120 History of Geology Division and History of the Earth Sciences Society Anniversary Celebration

Tuesday, 24 October 2006
8:00 AM - 12:00 PM

THE MARTIC LINE CONTROVERSY - HOW IT STARTED

Paper number: 120-4
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM

Authors

Abstract

During mapping for the Philadelphia and Trenton folios Bascom noted the Wissahickon formation contained a mica schist and a more highly metamorphosed mica gneiss. Her 1905 GSA paper assumed the Wissahickon formation was Ordovician. By 1909, the time the folios were published, she assigned the Wissahickon mica gneiss to the Precambrian and the mica schist remained Ordovician. In 1916, she and her ex-students Jonas and Bliss proposed the Doe Run fault thrust the gneiss over the schist in the Coatesville quadrangle creating the contact between the Wissahickon mica gneiss and the Octoraro mica schist; Bascom never abandoned this interpretation. Knopf (nee Bliss) and Jonas, 1923, proposed the Precambrian Glenarm Series which included the Wissahickon mica gneiss overlain by the Octoraro mica schist thus moving the Octoraro schist to the Precambrian. In 1929 they proposed the Martic Overthrust which placed the Glenarm Series over the Ordovician Conestoga limestone. As no fault is exposed at the type locality of Martic Forge many geologists believed the fault interpretation was based on the misconception the Octoraro and Wissahickon were Precambrian and suggested the Wissahickon-Octoraro-Conestoga contact is a conformable sedimentary one.

Wyckoff notes the controversy embraces two really distinct problems, and it is not true that settling one of them would necessarily settle the other. First, there is the question of the status of the Glenarm series – are there really two series of sediments, one Precambrian and one lower Paleozoic, or are the Glenarm rocks merely the metamorphic equivalents of the Cambro-Ordovician rocks? The second question is that of the Martic Thrust itself. Is this, a purely mental construct, designed to account of the position of supposedly Precambrian rocks on top of known Paleozoic rocks? Or is there independent evidence that large scale thrusting has actually occurred? Oh, if only the ages and interrelationships of the rock units lumped into the Wissahickon formation were truly known and their structural relations to other rocks beautifully exposed.