GEOLOGY OF SOUTH PARK, COLORADO

South Park is an intermountain basin of the Colorado Rockies. Three sides developed during late Cretaceous to middle Eocene time and the fourth during the Oligocene.West side has fault block mountains capped by lower Paleozoic rocks (the Mosquito Range) on the east flank of a huge anticline that centered at the Sawatch Range. The entire basin is on east flank and has a prevailing 20-degree east dip. North mountains are capped by sills or are plutons intruded from 70 to 28 m.y. East is the flank of the somewhat younger Front Range. Part of the Front Range has been thrust 7 miles west over the deepest part of the basin. By the end of the Eocene the area was low relief hills and valleys and drainage exited south. South side was closed during the Oligocene by debris from centers of the Thirtynine Mile Volcanic Field. The third center was the very large Guffey Volcano. Mudflows from this volcano dammed the south-flowing drainage and created a large lake. The eroded remnants of this volcano dominate the southern skyline. The dams forced an outlet that was superimposed across the Front Range. Miocene epeirogenic uplift rejuvenated the mountains. The mountains above 11000 feet were occupied by Pleistocene glaciers that sculpted the scenery we enjoy today.