GEOPHYSICAL LOGS AND LITHOLOGY FROM MANSON IMPACT STRUCTURE

by
P.H. Nelson, J. Mikesell, J.E. Kibler, R.R. Anderson, and B.J. Witzke

The Geological Society of America
1994 Annual Meeting
Seattle, WA, October 24-27, 1994
1994 Program with Abstracts, v. 26, no. 7, p. A-403

ABSTRACT


We compare gamma-ray and neutron logs with lithologic description and classification of impact materials obtained from five shallow cored boreholes drilling during 1991 and 1992 within the Mason Impact Structure at Manson, Iowa.

The gamma-ray response separates the identified classes of impact materials into two broad categories. Low gamma-ray response occurs within the two sedimentary classes of Phanerozoic Clast Breccia and Overturned Sedimentary Strata. High gamma-ray response occurs in those classes which contain a high fraction of crystalline material, either in clast or matrix form, and in Keweenawan shale breccia.

The neutron response, presented in terms of total water content, serves to differentiate lithologic type within these two low-gamma/high-gamma categories. Within the low-gamma category, water content is lowest in dolomite and increases progressively from sandstone to siltstone to shale. Within the high-gamma category, water content is lowest in gneiss, increases in the breccias, and is highest in the Keweenawan shale breccia.

Granite fragments are recognizable by extremely high gamma-ray spikes. Gneiss produces low neutron and intermediate gamma responses, so that gneiss fragments can be recognized within shaly breccias. Crystalline-clast breccias are characterized by intermediate neutron response. Blocks of shale and shaly breccia are characterized by high neutron and high gamma-ray responses.

In summary, variations in natural radioactivity and water content measured by the gamma-ray and neutron logs provide a supplemental means of discriminating among the lithologies described from core.