Ancona Anticline (New Name)
Part of the La Salle Anticlinorium
Old Name: Ancona Dome
Location
Southernmost La Salle and western Livingston Counties (C-6)
References
Buschbach and Bond 1967, 1974
Description
Buschbach and Bond (1967) used the names Ancona Dome and Garfield Dome for enclosed structural highs where gas storage fields were developed near the towns of Ancona and Garfield. A structure map by Buschbach and Bond (1974) shows that these structures are not domes, but separate areas of closure on a large southeast-trending anticline. The name Ancona Anticline is applied to the larger structure, and use of the names Ancona Dome and Garfield Dome should be discontinued.
The Ancona Anticline is mapped on the basis of deep drilling for gas storage. More than 125 wells have reached the basal Cambrian Mt. Simon Sandstone, the unit that serves as the gas reservoir. A structure map contoured on the top of the Mt. Simon shows closure in an area 10 miles (16 km) long and 2 to 3 miles (3-5 km) wide, encompassing the previously named Ancona and Garfield Domes (fig. 16). Maximum closure on top of the Mt. Simon near Ancona is 290 feet (88 m). Anticlinal nosing continues for more than 15 miles (24 km) southeast of the enclosed area to Graymont, in the southern part of T28N, R4E, and the northeastern part of T27N, R4E, where several small areas of closure have been mapped on the top of the Mt. Simon. On Buschbach and Bond's (1974) structure map of the top of the Galena Group (Ordovician), nosing at the southeast end of the Ancona Anticline continues southeast of Graymont into northern McLean County.
The Ancona structure differs from most folds in the La Salle Anticlinorium in that the steep flank is on the northeast rather than the southwest flank. The southwest flank dips at 2° or less, but the northeast flank locally dips at 25° or more. Rocks on the steep northeast flank evidently are fractured, but no faults have been detected. Cambrian units in wells on the flank are thicker than normal because of steep dips, but do not exhibit repeated section. Borehole data and proprietary seismic profiles across the anticline rule out faults of greater than 100 feet (30 m) displacement in the Paleozoic succession (Merle Williams, independent petroleum geologist, personal communications 1991 and 1993).
Cross sections prepared from borehole data demonstrate both pre-and early-Pennsylvanian uplift of the Ancona Anticline. Basal Pennsylvanian strata truncate Ordovician units with angular unconformity. At the crest of the anticline, pre-Pennsylvanian erosion removed as much as 420 feet (125 m) of Ordovician strata (Maquoketa, Galena, and Platteville Groups) that are present off the structure. In comparison, maximum structural relief at the base of the Pennsylvanian System is about 150 feet (45 m), and the Colchester Coal (Desmoinesian) is nearly horizontal across the fold (M. Williams, personal communication 1991). The timing of uplift on the Ancona structure is similar to that of the other elements of the La Salle Anticlinorium.
