Bedrock Geology of the Shin Pond and Stacyville Quadrangles Penobscot County, Maine GEOLOGICAL SURVEY PROFESSIONAL PAPER 524-1 Bedrock Geology of the Shin Pond and Stacyville Quadrangles Penobscot County, Maine By ROBERT B. NEUMAN SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 524-I A description of the lower Paleozoic stratigraphy and structure east of Mount Katahdin UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page Abstract__________________________________________ _ I 1 Stratigraphy-Continued Introduction ______________________________________ _ 1 Silurian System-Continued Purpose and major findings of the study __________ _ 1 Northwestern sequence-Continued History of investigations ________________________ _ 2 Volcanic rocks of Late Silurian or Early Fieldwork and acknowledgments _________________ _ 2 Devonian age _______________________ _ I 21 Regional setting _______________________________ _ 3 Southeastern sequence ______________________ _ 22 Glacial geology ________________________________ _ 3 Frenchville Formation _____ ~ ____________ _ 22 Distribution of bedrock exposures ________________ _ 4 Allsbury Formation ____________________ _ 23 StratigraphY--------------------------------------- 4 Sandstone member_________________ _ 23 Nomenclature _________________________________ _ 4 Slate member _____________________ _ 24 Cambrian(?) System ___________________________ _ 5 Unnamed conglomerate and associated Grand Pitch Formation ____________________ _ 5 rocks_______________________________ _ 25 Ordovician System_____________________________ _ 7 Conglomerate ______________ ~ ______ _ 25 Shin Brook Formation_________ !,. ____________ _ 7 Breccia ___________________________ _ 26 Unnamed volcanic rocks (greenstone) ________ _ 8 Limestone _____________ -- _____ --- __ 26 ~etadiabase ______________________________ _ 10 Devonian System______________________________ _ 27 Wassataquoik Chert_ ______________________ _ 11 Seboomook Formation _____________________ _ 27 Conglomerate and sandstone ________________ _ 13 ~atagamon Sandstone _____________________ _ 27 Ordovician or Silurian System___________________ _ 14 Katahdin Quartz ~onzonite ________________ - 28 Rockabema Quartz Diorite __________________ _ 14 Granophyre and diabase__ !__________________ - 29 Silurian System _______________________________ _ 15 Structure _________________________________________ _ 30 Northwestern sequence _____________________ _ 15 Northwestern syncline __________________________ _ 30 Conglomerate of Early Silurian (late Lland- Slate belt _________________________________ . ___ _ 31 overy) age __________________________ _ 15 Central anticlinorium _____________________ . ____ _ 31 Calcareous siltstone of Early or Late Fault zone between slate belt and central anti- Silurian (late Llandovery to Wenlock) clinorium ___________________________________ _ 33 age ________________________________ _ 18 Katahdin batholith ____________________________ _ 34 Limestone of Late Silurian (Wenlock or Paleogeographic interpretations ______________________ _ 34 early Ludlow) age ___________________ _ 19 References ________________________________________ _ 36 Calcareous siltstone of Late Silurian (early Ludlow) age ________________________ _ 19 ILLUSTRATIONS [Plates are in pocket] PLATE 1. Geologic and aeromagnetic map of the Shin Pond quadrangle. 2. Geologic and aeromagnetic map of the Stacyville quadrangle. 3. Explanation of the geologic and aeromagnetic maps of the Shin Pond and Stacyville quadrangles. Page FIGURE 1. Sketch map of type locality of the Wassataquoik Chert________________________________ I 11 2. Sketch map of Silurian sequence along Bowlin Pond Road_____________________________ 17 3. Photograph of folds in the Grand Pitch Formation-----------------~------------------ 32 m SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY BEDROCK GEOLOGY OF THE SHIN POND AND STACYVILLE QUADRANGLES PENOBSCOT COUNTY, MAINE By RoBERT B. NEuMAN ABSTRACT INTRODUCTION The bedrock of the Shin Pond and Stacyville quadrangles, PURPOSE AND MAJOR FINDINGS OF THE STUDY northern Penobscot County, Maine, is formed by Cambrian(?) to Lower Devonian sedimentary and igneous rocks in the chlorite Geologic mapping and related laboratory investiga­ zone of regional metamorphism and the nonmetamorphosed tions in the Shin Pond and Stacyville quadrangles have Katahdin Quartz Monzonite. yielded new information on stratigraphy, paleontology, Complexly deformed slate, quartzite, and related rocks of the igneous history, and structure that provides keys to the Lower Cambrian(?) Grand Pitch Formation are unconformably interpretation of the geologic