PETROGRAPHIC ANALYSIS OF THE MISSISSIPPIAN (CHESTERIAN) LOYALHANNA MEMBER OF THE MAUCH CHUNK FORMATION, KEYSTONE QUARRY, SOUTHWESTERN PENNSYLVANIA

The Mississippian (Chesterian) Loyalhanna Member of the Mauch Chunk Formation consists of mixed carbonate-siliciclastic sediments in southern Pennsylvania. At the Keystone Quarry, near Springs, Pennsylvania, a 15-m thick section of the Loyalhanna Member is interpreted as eolian dune, sabkha, and fluvial (wadi) deposits, and a paleosol with calcrete (Bk horizon), all of which formed in a semi-arid to arid climate. At thin-section scale, the eolian dune facies consists of laminae of medium- to fine-grained sand alternating with laminae of very fine-grained sand to silt. The sabkha facies consists of discontinuous laminae of medium-grained sand to silt. The fluvial facies is structureless coarse-grained sand to silt. The paleosol is structureless coarse-grained sand to silt with 0.5-1 cm thick micrite-rich laminae. Initial petrographic studies reveal that the eolian, sabkha, and fluvial facies consist primarily of framework grains and cement, and very little matrix or porosity. Framework grains are composed of mono- and polycrystalline quartz and calcite (peloids, broken and abraded skeletal grains, whole and broken ooids, and rare wackestone lithoclasts). Chert and minor feldspar, mica, and heavy minerals are also present. Carbonate and siliciclastic framework grains are present in relatively equal proportions in the eolian facies, siliciclastic grains predominate in the fluvial facies, and carbonate grains predominate in the sabkha facies. Petrographic characteristics of the eolian facies considered diagnostic of carbonate-rich eolian sediment include abundant quartz silt, diverse carbonate grain types (including broken ooids), lack of skeletal particles >4mm, evidence of solution packing, and lack of porosity. Petrographic characteristics of the paleosol include micrite matrix and microspar cement. Micrite-rich laminae in the paleosol have irregular upper and lower boundaries and contain floating framework grains. In all facies, carbonate grains are more rounded than siliciclastic grains, and straight and irregular grain contacts indicate that some compaction occurred prior to cementation. Intergranular areas are filled with spar, microspar, and local iron oxide, as well as late-stage fibrous calcite grain overgrowths that fill strain voids on the tops and bases of grains.