Sandstone Framework Mineralogy as Index of Orogenic Belt Basin Evolution and as Tool for Suspect Terrane Analysis
Frederic L. Schwab
The mineralogy of sand-sized detritus has been closely tied to specific modern plate settings. Consequently, sandstone framework mineralogy can be used to index precise stages of ocean basin and continental margin development in ancient orogenic belts. Analytical examples chosen from the Appalachian/Caledonian system independently imply an early stage of continental distension and a subsequent stage of passive, Atlantic-type continental margin development as the proto-Atlantic (Iapetus) Ocean opened. Other samples from the Western (French-Italian) Alps suggest a similar (though differently timed) evolution for that system, with a better preserved record of ocean basin closure and continental collision.
Suspect terrane models, developed to explain the circum-Pacific orogenes (especially the North American Cordillera) have recently been applied to the circum-Atlantic region. Within the Appalachian/Caledonian belt, recognizing terranes (distinct tectono-stratigraphic assemblages of rock units that extend across a specific region) is easier than establishing their suspect nature, i.e., the possibility that their original paleogeographic position differed greatly from their present setting with respect to one another and to the continental block or mobile belt of which they are a part. The mineralogy of sand-sized detritus, and the contrasts in most likely provenance and plate tectonic setting that can be inferred from such data provide the most promising means of evaluating the "exotici y" of individual terranes. Examples chosen from several highly publicized Appalachian "suspect terranes" demonstrate this premise, and point out the skimpiness of useful, existing data on sandstone petrology in the Appalachians.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.