TEMPORAL AND GEOGRAPHIC TRENDS IN A SYNOROGENIC CONGLOMERATE

The Upper Ordovician, Lost Run Member of the Bald Eagle Formation of central Pennsylvania is a synorogenic conglomerate. As the only Ordovician synorogenic conglomerate in the central Appalachians, it contains information about the dynamics of the Taconic orogeny that is not otherwise accessible. To better understand the Late Ordovician Appalachian orogen, we prepared measured sections of the Lost Run Member throughout the outcrop belt, completed 300-clast counts in the field, and petrographically examined clasts. As a result of this study, we are able to recognize temporal and geographical trends in this unit.

Clast counts show that vein quartz clasts are most abundant, while sandstone clasts are common as well. Although less abundant than quartz and sandstone clasts; siltstone, slate, and chert clasts are also typical of this conglomerate. Apparent trends across the outcrop belt include two major pulses of lithic clasts in the lower half of the Lost Run Member separated by a pulse of predominantly quartz clasts.

The most conglomeratic interval of the Lost Run Conglomerate is in the southeastern portion of the outcrop belt, near Harrisburg, where most of the rock is clast supported. The ratio of quartz clasts to other clasts at Harrisburg is as low as 1:8 in some portions of the section. Chert and slate clasts from the Harrisburg cut appear identical to rocks found within the Taconic allochthon (Hamburg Klippe), which crops out less than ½ mile east of the Harrisburg exposure. Additionally, some intervals were represented by predominantly red sandstone clasts (similar to the Juniata facies); and clasts of conglomerate that, in turn, contain sandstone clasts are characteristic of proximal Lost Run deposits. These proximal conglomerates provide evidence of cannibalization of older molasse, and suggests a structurally complex source area in which both Klippe material and recycled molasse were being eroded. Distal conglomerates contain higher proportions of quartz clasts and significantly smaller pebbles.