Chapter 18
Gamma-ray, Spectral Gamma-ray, and Neutron-density Logs for Interpretation of Ordovician Volcanic Rocks, West Cumbria, England

David Millward
Simon R. Young
Brett Beddoe-Stephens
Emrys R. Phillips
British Geological Survey, Edinburgh, U.K.

Chris. J. Evans
British Geological Survey, Nottingham, U.K.

ABSTRACT

Borehole geophysical logs are a major source of information about rocks of the Ordovician Borrowdale Volcanic Group, which is concealed beneath a Carboniferous and/or Permian-Triassic cover sequence in west Cumbria, northwestern England. The volcanic succession is dominated by more than 1000 m of pyroclastic rocks, composed of thick andesitic, dacitic, and rhyolitic ignimbrites; volcaniclastic sedimentary rocks, intrusive andesite sheets, and thin dikes of basalt and rhyolite are subordinate constituents. Comparison of lithologic logs with gamma-ray, spectral gamma-ray, and neutron-density geophysical logs from 22 fully cored boreholes provides a robust interpretation of many aspects of the volcanic facies, lithostratigraphy, geochemistry, and mineralogy. Distinctive gamma-ray and spectral gamma-ray responses characterize volcanic units that vary from substantial lithostratigraphic units several hundred meters thick to marker beds only a few meters thick. Gamma-ray and spectral gamma-ray logs indicate the bulk chemical composition of the rocks, and show that some ignimbrite sheets are internally homogeneous, whereas others are zoned geochemically. Detailed comparison of the same lithologic units and their gamma-log responses in multiple boreholes facilitates interpretation of complex lateral variation in thickness and facies. Wireline-log responses allow recognition of dikes where these differ distinctly in composition from country rock, and they facilitate correlation among closely spaced boreholes. Bulk-density and neutron-"porosity" logs aid in identification and characterization of mineralogically and chemically altered zones within cleaved, unwelded lithofacies. In rocks of this kind, this combination log is also an indicator of lithology.