

Journey to a ~500 Ma Ooid Shoal-Shoreface Sand-Microbial Reef System
Ever wonder what the outermost extent of the Laurentian shelf looked like in the Cambrian? Wonder no more...
LWB Refractories Quarry is a 240 foot deep, 86.5 acre surface mining operation in York County, Pennsylvania extracting high purity dolomite used in the manufacture of refractory brick. The deposit was discovered in 1946 and active mining commenced in 1950. The two major carbonate facies include a massive ooid shoal and neighboring microbial patch reefs. These deposits accumulated under high energy conditions at the shelf margin of the Laurentian continent during the Middle Cambrian (500 to 520 Ma). A modern analog is the well-studied, Late Pleistocene Great Bahama Bank. Exposure of the ancient patch reefs in-situ makes this location remarkably unique. Such Cambrian reef deposits have been noted as off-shelf slump blocks at other locations, but have rarely been observed in place. No other location in Pennsylvania is known to have in-situ exposures of the Ledger reef facies (de Wet, personal communication).
Contrasting permeability characteristics between the facies transformed the ooid shoal into dolomite while leaving the relatively impervious reef facies as limestone. At least three stages of dolomitization have altered the ooid shoal substrate. One initial phase occurred early on and is believed to be related to periods of subaerial exposure resulting from fluctuating sea levels. Evidence of such emergent conditions are not pervasive in the deposit, but have resulted in a shoreface interpretation for one area flanking the ooid shoals (de Wet, personal communication).
Other interesting exposures at the quarry include the Ledger Formation "white marble", Triassic fanglomerate, and the off-shelf Kinzers Formation. The "white marble" comprises a series of dedolomitized zones believed to be associated with Triassic extensional faulting that marked the beginning of tectonism associated with the break-up of Pangaea. The fault zones were higher permeability and preferentially exposed portions of the Ledger dolomite to circulating calcite-rich fluids (de Wet, personal communication). In some areas, undersaturation of migrating fluids opened up cavities and vugs (paleokarst) that later received an influx of Triassic hematitic sediment (de Wet and Hopkins, 1). A fault contact between the Ledger and the Triassic fanglomerate facies is revealed near the northwestern corner of the quarry (de Wet, personal communication). Such conglomeritic deposits were known to have formed the northern boundary of the Gettysburg basin, a lacustrine setting during most of the Triassic (Smoot, 198-199). The fanglomerate is composed of coarse clastic detritus derived from the Ledger and Triassic sediments and is cemented with mud- to silt-sized hematitic material (de Wet, personal communication). Finally, the Kinzers Formation, which is exposed at the base of the quarry, is well known for its lowermost member, the Emigsville, a deposit of exceptional preservation that captured the "Cambrian Explosion" (Skinner, 2005).
For a more detailed discussion of the deposits being mined at the LWB Refractories Quarry and their origins along with some excellent figures, please check out Dr. Carol de Wet's website at:
http://www.fandm.edu/x12035.xml
For more pictures from the field trip and reference information navigate to:
http://udel.edu/~spelepko/my_homepage/
(select links near lower left hand side of page)