Martinsburg Formation

Description

The Martinsburg is defined as a gray to dark gray, and infrequently tan and purple shale and slate. Local members of impure sandstone; thin, argillaceous limestone or phyllitic shale are present in the Cumberland Valley and the Lehigh Valley.

The Martinsburg has three defined members in the Lehigh Valley: The Bushkill Member which is the basal slate unit. The Ramseyburg Member which is composed of alternating turbidite sandstone units with interbeds of shale and siltstone. And the Pen Argyl Member which is the uppermost slate unit with some minor phyllite and shale beds.

Depositional environment

The shales (and slates) of the Martinsburg were deposited in a large forearc basin resulting in a flysch deposit. This basin was the result of a deepening sea due to the closing of the Iapetus Ocean. Turbidites are common in the Martinsburg due to underwater landslides stirring up sediments and rushing down a slope. Limestones were deposited upon peripheral bulges in the deep seas. A modern analog to the depositional history of the Martinsburg might be the Australia-East Timor tectonic setting.

Fossils

In the Pen Argyl Member, both graptolites and conodonts are found. The trilobite Cryptolithus and the brittle star Protasterina are common in the Swatara Gap area.