St. Jacob Domes

Location

T3N, R6W, Madison County (H-4)

References

Lowenstam 1948, Bell 1961, Bristol and Buschbach 1973, Buschbach and Bond 1974, Sargent 1991

Description

The St.Jacob structure has two domes, one centered in Section 16 and the other centered in Section 27, T3N, R6W, about two miles south-southeast. The northern dome is nearly circular and approximately 1 mile (1.6 km) in diameter. Closure is about 100 feet (30 m) on top of the St. Peter Sandstone (Buschbach and Bond 1974), 75 feet (23 m) on top of the Galena Group (Bristol and Buschbach 1973), and less than 50 feet (15 m) on top of the Silurian System (Lowenstam 1948). The southern dome, elongated from north to south, is 2 miles (3 km) long and 1.25 miles (2 km) wide. It has closure of 75 to 100 feet (23-30 m) on the Galena and 50 to 75 feet (15-23 m) on the Silurian.

A test hole on the southern dome was drilled directly from the Cambrian Eau Claire Formation into Precambrian granite and encountered no Mt. Simon Sandstone. A second test hole penetrated an abbreviated section of Eau Claire and a thin section of Mt. Simon above the Precambrian. An off-structure test hole encountered a normal section of Eau Claire overlying thin Mt. Simon and granite. These findings suggest that the southern dome overlies a Precambrian paleotopographic high (Sargent 1991). Doming may be the result of differential compaction over the buried knob. A gas storage field has been developed in the St. Peter Sandstone on the northern dome.