Figure 1.
Research Area Showing Three Type Sections.
Three locations separated by as much as twelve miles were used in the formal descriptions: one location for the type section of the Rockyford Member of the Sharps Formation, a second for the lower type section of the Sharps Formation, and a third for the upper type section of the Sharps Formation.
Figure 2.
Stratigraphic Relationships of Some Great Plains Formations.
Both the Poleslide Member, Brule Formation in South Dakota and the Whitney Member, Brule Formation in Nebraska are overlain conformably by lithologic bodies of approximately the same thickness: the Sharps Formation and the Brown Siltstone Beds [3], [7]. In the vicinity of the Chadron Arch, a thick tephra is identified at the base of both: the Rockyford Ash Zone (RAZ) in South Dakota [9] and the Nonpareil Ash Zone (NPAZ) in Nebraska [7]. The lower portions of the Brown Siltstone Beds and Sharps Formation are more stratified than their upper portions. The upper portions of each are massively bedded siltstone with intermittent, vertically oriented, spindle/potato-shaped nodules. The lithologies of the Brown Siltstone Beds and the Sharps Formation are similar. These data are consistent with the subsurface evidence of lateral continuity across the Chadron Arch [12]. The Nonpareil Ash Zone (NPAZ) of the Brown Siltstone Beds can be traced in the subsurface across the Chadron Arch. This ash complex northeast of the Chadron Arch is known as the Rockyford Ash Zone (RAZ) [9]. Swinehart, et al. (1985) provides direct subsurface evidence of the correlation of the NPAZ with the RAZ and the subsequent correlation of the Sharps Formation with the Brown Siltstone Beds.
Figure 3.
Stratigraphic Correlation of Sediments.
At the southernmost extension of the Brown Siltstone Beds in Nebraska, these beds are quite thin and overlain by 200+ feet of the Gering Formation. The Brown Siltstone Beds are thicker northward. At the Round Top to Adelia section a complete section of the Gering Formation displays a basal disconformity with the underlying Whitney Member of the Brule Formation. There is evidence of an Upper Whitney Ash (UWA) and an Upper Ashy Zone (UAZ). Proceeding to the northeast, the Gering Formation pinches out, the Whitney ash beds disappear, and the NPAZ appears at the base of the Brown Siltstone [7] so that in the vicinity of the Chadron Arch and the town of White Clay, Nebraska, there is a fully developed section of the Brown Siltstone Beds with no overlying Gering Formation and no Whitney ashes. The NPAZ is a stratigraphic correlative of the RAZ, and the Brown Siltstone Beds in Nebraska occupy a position between the top of the Brule Formation and the Monroe Creek Formation identical to the Sharps Formation in South Dakota. Where the Gering Formation occurs in Nebraska, it disconformably overlies either Brule Formation sediments or the Brown Siltstone Beds. The Gering sediments are a stratigraphic equivalent, of a part of the upper Sharps Formation [12] All of the evidence indicates the Brown Siltstone Beds are strongly associated with the Sharps Formation and the Nonpareil Ash Zone within the Brown Siltstone Beds is an extension of the Rockyford Ash Zone.
Figure 4.
Composite Stratigraphic Column of the Sharps Formation.
