"Haswell Hole", A Previously Unknown Impact Structure in Southeast Colorado

S. Parker Gay, Jr.

A high-resolution aeromagnetic survey of southeast Colorado flown by Applied Geophysics, Inc. in 1984 shows a circular area of low residual magnetic intensity straddling the border of Bent and Kiowa Counties. The circular feature is 22 miles (35 kms) in diameter and is interpreted as an impact structure of late Precambrian age. The host Precambrian basement rocks here, as deduced by the late Ogden Tweto, are ^approx1.4 Ga granites under the south half and ^approx1.8 Ga quartzites and schists under the north half. The magnetic low in the center may result from infill of ^approx1.0 Ga Late Proterozoic "slightly metamorphosed sedimentary and volcanic rocks" that occupy basement grabens 30 miles to the southwest, according to the basement mapping. However, no basement well in ercepts lie within the structure.

Studies of the overlying sedimentary section are currently being undertaken to determine what effects the ancient impact may have had on oil and gas production. Two types of effects may have occurred: 1) the structure of the sedimentary section may mimic, through gravitational compaction, the underlying basement topography, which could include a rim anticline and rim synclines, and 2) re-compaction of the infilling Precambrian sediments, if they exist, could have occurred in post-Precambrian time, lowering the relative land surface and affecting fluvial, lacustrine, or marine deposition. Four Morrow gas fields - Haswell, Salt Lake, Colt, and Bent's Fort - occupy the area of the supposed ring syncline interior to the interpreted rim and may result from the Morrow fluvial system following Mississippian-age topographic lows resulting from compaction by either of the two mechanisms mentioned.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California