CAPE ST. MARY'S

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GEOLOGY

NEWFOUNDLAND CANADA

 

Newfoundland is split into 4 Tectonostratigrapic Zones, Cape St. Marys is located in the coastal portion of the Avalon Zone, which is the most eastern zone out of the 4. The majority of the rock strata located at Cape St. Mary's is mainly subaerial sedimentary rocks. 

Techtonostratigrapic Zones (East to West)

1. Avalon Zone

2. Gander Zone

3. Dunnage Zone

4. Humber Zone

 

Cape St. Mary's is geologically located in the Northeastern section of the tectonostratigraphic Avalon zone of the Appalachian Orogen. Most rocks around the Cape St. Mary's Peninsula are Ediacaran age (635 Million years old) and Cambrian age (485 Million years old). Volcanic Sills of intrusive Silurian magma are also apart of the main mass of the Cape St. Mary's Peninsula.    

The Silurian Sills that were intruded in the Siluro-Devonian times (419 - 358 million years old) and were then folded and faulted, these sills were later intruded by basic dykes possibly in the Triassic age (252 - 201 million years old). These dykes form the border of the tectonostratigraphic Avalon Zone which lies next to the more Western Gander Zone.

ROCK LAYERS

Both layers visible along the Cape St. Marys Peninsular are of Ediacaran age (635 - 541 million years old), the layers are part of the Musgravetown group which contain the oldest volcanic rocks which are within the Bull Arm Formation with the slightly younger sedimentary Big Head Formation lying above.

 

The youngest part of the Musgravetown group is the Crown Hill Formation which contains red conglomerate and other unusually bright red siltstones along with dull red arkose sedimentary rocks. The Crown Hill Formation is not visible in the Cape St. Mary's park. 

The Nearby town of St. Bride's has a good example of the younger bright red Brigus Formation, it was deposited in the early to middle Cambrian age (510 - 490 million years old). It is a mix of red mudstone interbedded with green and purple mudstone along with fossiliferous nodular limestone. 

Brigus Formation Viewed from the town of  Saint Bride's along the Muserat Brook

Bull ARM FORMATION

The oldest and lowest rocks along the Peninsula are bimodal volcanic rocks, which is lava that's Rhyolite (Felsic) and Basalt (Mafic) from the same volcanic centre. The volcanic rocks formed underwater as submarine volcanoes, they erupted within a shallow-marine setting. 

The portion of the volcanic Bull Arm Formation that is visible along the Cape St. Mary's cliffs contain andesites, basalts and siltstone. Green and grey-green arkoses, which are a type of sandstone that contains at least 25% feldspar, along tuffaceous arkoses which are a slightly different composition as it contains volcanic ash.

Majority of the rocks are interbedded volcaniclastic rocks, which means that they contain some form of volcanic debris like felsic ash-flow tuffs and pyroclastic sediments. 

Only 60 meters (196 ft) are visible of the formation along the Cape St. Mary's Peninsula, there has been no detailed study of the volcanic layers in the Bull Arm Formation so the local sequence at Cape St. Mary's is mostly unknown. 

BIG HEAD FORMATION

The younger Big Head Formation is comprised of siltstone and green sandstone but at the base of the formation, it's interbedded with volcanic breccia from the Bull Arm Formation. The top of the formation is comprised of thinner bedded rock compared to the lower portion.

 

Many of the beds have ripple-marked tops from the past ocean setting. Most of the sandstone is well sorted and is classified as feldspathic which is a sandstone that contains less than 75% quartz and the rest being feldspar. The lower layers have been metamorphosed as it's at the contact of the volcanic Bull Arm Formation.

The upper portion of the Formation contains the less common arkosic greywackes which are visible by the whiteness of weathered rocks, arkosic sandstone contains at least 25% feldspar while greywackes are hard, dark and poorly sorted sandstone in a clay matrix. This is more typical of the greenish sandstone at the top of the Big Head Formation.

REFEreNCES

Terence Patrick Fletcher BEDROCK GEOLOGY OF THE CAPE ST. MARY’S PENINSULA, SOUTHWEST AVALON PENINSULA, NEWFOUNDLAND

(INCLUDES PARTS OF NTS MAP SHEETS 1M/1, 1N/4, 1L/16 and 1K/13)

Report 06-02 St. John’s, Newfoundland 2006

Bedrock of the Cape St. Mary's Peninsula, Southwest Avalon Peninsula, Newfoundland, Map 2006-02, (Includes parts of NTS map sheets 1M/1, 1M/4, 1L/16 and 1K/13), Report 06-02, Newfoundland and Labrador, Department of Natural Resources Geological Survey  

Andrea Mills, Logistics Coordinators: Stephanie Lode and Anne Westhues  DEPOSITIONAL SETTINGS AND TECTONIC EVOLUTION OF ROCKS OF THE BONAVISTA PENINSULA: NEW CONSTRAINTS AND QUERIES FOR AVALONIA

2017 FALL FIELD TRIP September 29 – October 1, Open File 002C/0237  GEOLOGICALASSOCIATION OF CANADA NEWFOUNDLAND and LABRADOR SECTION

R.J. Bullock, J.R. Morris and, D. Selby NEW FINDINGS OF BODY AND TRACE FOSSILS IN THE ST. BRIDE’S AREA, CAPE ST. MARY’S PENINSULA, NEWFOUNDLAND 1 Department of Earth Sciences, University of Durham, Durham, UK, DH1 3LE 2 Department of Earth Science and Engineering, Imperial College London, London, UK, SW7 2AZ, Current Research (2011) Newfoundland and Labrador Department of Natural Resources Geological Survey, Report 11-1, pages 241-252

THE ARCHES
NEWFOUNDLAND, CANADA
SAN JON CANYON
NEW MEXICO, U.S.A
TROPICAL STORM ISAIAS
MARYLAND, U.S.A