AIPG FIELD TRIP - ELEPHANTS • NATURAL WATER PARK • MONUMENTS
General Geology of the St. Francois Mountains The rocks in the St. Francois Mountains include extrusive and
intrusive igneous, marine sedimentary, and minor metamorphic rocks. The oldest are extrusive igneous rocks, generally rhyolites and felsites. During Precambrian time these rocks accumulated as a series of lava flows, tuffs, and associated volcanic rocks, aggregat- ing many thousands of feet in thickness. These rocks are partly exposed at Johnson Shut-Ins Park. The initial extrusive rocks later were intruded by granite batholiths, one of which will be seen at Elephant Rocks State Park. These granites have been dated by several methods, and the determinations are between 1.5 and 1.3 billion years. Later in Precambrian time there was intrusion of small mafic dikes along more or less vertical fractures. The rock formed is basaltic in composition and is diabase and/or gabbro. Several dikes will be seen a few miles from Fredericktown.
Later in Precambrian time the St. Francois Mountains area was
eroded to a landscape which had a series of ridges and knobs separated by val- leys several hun- dred feet deep. It was onto this irreg- ular topography that the sea trans- gressed in late Cambrian time, about 510 million years ago. During Late Cambrian time sea level rose slow- ly, turning the knobs and ridges into a series of islands. Sediment was erod- ed from the islands and deposited in the intervening val- leys as sandstone and shale. Some limestones and dolomites were also deposited dur- ing Late Cambrian time. By the Early Ordovician, the whole area was cov- ered by the sea and all the islands were buried by sedimen- tary strata. At several of the stops on the trip we will see various representatives of these strata.
Figure 1. Stratigraphic column showing rocks pres- ent in the St. Francois Mountains area.
The Paleozoic exposures on the Ozark Plateau include Upper Cambrian, Ordovician, Mississippian, and very minor amounts of Silurian and Devonian strata (figure 1). The Ordovician strata are very widely distributed throughout southeastern Missouri, and are generally overlain by Mississippian strata. In most of the Midcontinent (read: Midwest), fairly thick (hundreds of feet) strata of Silurian and Devonian age are found between the Ordovician and Mississippian strata. The absence of these strata in the Ozark Plateau is unusual. Regional geologic studies show that in some parts of the Ozarks, Silurian and Devonian strata are found in small patches where they have been preserved in down-dropped fault blocks. These strata apparently were deposited in the Ozarks. However, sometime in the Late Devonian, the Ozarks were gently uplifted above sea level
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into a dome-like structure (figure 2). Faulting and fracturing was associated with this uplift.
In some places, fault blocks were
dropped downward by several hundred feet. Erosion then removed most of the Silurian, Devonian, and some of the Ordovician strata. Silurian and Devonian strata are preserved only on small, down- faulted blocks. During Mississippian time, the Ozark Dome was covered by the sea, and marine limestones were deposited above the older strata.
Figure 2. Generalized geologic map of Missouri. Since Mississippian time the Ozark Dome has been gently
uplifted, probably several times. Today the area is far above sea level. Taum Sauk Mountain is the highest point in the state, at 1,772 feet above MSL. Erosion has removed mores strata from the uplifted area. In the center of the uplift the Precambrian rocks are exposed as the St. Francois Mountains. The mountains consist of a series of ridges and knobs with valleys between. Some of the valleys are remnants from Precambrian time and are partially filled with Cambrian strata. On a geologic map (figure 2), the St. Francois Mountains appear to be the center of a “Bulls-eye” pattern, sur- rounded by concentric bands of progressively younger rocks. The rocks nearest the center are Cambrian in age surrounded by con- centric bands of Ordovician, and Mississippian strata.
As streams erode through the Paleozoic strata, they occasion-
ally encounter one of the buried knobs of Precambrian rocks. Where the stream flows over the Paleozoic (generally Cambrian) strata, it flows in a fairly wide valley. Where the stream flows through a valley cut in the Precambrian rocks, the valley is nar- rower. (Why would this be the case?) Thus a stream may flow through a wide valley on either end of a narrower stretch underlain by Precambrian igneous rocks. Such a narrow place is called a ”Shut-Ins”, a term that appears to be unique to the Missouri Ozarks.
Elephant Rocks State Park Preserved within the park boundaries are large, elephant-sized
boulders of Graniteville Granite. The boulders formed as a result of extensive weathering along joint planes. Weathering along the joints has rounded the rock adjacent to the joints, and created spheroidally weathered rocks. The Missouri Department of Natural Resources (MODNR) has a nice diagram illustrating the evolution of the elephant rocks through time. The weathering process left behind these giant boulders and other, subtle weathering features such as weathering pits, etched crystals, and residual mineral grains (or when large, residual knobs.
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