soon be published as a wall-chart by AAPG. In our current mapping effort,
1:10,000,000 paleogeographic reconstructions will be produced for the Late Paleozoic and
Early Mesozoic (Project Pangea), and the Late Cretaceous through Cenozoic (PALEOMAP Project, the Last One Hundred
Million Years).
Regional Paleogeographic Studies. The global plate tectonic and paleogeographic maps
provide an ideal framework for more detailed, regional tectonic & geologic studies.
Current projects include a tectonic model of Southeast Asia, and regional paleogeographic
studies of Asia, the former Soviet Union, the northwestern margin of Australia, and the
margins of the South Atlantic.
Paleoclimate Modeling. The global paleogeographic reconstructions also provide an
important boundary condition for paleoclimate modeling. In collaboration with Bette
Otto-Bliesner (NCAR) and Malcolm I. Ross (Rice), the
PALEOMAP Project is producing an atlas of paleoclimatic reconstructions. The predictions
of these climate models are being tested using a global database of climatically sensitive
lithofacies (i.e. coals, tillites, evaporites, calcretes) which has been assembled by A.
Boucot (Oregon)).
Dinosaur Biogeography. Paleogeographic maps also provide a framework for understanding
biogeographic patterns. A data base has been assembled of all known dinosaur localities.
Using this database we hope to refine our paleogeographic reconstructions for the
Mesozoic, and study the relationships between dinosaurs, paleogeography, and climate.
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direct comments and inquiries about this page to:
crick@uta.edu
This Page was last updated
02/18/03
Department of Geology, University of Texas at Arlington
UTA Box 19049
Arlington, Texas 76019, U.S.A.
phone: 817-272-2987, fax: 817-272-2628
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