Post Creek Formation
Harrison, Richard W. and Ronald J. Litwin, 1997, Campanian coastal plain sediments in southeastern Missouri and southern Illinois— significance to the early geologic history of the northern Mississippi Embayment: Cretaceous Research 18, no. 5, p. 687-696.
The first detailed description of the Post Creek Formation in Missouri and Illinois was under the name Tuscaloosa Formation (Pryor 1960; Pryor and Ross 1962).
Named for the Post Creek Cutoff, directly south of the Joppa to Grand Chains Highway, in the middle of sec. 2, T 15 S, R 2 E, Bandana 7.5-minute quadrangle, Pulaski County, Illinois.
Until 1997, the Post Creek Formation was referred to as the Tuscaloosa Formation, after the exposures along the Tuscaloosa River in Tuscaloosa, Alabama.
Smith and Johnson (1887) first described the Tuscaloosa Formation near the Tuscaloosa River in the 1880s. The Tuscaloosa Formation was a pebbly, micaceous sand with nonmarine chert gravels in a white clay and fine-grained silica matrix. Pryor (1960) located an outcrop in Missouri with similar lithology and stratigraphy to the Tuscaloosa Formation and identified it as the Tuscaloosa Formation. Pryor and Ross (1962) found a comparable outcrop in Post Creek in Pulaski County, again naming it the Tuscaloosa Formation. The outcrop was a white to light gray gravel with a variety of subrounded to angular, pebbles and cobbles ranging from ½ inches to 20 inches in diameter. The pebbles and cobbles are composed of either a light gray, fossiliferous chert or black, denser chert, set in a light gray to white fine-grained silica and/or clay matrix. The same formation was also identified in surrounding areas. However, Willman et al (1975) noted that the exposures in Post Creek, Pulaski County and Thebes Gorge, Alexander County were a transgressive deposit that were possibly a different age from the Tuscaloosa found in Kentucky and Tennessee.
In 1993, as part of a USGS investigation into the Midcontinental United States and New Madrid seismic zone tectonic history, an exploration core drilled in the Benton Hills in southwest Missouri perforated Tertiary and Cretaceous stratigraphic units. Palynologic analyses from the core proposed an age of middle to late Campanian for the basal Cretaceous deposits—the first age identification in this locality. Palynological analysis of the preserved palynomorphs in the Tuscaloosa in Missouri proved substantially different from the palynomorph assemblages from the Alabama type section (Harrison and Litwin 1997). The species found in the Missouri core showed species only present in Campanian or younger aged strata in this locality (Harrison and Litwin 1997). Absence of Complexiopollis in the Missouri section further supported a date of middle to late Campanian age, as this genus tends to be common throughout pre-Campanian strata in the Mississippi Embayment. The palynological analysis showing Campanian age for deposits in southeastern Missouri and southern Illinois indicated a significant age gap between the “Tuscaloosa” found in Missouri and the Tuscaloosa identified in Kentucky and Tennessee, which led to the proposal to change the name from Tuscaloosa to Post Creek. The Campanian section in the area was composed of the newly renamed Post Creek Formation and the overlying Coffee Sand (Harrison and Litwin 1997).
Post Creek Cutoff, directly south of the Joppa to Grand Chains Highway, in the middle of sec. 2, T 15 S, R 2 E, Bandana 7.5-minute quadrangle, Pulaski County, Illinois (Harrison and Litwin 1997).
Richard W. Harrison and Ronald J. Litwin designated the type section in Post Creek Cutoff (Harrison and Litwin 1997).
The type section is still visible as an outcrop in the Post Creek Cutoff in Pulaski County.
Extent and thickness
The Post Creek Formation is exposed only locally in southern Illinois and appears to have a patchy distribution (Pryor 1960; Pryor and Ross 1962; Ross 1964). It has maximum thickness of 20-57 feet but is typically much thinner. The Post Creek is well exposed along the Post Creek Cutoff, Pulaski County cen. sec. 2, T15S-R2E, and in railroad cuts in the Thebes Gorge, 1 mile south of Rock Springs Hollow, Alexander County SW sec. 21, T 15 S, R 3 W (Harrison and Litwin 1997).
It is largely chert gravel, generally moderately cemented with fine silica or white clay. The chert pebbles, in a matrix of coarse quartz sand, are mainly white or gray, but black pebbles are locally common. Beds of pebbly sand and white clay also are common. The clays are dominantly kaolinite (Pryor and Glass 1961). The Little Bear Soil occurs in deposits at the base of the Post Creek Formation. Gravel found northeast of Elco in northern Alexander County (SW SE sec. 7, T14S, R1W) is probably Post Creel preserved in sink holes. Local concentrations of gray and black chert pebbles on the upland bedrock surfaces as far north as Kaolin, Union County (SW NW sec. 2, T 12 S, R 2 W), are probably residual or reworked from the Post Creek.
Well log characteristics
These fossils identified in the Post Creek were significantly different from those found in the Tuscaloosa Formation type section in Alabama. The fossil palynomorph assemblage obtained from the Post Creek Formation contained Semioculopollis sp, Trudopollis variabilis, Osculapollis perspectus, Choanopollenites consanguineus, C . transitus, Extremipollis vivus, Proteacidites spp., and Vacuopollis munitus These species were only present in strata of Campanian age or younger in the Mississippi Embayment. The samples lack Osculapollis aequalis, Pseudoplicapollis serena, and Trudopollis sp., all of which are present in the younger, late Campanian Coffee Sand that lies above it. In addition, Complexiopollis species are absent in the Post Creek Formation sample, offering further support for assignment of a middle to late Campanian age, as this genus is common in pre-Campanian strata of the Mississippi Embayment (Harrison and Litwin 1997).
Age and correlation
The Post Creek Formation was previously correlated to the Tuscaloosa Formation of Kentucky and Tennessee, but the formation has now been dated as middle to late Campanian in age (Harrison and Litwin 1997).
Environments of deposition
The Post Creek Formation is interpreted to be residual material reworked by nonmarine fluvial processes and mass wasting. Many of the gravel pebbles are thought to be from the local Mississippian bedrock, with the more rounded selection travelling a longer distance. Plant rootlet impressions have also been identified, indicating a local paleosol development (Harrison and Litwin 1997).
- Harrison, Richard W., and Ronald J. Litwin, 1997, Campanian coastal plain sediments in southeastern Missouri and southern Illinois— significance to the early geologic history of the northern Mississippi Embayment: Cretaceous Research 18, no. 5, p. 687-696.
- Pryor, W.A., 1960, Cretaceous sedimentation in upper Mississippi Embayment: American Association of Petroleum Geologists Bulletin, v. 44, p. 1473-1504; Illinois State Geological Survey Reprint 1960-J.
- Pryor, W.A., and H.D. Glass, 1961, Cretaceous-Tertiary clay mineralogy of the upper Mississippi Embayment: Journal of Sedimentary Petrology, v. 31, p. 36-51; Illinois State Geological Survey Reprint 1961-M.
- Pryor, W.A., and C.A. Ross, 1962, Geology of the Illinois parts of the Cairo, La Center, and Thebes Quadrangles: Illinois State Geological Survey Circular 332, 39 p.
- Ross, C.A., 1964, Geology of the Paducah and Smithland Quadrangles in Illinois: Illinois State Geological Survey Circular 360, 32 p.
- Smith, E.A., and L.C. Johnson, 1887, Tertiary and Cretaceous strata of the Tuscaloosa, Tombigbee, and Alabama Rivers: USGS Bulletin 43, 189 p.
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