history of northern Maine. overlain by tuffaceous sandstone, tuff, and flows of the Lower or Middle Ordovician Shin Brook Formation in an anticlinorium The salient structural feature of the area is a large in •the central part of the area. Ordovician greenstone, largely southward-plunging anticlinorium in which Lower of extrusive origin, and the Middle Ordovician Wassataquoik Cambrian ( ?) and, Ordovician sedimentary and vol­ Chert (new) lie on the eastern and southern flanks of the anti­ canic rocks and Late Ordovician or Early Silurian in­ clinorium. The Rockabema Quartz Diorite, altered and locally trusive rocks are exposed. The Silurian sedimentary sheared and mylonitized, intrudes the Grand Pitch Formation and the Ordovician greenstone. sequence on the northwestern flank of this anticlinorium Upper Ordovician and Lower Silurian conglomerate on the includes distinctive calcareous sedimentary and volcanic northwestern flank of this anticlinorium and Lower Silurian rocks, whereas correlative rocks on the southeastern conglomerate and sandstone on its southeastern flank were prob­ flank consist of a contrasting assemblage of slate, ably derived from an ancestral uplift in the position of the sandstone, and conglomerate. A major fault system on present structure. The presence of this anticlinorium through­ out the Silurian is indicated by the contrast in facies of Silurian the southeastern flank of this fold has complexly rocks on opposite flanks. Distinctive rocks on the northwest shuffied these rocks. The Katahdin Quartz Monzonite, are calcareous siltstone, limestone, and volcanic rocks, whereas the youngest unit in the area, cuts across all structures; on the ·southeast, noncalcareous slate and siltstone (A.llsbury that this rock was emplaced by stoping is indicated by Formation) predominate. the breccia zone along its contact. Lower Devonian rocks, consisting of dark slate and siltstone of the Seboomook Formation and the thick-bedded Matagamon The area was selected for mapping because it seemed Sandstone, overlie Silurian rocks to the northwest of the anti­ potentially rewarding for the deciphering of pre­ clinorium, but not those to the southeast. Silurian stratigraphy. Dodge (1881), long ago, found The tectonic history of the area is complex. The Grand Pitch Ordovician graptolites in exposures along W assata­ Formation was first deformed before the deposition of the Shin quoik Stream in the central part of the Stacyville quad­ Brook Formation. This folding event, here named the Penobscot disturbance, probably affected a widespread area through Maine rangle; Smith (1928) found Oldhantia, which he be­ and southeastern Quebec. The ancestral anticlinorium of the lieved to be a Cambrian fossil, at Bowlin Falls of the Shin Pqnd-Stacyville area probably dates from the later East Branch of the Penobscot River in the southeastern Ordovician Taconic orogeny, as does the intrusion of the Rocka­ part of the Shin Pond quadrangle; and Boucot bema Quartz Diorite. Acadian (Middle Devonian) orogeny identified the Silurian-pre-Silurian boundary, trending considerably modified the central anticlinorium and the adjoin­ ing basins. A. complex pattern of folds and faults, including a diagonally across the northern part of the Shin Pond large right-lateral strike-slip fault system along its southeastern quadrangle, in the course of reconnaissance mapping flank, was formed during this event, and the area was regionally ( Boucot, Griscom, and others, 1964) . Identification of metamorphosed at the same time. The posttectonic Katahdin this boundary in detail and the determination of the Quartz Monzonite, which has a wide bordering breccia in this area, cuts across all these structures and represents the final relations of Dodge's and Smith's localities to it were phase of Acadian o:r:ogeny. the original goals of the investigation. 11 12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY The sequence of dark slate and chert in which Dodge cock, 1862, p. 359-376) traveled by road from Patten found the graptolites, named W assataquoik Chert in to Snowshoe Lake and Scraggly Lake, making geologic this report, is in a fault block on the southeastern flank observations incidental to his primary purpose of col­ of the anticlinorium. Rocks bearing the Oldhamia, the lecting zoologic specimens. Most of his remarks con­ Grand Pitch Formation (Neuman, 1962), form the core cern the physiography, but he commented on the of the anticlinorium. The Grand Pitch Formation lies distribution of the "Helderberg limestone," giving beneath Silurian rocks traced by Boucot along the localities where it had been seen. northwestern flank of the