Abstract

The Lower Ordovician series (Beekmantown group) is at least 3500 feet thick near Chambersburg, Pennsylvania, approximately 1200 feet thicker than was earlier estimated. Reverse faulting in the upper part of the section may have obscured the youngest Early Ordovician and oldest Middle Ordovician beds in the area, so that the total thickness of the Lower Ordovician series may be comparable to the thickness measured in Maryland.

The Beekmantown group is represented by the Stonehenge limestone and the Rockdale Run formation. Moving the base of the Stonehenge downward to include about 200 feet of strata previously referred to the underlying Conococheague formation facilitates regional correlation without impairing mappability. Newly discovered fossils from the Rockdale Run formation add notably to the faunas of the upper part of that formation. Comparison of a trilobite-brachiopod faunule near the top of the Diparelasma zone (Cotter-Powell equivalent) with similar fossils in Zone J of the Utah-Nevada sequence suggests that Zone J may be no younger than the Powell dolomite of the Ozark region. Difficulty in recognizing correlatives of the Smithville and Black Rock formations suggests the need for restudy of these units in the Ozark region.

Evidence from the Chambersburg section neither proves nor contradicts the hypothesis of continuous deposition from Early Ordovician to Middle Ordovician time in the central Appalachian area.

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