Geologic Features of Urling Mine Complex, Keystone Coal Company, Indiana County, Pennsylvania
Stephen J. Harvan
The Urling mine complex of the Keystone Coal Mining Company exhibits a variety of geologic features that adversely effect the mineability of the Upper Kittanning coal seam. Seven features were chosen to illustrate the disruption of coal quality, coal continuity, and roof stability. Interpretations are based on in-mine mapping and core-hole information.
Paleochannels remove some or all of the coal seam by erosion. Other problems can be an increase in roof water, an increase in ash and sulfur content, and roof instability owing to compaction or slumping. Small paleochannels, narrower than the average drill-hole spacing, are especially very difficult to predict.
A crevasse-splay deposit consisting of a hard flat-bedded sandstone actually improves roof stability in the Urling 1 mine. However, channel rolls associated with this splay deposit cause problems typically found with other paleochannels.
A severe depositional dip is caused by the compaction of a thick bottom bench. A vertical drop of 20-30 ft is expected. Consequently, a paleochannel is attracted to the topographic low and will cause the usual mining problems.
A coat split occurring in Urling 3 marks the end of the mineable field. A shale parting rapidly expands from 0 to 3 ft over the entry width (20 ft). The top bench thins to zero as it continues to rise to about 15 ft above the bottom bench.
Multiple coal splits occur on the east side of Urling 1. The numerous shale partings gradually increase in number and thickness to the extent that the ash content is 60% near a channel.
Shale rolls and unstable roof of unknown origin occur in a wide zone west of the Urling mines. In Urling 3, attempts have been made to mine the area with little success. The immediate roof consists of about 1 ft of a rashy clay and coal mixture that grades into a dark gray shale with slickensides.
Clay veins (clastic dikes) are a common occurrence in most mines in the region. There are areas where the density of clay veins is to great as to hinder mining. Such an area was mined in 1951 in the Kent 1 mine. Clastic dikes break the roof beam created by bolting and cause unstable conditions.
These seven geologic features have a dramatic effect on the present and future shape of the Urling mines. Knowledge of their cause and extent is an invaluable aid to mine planning.
AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.