CHANGES IN MARTINSBURG FORMATION PROVENANCE BASED ON SPECTRAL GAMMA RAY ANALYSIS OF WELL CUTTINGS, EASTERN PENNSYLVANIA, USA

The Upper Ordovician Martinsburg Formation (~455-450 Ma) in eastern Pennsylvania accumulated in a foreland basin during the Late Ordovician Taconic Orogeny. It is subdivided into three members. The lower Bushkill Member (mostly mudstone turbidites) is gradational to the middle Ramseyburg Member (mudstone, siltstone, and sandstone turbidites). Together they form an overall coarsening upward sequence. The upper Pen Argyl Member consists of mostly mudstone turbidites. Potassium, uranium, and thorium in bulk samples (cuttings composited over 5 foot intervals) from wells drilled at the transitions between the three members of the Martinsburg formation were quantified by spectral gamma ray analysis. Two wells at the base of the Pen Argyl Mbr. showed fine sandstone (fss)=4%, siltstone (si)=13%, mudstone (md)=83%. Samples with >80% md (n=75) had K=1.7±0.3 %, U=1.3±0.6 ppm, Th=2.4±0.6 ppm. The underlying rocks of the Ramseyburg Mbr. (medium sandstone (mss)=4%, fss=15%, si=22%, md=60%) had K=2.3±0.3 %, U=2.3±1.1 ppm, Th=3.0±1.0 ppm in samples with >80% md (n=24); higher than in the overlying rocks (95% CI). Samples from a well near the base of the Ramseyburg Mbr. (fss=8%, si=9%, md=83%; K=2.3±0.3 %, U=1.8±0.9 ppm, Th=3.3±0.7 ppm in samples with >80% md (n=94)) were indistinguishable (95% CI) from the upper part of the member. Samples from a well at or near the top of the Bushkill Mbr. (mss=2%, fss=7%, si=7%, md=84%) had K=2.6±0.3 %, U=2.5±0.9 ppm, Th=3.5±0.7 ppm in samples with >80% md (n=95); higher than in the overlying Ramseyburg member (95% CI).

The three lithologic units exhibited distinct gamma ray spectra. Each was apparently deposited in response to tectonic activity within the orogen that exposed compositionally distinct source areas to erosion and transport into the foreland basin.