Reorientation mechanisms of phyllosilicates in the mudstone-to-slate transition at Lehigh Gap, Pennsylvania

摘要: Abstract The mudstone-to-slate transition of the Martinsburg Formation at Lehigh Gap, Pennsylvania, was re-examined using a new transmission-mode X-ray texture goniometer, supplemented by SEM, XRD, and optical studies. Three mesoscopic zones are recognized in outcrop: (I) mudstone, (II) transition, (III) slate zone. In mudstone zone, mica basal planes parallel to bedding whereas preferred orientation chlorite is up 30° shallower than bedding. angular difference between decreases towards becoming subparallel ca. 50 m from contact with overlying Shawangunk Formation. orientations both intermediate cleavage orientations, which consistent mechanical reorientation phyllosilicates. This supported decrease March strain minimum 95 for chlorite. SEM observations similarly show importance grain rotation large detrital grains. cleavage. Chlorite zone have high Fe contents, low-Fe Mg > dominate indicates that dissolution-neocrystallization dominant mechanism Thus, grains important early stages development, dissolution—neocrystallization occurring all more evolved stages.