The Monroe Creek Formation forms vertical cliffs above slopes of the Sharps Formation. It weathers gray and presents as a massive, compact body of siltstone with no concretions. Sharps Formation (from top to bottom). 21 Silt, pinkish-tan, weathers to smooth steep slope; random vertically elongate light-tan nodules to 2 inches in diameter, 1–3 inches in length; nodules less abundant in upper 50 feet and almost absent in upper 20 feet; upper few feet grades into overlying Monroe Creek Formation. 115′; 20 Silt, tan, massive; small amount of very fine-grained sand. 28′; 19 Silt, tan, with randomly scattered vertically elongate nodules, 1–2 inches in diameter and 1–4 inches in length, more abundant upward. 60′; 18 Siltstone, red-brown, thinly bedded. 2′; 17 Silt, tan, with randomly scattered vertically elongate nodules, 1–2 inches in diameter and 1–4 inches in length; chalcedony and clastic dikes. 28′; 16 Claystone, red-brown, thinly bedded, with 2 thinly bedded clayey gray siltstone layers interspersed. 4′; 15 Siltstone, gray, massive 4′; 14 Siltstone, clayey, red-brown 3′; 13 Siltstone, greenish gray, sandy 3′; 12 Siltstone, gray 3′; 11 Siltstone, gray, with intermittent red-brown clayey layers; 1′ red-brown clayey siltstone at top, 3′ red-brown clayey layer at base. 12′; 10 Siltstone, gray. 3′; 9 Limestone; fresh surface shows scattered black dots; widespread but not continuous. May be multiple layers locally separated by gray siltstone. 8′; 8 Siltstone, tan with randomly scattered light-gray vertically elongate nodular concretions, 1–2 inches in diameter, 2–3 inches in length; upper 5′ massive with green tinge. 20′; 7 Sand, greenish gray; weathers white. 6′; 6 Silt, tan to light-brown, with vertically elongate nodular concretions, 1–4 inches in diameter, 2–6 inches in length. Two nodular layers at base. 34′; 5 Silt, somewhat clayey, pinkish gray; nodules that are sometimes traceable laterally as a layer; chalcedony and clastic dikes. 21′; 4 Claystone, reddish brown; lower 1.5′ stratified. 3′; 3 Ash with some silt, greenish white to white; upper 3′ clayey and stratified. 6′; 2 Silt, pinkish gray, clayey with 2″ red-brown nodular layer at base. 31′; 1 Ash with some silt, white to greenish white; upper portions stratified. 8′; Total Sharps Formation thickness 402 ft. Brule Formation Siltstone, sandstone, and clay pinnacles; stair-steps as result of alternating layers; upper 20–30 feet contain several layers of siltstone and sandstone showing some steep cross-bedding.
Figure 5.
Wolff Camp Member of the Sharps Formation.
Wolff Camp Member units (from top to bottom). 16 Claystone red-brown, thinly bedded, with 2 thinly bedded clayey gray siltstone layers interspersed. 4′; 15 Siltstone, gray, massive 4′; 14 Siltstone, clayey, red/brown 3′; 13 Siltstone, greenish gray, sandy 3′; 12 Siltstone, gray 3′; 11 Siltstone, gray, with intermittent red-brown clayey layers; 1′ red/brown clayey siltstone at top; 3′ red/brown clayey layer at base. 12′; 10 Siltstone, gray. 3′; 9 Limestone; fresh surface shows scattered black dots; widespread but not continuous. May be multiple layers locally separated by gray siltstone. 8′; 8 Siltstone, tan with randomly scattered light-gray vertically elongate nodular concretions, 1–2 inches in diameter, 2–3 inches in length; upper 5′ massive with green tinge. 20′; 7 Sand, greenish gray; weathers white. 6′; 6 Silt, tan to light-brown, with vertically elongate nodular concretions, 1–4 inches in diameter, 2–6 inches in length. Two nodular layers at base. 34′; 5 Silt, somewhat clayey, pinkish gray; a few nodules that are sometimes traceable laterally as a layer. 21′; 4 Claystone, reddish brown; lower 1.5′ stratified. 3′; 3 Ash with some silt, greenish white to white; upper 3′ clayey and stratified. 6′; 2 Silt, pinkish gray, clayey with 2″ red-brown nodular layer at base. 31′. Total Wolff Camp Member thickness 161 ft.
Figure 6.
Gooseneck Road Member of the Sharps Formation.
Gooseneck Road Member units (from top to bottom). 21 Silt, pinkish-tan, weathers to smooth steep slope; random vertically elongate light-tan nodules to 2 inches in diameter, 1–3 inches in length; nodules less abundant in upper 50 feet and almost absent in upper 20 feet; upper few feet grades into overlying Monroe Creek Formation. 115’; 20 Silt, tan, massive; small amount of very fine grained sand. 28′; 19 Silt, tan, with randomly scattered vertically elongate nodules, 1–2 inches in diameter and 1–4 inches in length, more abundant upward. 60′; 18 Siltstone, red-brown, thinly bedded. 2′; 17 Silt, tan, with randomly scattered vertically elongate nodules, 1–2 inches in diameter and 1–4 inches in length. Calcedony and clastic dikes. 28′. Gooseneck Road Member thickness 233 ft.