QUANTITATIVE MEASUREMENTS AND SPATIAL STATISTICS OF IGNEOUS LAYERING IN THE MORGANTOWN SHEET, SE PA

Quantitative analysis of cm- to decimeter-scale modal layering in the Jurassic Morgantown Sheet (located in SE PA and associated with the failed rift of the Central Atlantic Magmatic Province) is consistent with magmatic flow, settling of crystal rafts from the chamber roof, and compaction of crystal mush. Plagioclase-rich layers commonly overlie orthopyroxene-enriched layers within diabase cumulates exposed in the walls of a dimension stone quarry 100-150m above the inferred bottom contact of the ~500-m thick sheet. Thick dark layers parallel to plag layers and with lower cuspate borders are interpreted as replenishing flows into basal crystal mush; dark zones cross-cutting the layers are interpreted to be channels of mobile magma. Sorting of crystals by magmatic flow (e.g., Marsh, 2006, Elements v. 2, 287-92) could explain localization of plag- and opx-rich layering near a replenishing feeder.

A total of 1452 plag layers were digitized on photographs of quarry walls oriented normal to strike (SN, 4.9 m x 3.1m) and parallel to strike (SP, 4.5 m x 3.7 m), and were grouped into 3 categories, with Series 1 being the most distinct (greater plag proportion). Analysis of layer sequence along vertical traverses shows a non-random distribution with Series 2 and 3 layers preferentially occurring between Series 1 layers. Layer dips average ~0° (horizontal) on the SP wall and ~18°N on the SN wall; dips of layers and cross-cutting dark channels suggest post-magmatic tilting of roughly horizontal layering by ~20° north. Total length range is 3 cm to ~3 m and average lengths vary with Series 1 > Series 2 > Series 3 for both walls; the SN wall contains more long layers. Most layers on both walls are concave-upward with low sinuosity. Layer length distributions are consistent with random sections through many “wok-shaped” layers with median dimensions of 0.27 m on the SP wall and 0.4 m on the SN wall. Settling of crystal rafts formed at the chamber roof could result in layers with upward concavity reflecting the shape of initial thermal boundaries (Philpotts & Dickson, 2002, J. Str. Geol. v. 24, 1171-7). Plag layers are deflected upward more than downward (78% vs. 22%) next to cross-cutting dark channels, possibly due to migrating liquids from compaction of crystal mush.