Precambrian Hills
Location
See Plate 1
References
Thwaites 1931, Dake and Bridge 1932, Hayes 1961, Bradbury and Atherton 1965, Atherton 1971, Wharton et al. 1975
Description
Buried Precambrian hills are not structural features, but because they are probably responsible for several named domes in Illinois, they merit discussion here.
On the Ozark Dome the Precambrian surface is rugged and has as much as 1,500 feet (450 m) of local relief (Dake and Bridge 1932). The highest knobs protrude through sedimentary cover in the St. Francois Mountains and in numerous isolated hills to the south and west. Many additional Precambrian buried hills and ridges in the Ozarks have been located by drilling and mining operations. Most of the largest hills are composed of rhyolite porphyry, an extremely resistant rock. Dake and Bridge (1932) showed that the Precambrian topography was the product of a dendritic drainage system established prior to deposition of the basal Cambrian Lamotte (Mt.Simon equivalent) Sandstone.
The Lamotte and younger Cambrian formations of the Ozarks thin and pinch out against the flanks of the Precambrian hills and display original dips as steep as 30°. Mapped structural relief on Cambrian horizons near buried hills locally exceeds 300 feet (90 m). The presence of residual chert of the Gasconade Formation (base of Ordovician) on the tops of some of the higher knobs suggests that the Precambrian surface was mantled in sediments early in the Ordovician Period, but it was subsequently exhumed by erosion (Dake and Bridge 1932).
Information on the Precambrian surface in Illinois is limited, because only 34 boreholes to date have reached Precambrian rocks (fig. 4). Some of these wells do, however, provide evidence for considerable relief on the Precambrian surface. Two wells drilled 8 miles (13 km) apart in Pike County indicated about 800 feet (240 m) of local relief on the Precambrian. In the higher well the Mt. Simon Sandstone was absent and the Eau Claire Formation was found resting directly on red granophyre. Three Precambrian tests were drilled within 1 mile (1.6 km) of one another on the southern St. Jacob Domes in southeastern Madison County. These revealed that local relief was at least 347 feet (107 m) and that the Mt. Simon was absent on top of one of the domes. This evidence strongly implies that the domes, which affect Ordovician strata, resulted from uneven deposition and compaction across Precambrian knobs. A similar origin of the Dale Dome in Hamilton County is suggested by the absence of the Mt. Simon in a Precambrian test drilled on top of the dome. Other Precambrian hills, indicated by wells that encountered Eau Claire or younger formations resting directly on basement, are shown on plate 1. These wells are isolated from other Precambrian tests and not associated with named domes.
Domes related to Precambrian hills would be expected to occur alone or in small groups. They should have irregular outline, random orientation, and abnormally thin or missing basal Cambrian formations. Structural relief should increase with depth.
Such domes obviously are targets for petroleum exploration and for gas storage reservoirs. Many zinc and lead deposits in the Ozarks occur on the flanks of buried hills. Paleotopography influenced facies distribution of the ore-bearing Bonneterre Formation (Eau Claire equivalent) and structural irregularities around buried hills probably controlled movement of the ore-forming fluids (Wharton et al.1975).
Figure(s)
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