Crystalline bedrock geology of the lower Susquehanna Gorge: Conowingo to Havre de Grace, Maryland

Abstract

The crystalline bedrock of the lowermost Susquehanna River Gorge, Conowingo to Havre de Grace, Maryland, consists of two discrete structural blocks, each with its own unique history prior to juxtaposition. The southern Havre de Grace Block is a Cambrian magmatic arc association (James Run Formation and Port Deposit Intrusive Complex), possibly developed on a rift fragment of Laurentia (Canal Road Formation). The northern Conowingo Block represents a precursory mélange (Conowingo Dam Formation) intruded during the Ordovician by a layered mafic complex (State Line Mafic Complex) and associated plutons (Basin Run Tonalite). Many olistoliths within the Conowingo Dam Formation could have originated in the Havre de Grace Block. The Havre de Grace Block was metamorphosed and deformed under amphibolite facies conditions during Middle Ordovician to Early Silurian time. It was subsequently thrust upon the Conowingo Block along the Elbow Branch Thrust, probably during the Late Ordovician or Early Silurian. From as early as the Middle Devonian, through the Pennsylvanian, dextral strike-slip shearing took place under greenschist facies conditions along the Rock Run Shear Zone, within the northern part of the Havre de Grace Block. Analysis of SC fabrics in the zone yields a minimum offset of 2 to 6 kilometers. From Late Paleozoic to Mesozoic time, strike-slip shearing gave way to dip-slip shearing across very thin, low grade, ductile shear zones.