Appendices & Glossary - Botanical Research Institute of Texas

EAST TEXAS AS A UNIQUE HABITAT/APPENDIX TEN
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exists primarily at the generic level implies “a substantial period of isolation” of the Mexican components—a
conclusion in line with recent paleobotanical evidence showing that the relationship was established
long before the glacial cycles of the Pleistocene (Graham 1999). In actuality, this relationship
represents a middle Miocene (Miocene epoch—24.6–5.1 million years ago) extension of deciduous forest
and associated fauna (particularly amphibians) into Mexico during a period of widespread climatic
cooling (Burnham & Graham 1999; Graham 1999). In the words of Graham (1999), a “major decline
in temperature occurred in the middle Miocene” and “global temperatures reached new lows,
allowing the deciduous forest to reach its southernmost extent.” Subsequently, during the Pliocene
(Pliocene epoch—5.1–1.8 million years ago) and later, the climate warmed and dried, and other types of
vegetation (e.g., prairie or thorn scrub communities) spread replacing deciduous forest. As a result, the
once continuous deciduous forests became disjunct, surviving only in the eastern U.S. and eastern
Mexico. During the following millions of years, evolution resulted in the differentiation of some of the
disjunct populations into separate species, while others remained similar enough to be considered one
species. Ultimately, with continued climatic change, this forest type has been further reduced in
Mexico until at present it survives in only limited isolated pockets of appropriate microclimate in the
highlands (Miranda & Sharp 1950; Graham 1993a, 1993b, 1999).
The influence of the eastern deciduous forest on the flora of East Texas is thus an extremely complex
and interesting story written across tens of millions of years, untold upheavals in geology and climate,
and profound evolutionary change. And yet, it is a story that can be deciphered using modern
concepts and techniques in geology, paleobotany, paleoclimatology, and evolutionary biology.
6) Unique geology—While there are many aspects of the geology of East Texas that are interesting
and important biologically, several unusual phenomena deserve special mention. First, the salt domes
scattered in East Texas are an interesting geologic feature, though very limited in total area. Millions of
years ago in the area we now call East Texas, the forces of plate tectonics pulled a continent apart and
the earth’s crust began to sag. During the Jurassic Period (208–145 million years ago) huge shallow
seas, eventually retreating to become the present-day Gulf of Mexico, began to form in this sagged
area. This water was shallow, the area was not well connected to the ocean, and it sometimes virtually
dried up leaving vast salt flats—the result over long periods of time was the deposition of tremendously
thick layers of salt (known now as Louann salt). These deposits are the source of the salt domes
of modern day East Texas. Eventually, as sedimentation continued and more and more material was
laid down over the salt, the tremendous pressure of the overlying younger sediments (Cretaceous and
Tertiary in age) caused the salt layers to become distorted. Acting under pressure almost like toothpaste
being squeezed out of a tube, the salt formed upward thrusting columns and spires, which in
some areas broke through the covering sediments to reach to or near the surface in the form of isolated
domes of salt (e.g., near Palestine or Grand Saline—in the latter case, the salt is mined) (Spearing 1991).
Ecologically, these salt domes are significant because of the effect they have on plant and animal life—
coastal salt marsh plants, for example, can be found hundreds of miles inland (e.g., Bulboschoenus
robustus, salt-marsh bulrush, at an inland salt marsh near Grand Saline in Van Zandt Co.). Economically,
they are also very important because rich deposits of oil often collected around them from
nearby oil-bearing layers—e.g., Spindletop, the first gusher that in 1901 ushered in the Texas oil boom.
Evidence for an extraterrestrial object hitting East Texas is a second fascinating and unusual geologic
phenomenon. 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 the area. Limestone rocks, unlike those anywhere nearby, occur at the surface. In fact, there
is a roughly circular 1.2 km diameter surface outcrop of disturbed Upper Cretaceous limestone rocks
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 relatively common salt domes
found widely in East Texas (e.g., Spearing 1991). Recently, however, geologists have more accurately explained
this unusual geologic feature as an impact crater, 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. It is thus one of fewer than 200 such craters known world-wide
(Perkins 2002b). Approximately 58 ± 3.1 million years ago near the Paleocene-Eocene boundary, when