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Measured sections and discussion of the main turbidite member, Middle Pennsylvanian Minturn Formation, northern Sangre de Cristo Range, Custer and Saguache counties, Colorado

Miscellaneous Field Studies Map 1622-B

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Abstract

Turbidites are sediments deposited by turbid density currents. The turbidites described here are interpreted as prodelta deposits that formed in front of fan deltas and alluvial fans during the uplift of the ancestral Rocky Mountains in Pennsylvanian time. Laterally extensive, lenticular sand bodies deposited by turbidity flows crop out in the Middle Pennsylvanian Minturn Formation in the northern Sangre de Crísto Range, Custer and Saguache Counties, Colo. (figs. 1, 2). One of the turbidite-bearing intervals, informally designated the "main turbidite member," lies 90.6 m above the base of the Minturn; it reaches 150 m in thickness and extends more than 13 km along strike. The internal stratigraphy and sedimentary structures of the main turbidite member are described from the measured sections presented here.

The Middle Pennsylvanian Minturn Formation of the Spread Eagle Peak thrust plate (Lindsey, Johnson, and Andriessen, 1983) consists of about 2,000 m of interbedded conglomeratic sandstone, sandstone, siltstone, and lesser amounts of conglomerate, shale, and limestone. The lower 1,500 m of the Minturn is composed mainly of prograding deltaic-alluvial cycles that contain interva1s of prodelta turbidites. The upper 500 m of the Minturn consists of deposits of small deltas and bralded streams interbedded with shallow marine limestones Interpreted as a san-delta deposit (Clark, 1982). Above the Minturn Formation, the Pennsylvanian and Permian Sangre de Cristo Formation consists of more than 1,700 m of coarse conglomerate and sandstone interpreted as an a 11uvialfan deposit (Lindsey and Schaefer, 1984).

The main turbidite member is the thickest and most extensive of the intervals of prodelta turbidites in the Minturn Formation. Each turbidite interval is part of a coarsening upward sequence interpreted as a prograding fan delta. A typical prograding cycle consists of prodelta marine shale and siltstone, prodelta turbidite sandstones, delta-front sandstone and conglomerate, and deltaic and alluvial-plain sandstone, siltstone, and shale. Fossils of 1and plants (mostly Calamites, including some in growth position) are locally abundant in deltaic and alluvial sediments of the prograding cycles. The turbidites are regarded as having been deposited offshore from alluvial systems.

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Additional publication details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Measured sections and discussion of the main turbidite member, Middle Pennsylvanian Minturn Formation, northern Sangre de Cristo Range, Custer and Saguache counties, Colorado
Series title:
Miscellaneous Field Studies Map
Series number:
1622
Chapter:
B
ISBN:
0607814276
Year Published:
1984
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Reston, VA
Description:
Plate: 53.05 x 36.78 inches
Country:
United Staes
State:
Colorado
County:
Custer County, Saguache County
Other Geospatial:
Middle Pennsylvanian Minturn Formation
Scale:
42850
Online Only (Y/N):
N
Additional Online Files (Y/N):
N