Age dating of basalt

03-Sep-2019 12:14

Its upper part forms nearly continuous low cliffs that are parallel to the general course of the Colorado River. The Unkar Group is about 1,600 to 2,200 m (5,200 to 7,200 ft) thick and composed, in ascending order, of the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt.The most complete, readily accessible, and easily studied exposure of the Cardenas Basalt lies in Basalt Canyon. In ascending order, the Cardenas Basalt is overlain by the Nankoweap Formation, about 113 to 150 m (371 to 492 ft) thick; the Chuar Group, about 1,900 m (6,200 ft) thick; and the Sixtymile Formation, about 60 m (200 ft) thick.Within the lower part of the Cardenas Basalt, the lava is highly altered and might have been glassy at one time.Near the top of the lower unit the basalt is more massive and less altered.It is about 200 m (660 ft) thick and contains four to six, prominent lava flows that range in composition from quartz tholeiite to tholeiitic andesite (icelandite).

The Cardenas Basalt has also been called the Rama Formation.In addition, in places, the uppermost 60 cm (2.0 ft) of the Dox Formation is mildly baked.A thin lava flow occurs within the uppermost part of the Dox Formation.Along the length of the outcrop of this unconformity, it cuts as much as 100 m (330 ft) down into the Cardenas Basalt.The lowest part of the Nankoweap Formation consists of a basal conglomerate that is composed chiefly of gravel derived from the Cardenas Basalt.

The Cardenas Basalt has also been called the Rama Formation.In addition, in places, the uppermost 60 cm (2.0 ft) of the Dox Formation is mildly baked.A thin lava flow occurs within the uppermost part of the Dox Formation.Along the length of the outcrop of this unconformity, it cuts as much as 100 m (330 ft) down into the Cardenas Basalt.The lowest part of the Nankoweap Formation consists of a basal conglomerate that is composed chiefly of gravel derived from the Cardenas Basalt.The basal contact of the Cardenas Basalt with the underlying Dox Formation is smooth, planar, parallel to bedding and locally interfingering.