PALEOLIMNOLOGY OF A LATE TRIASSIC LACUSTRINE SYSTEM IN THE NEWARK CONTINENTAL RIFT BASIN, POTTSTOWN, SOUTHEASTERN PENNSYLVANIA

The purpose of this undergraduate study is to determine if valuable paleoclimatic data can be gleaned from the structurally complicated northwestern corner of the Newark continental rift basin. The exposure being studied is located near Pottstown in southeastern Pennsylvania. The layers are believed to correlate with the Metlars Member of the Passaic Formation. A set of three beds consisting of laminated, calcareous shale representing the ancient lake bottom are the focus of the project. Beds immediately above and below will also be examined to provide a frame of reference.

Generally speaking, the sediment packages are very repetitive throughout the exposure. Preliminary analysis has found evidence of wet and dry periods which other researchers have attributed to climate fluctuation in a hydrologically closed basin. Laminated beds of shale, roughly 1-2 meters in thickness, indicate quiescent settling of particles in deeper water. These are succeeded vertically by sets of massive mudstone to sandstone beds, totaling approximately 4-6 meters in thickness. The last meter is a massive and rooted mudstone, sometimes containing carbonate nodules, followed by a sharp contact with the next package of shale.

There are features that suggest episodes of extreme dry climate broken by periods of ample precipitation. Indicators of aridity include: evaporite cavities filled with secondary mineral psuedomorphs of halite, gypsum and calcite; intrasediment crystal growth of euhedral to anhedral crystals of gypsum, occasionally removed by dissolution; desiccation marks and brecciated fabric within shale laminations that are filled with material similar to vertically adjacent mudstones; lastly, ripples with deformed crests believed to be a result of efflorescent salt crust growth. Indicators of flooding and/or humidity include: abundant rip-up clasts of variable color comprised of silt; buckled and crumpled laminations with bumpy outlines and fine vertically-aggrading amorphous sand bodies; thick beds of laminated shale; abundant localized root structures; domal stromatolites perched on carbonized tree roots.

Additional data about micromorphology is being obtained using the petrographic microscope. There will be an opportunity to use x-ray diffraction and an electron microprobe to gather mineral data.