HYDROGEOLOGIC CHARACTERIZATION OF FRACTURED 
SEDIMENTARY BEDROCK IN THE HARTFORD RIFT BASIN 
USING BOREHOLE-GEOPHYSICAL TECHNIQUES 

	STONE, Janet Radway,  STARN, J. Jeffrey,  MELVIN, 	
		Robert L.,  U. S. Geological Survey, 450 Main St., 	
		Hartford CT 06103

Recent investigations were conducted by the U. S. Geological Survey 
at two ground-water contamination sites, one in the Portland 
Formation in Durham, Connecticut (Melvin and others, 1995) and one 
in the New Haven Arkose in Cheshire, Connecticut (Stone and others, 
1996).  Borehole-geophysical logging, coupled with geologic analysis 
and hydraulic testing, reveal that bedding-parallel partings and 
fracture planes provide the major pathways for ground-water flow in 
the sedimentary bedrock of the Hartford Basin. The studies also 
indicate that only a small number of these geologic features are 
hydraulically significant. Ground water moves along some 
subhorizontal, bed-parallel features such as bedding-plane partings at 
the base of coarse channel-sandstone units and fissile zones within 
finer-grained mudstone units, and moves across bedding planes along 
open, high-angle fractures. The direction of ground-water flow and 
contaminant transport within the bedrock is greatly affected by the 
resultant geometry of intersecting bedding planes and fracture planes. 
	Borehole-geophysical logs used to interpret lithology, fracture 
type and fracture orientation include natural gamma, formational 
resistivity, caliper, TV camera, and acoustic-televiewer. At the 
Cheshire site, in the New Haven Arkose, logs showed a strong inverse 
correlation between gamma and formational resistivity, and 
comparison to rock core indicated that gamma values were a good 
indicator of textural variation between beds. In the Portland 
Formation, in the Durham Center area, the presence of higher amounts 
of K-feldspar in many textural units, precluded the use of gamma logs 
as an indicator of grain-size. In the boreholes at Durham, the highest 
gamma peaks were commonly present where high-angle fractures 
intersect the borehole wall. At both localities, the downhole TV log 
provided important lithologic and fracture information, but acoustic 
televiewer logging was required to determine the orientation of 
fractures and bedding. Borehole-geophysical logs used to identify 
significant water-bearing zones in bedrock wells include fluid 
conductivity, fluid temperature and heat-pulse flowmeter. Flowmeter 
logging was done under natural, nonstressed conditions, and single- 
and multi-well logging was also done under pumping conditions to 
interpret hydraulic connectivity of fractures.
     At both Cheshire and Durham, rock units strike nearly N-S and dip 
gently eastward at <5o to 20o. In Cheshire, fracture orientations were 
measured in outcrop and in four bedrock wells. The dominant fracture 
set strikes NNE and dips steeply WNW, perpendicular to bedding; the 
paleovertical orientation appears to be a result of syndepositional, rift-
basin extension. These high-angle fractures have limited vertical 
extent and occur mostly in the more competent channel-sandstone 
units, extending only locally into the mudstone units. In Durham, 
high-angle fractures measured in 13 bedrock wells strike dominantly 
NE and dip both NW and SE.  Fracturing at this location is associated 
with postdepositional faulting; individual high-angle fractures have 
greater vertical extent and are more significant hydraulically than are 
bed-parallel features. The investigation strongly indicates that a NE-
striking, NW-dipping normal fault (shown by mapped offsets in 
Hampden Basalt to the west on geologic maps) extends into the 
Portland Formation through the Durham Center area, where we refer 
to it as the Ball Brook Fault. Yields of bedrock wells penetrating the 
fault zone are anomalously high.

Melvin, R. L., Stone, J. R., Craft, P. A., Lane, J. W., and Davies, B. S., 1995,                
Geohydrology and water quality of the Durham Center area, Durham, 
Connecticut: U.S. Geological Survey Water-Resources Investigations Report 
94-4237, 97 p. 

Stone, J. R. , Barlow, P. M., and Starn, J. J., 1996, Geohydrology and conceptual 
model of a ground-water flow system near a Superfund site in Cheshire, 
Connecticut: U. S. Geological Survey Open-File Report 96-162, 88 p.