ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...

Finally, while such major impact events may be relatively rare, we have evidence of at
least one smaller impact which occurred in East Texas. Local farmers and ranchers (e.g.,
family of J.A. [Jack] Lincecum, J.B. Lincecum, pers. comm.) in Leon county near Marquez
have long known that there was something strange about the geology of their area (Fig. 24).
A surface outcrop of disturbed Upper Cretaceous limestone rocks, unlike those anywhere
nearby, occurs in an area otherwise characterized by much younger Paleocene sandstone.
Some geologists thought the structure, known as the Marquez Dome, was another example
of the salt domes found widely scattered in East Texas (e.g., Spearing 1991). Recently, however,
based on a variety of evidence, including core samples (petroleum industry well-log data),
gravity anomaly data, seismic reflection data, surface geology, faults, and steeply sloping strata,
geologists have more satisfactorily explained this unusual geologic feature as an impact crater.
It is thus one of fewer than 200 such craters known worldwide (Perkins 2002b). Approximately
58 million years ago near the Paleocene-Eocene boundary, when the region was a
near-shore environment, an extraterrestrial object (e.g., a meteorite) slammed into the soft
unconsolidated sediments, producing a 13 km diameter impact crater. Subsequently buried
by sediments and later partially uncovered by erosion, the only present-day surface exposure
of the crater is an uplifted area at its center—a 1.2 km diameter outcrop of blocks of
Cretaceous Pecan Gap limestone (Taylor Group) in a sand and clay matrix. The presence
of this anomalous limestone at the surface represents at least a 1,120 m vertical uplift of
the Cretaceous rocks, and is an indication of the tremendous energy released during the
impact (Gibson & Sharpton 1989; Gibson 1990; McHone & Sorkhabi 1994; Buchanan et al.
1998; Wong et al. 2001).
SOILS AND SOIL-RELATED GEOLOGY OF EAST TEXAS
BY RAYMOND “CHICK” DOLEZEL
FORMERLY SOIL SCIENTIST, U.S. DEPARTMENT OF AGRICULTURE & TEXAS STATE LICENSED GEOSCIENTIST
INTRODUCTION AND SOIL FORMATION
AND GEORGE DIGGS
SOILS/INTRODUCTION 45
Soils of East Texas vary dramatically, ranging from loose, drought prone sands to rich
Blackland Prairie soils to the flooded soils of swamps and bogs. These different soils have
profound impacts on vegetation, in some cases being the determining factor in plant distribution
and abundance. The varied soils have formed as a result of a complex series of
interactions involving the effects of parent material, time, climate, plant and animal life,
and topography (Dolezel et al. 1988). Key processes include erosion, weathering, leaching
of soluble bases, accumulation of organic matter in surface layers, and the downward
movement of clay particles. It is obvious that many of these factors are interrelated. For
example, time has relatively little effect on soil development in very dry areas such as
deserts. In East Texas, however, with annual rainfall of 28 to nearly 60 inches (71 to 152
cm), subtropical temperatures, and an abundance of plant and animal life, there has been
significant soil development. One of the most important factors in determining the type of
soil that develops in a given area is parent material. In East Texas there are three major categories
of parent material: 1) the alluvial fills of flood plains, 2) the Pleistocene Age mounded terraces,
and 3) extensive areas of older marine and fluvial (= associated with a river or stream) sediments.
The varying makeup of these parent materials and their complex distributions
across East Texas have resulted in numerous different soil types arranged in an extremely
complex pattern. During the process of soil development, different layers (called horizons)
form, resulting in characteristic soil profiles (= cross-section of a soil including all horizons).
Such differences are the basis for soil classification.