Sears Island Geologic Fault Investigation Results 1976

SEARS ISLAND FAULT INVESTIGATION
SEARS ISLAND, SEARSPORT, MAINE
March 19, 1976  (263 page pdf; Maps start pg 41

by John R. Rand, Consulting Geologist

Robert G. Gerber, Geologist, Central Maine Power Company for Maine Nuclear Power Station Central Maine Power Company

Augusta, Maine  19 March 1976

Executive Summary

During the course of geologic studies for Central Maine Power Company’s proposed nuclear
power plant on Sears Island in Searsport, Maine, a northeasterly trending, highly weathered rock
zone was inferred about 1,000 feet (300 m.) from the reactor site. 

From April through November 1975, an investigation was conducted centering around two large trenches across the trend of the weathered zone. This report is a summary of the results and interpretations of the investigations performed.

The trenches exposed an ancient fault zone containing highly weathered phyllitic rock. In the

more westerly of the two large trenches, this weathered rock material had locally intruded and
caused minor deformation of Laurentide lodgment till. 

In the more easterly trench, a small bedrock reverse offset was found on the east wall with associated disturbance of overlying glacial till. On the west wall of the easterly trench, a small monoclinal flexure was found at the  till/bedrock interface. 

The investigators conclude that the bedrock fault zone experienced its last tectonic movement in Pre-Cenozoic time. 

The deformation of the tills over the fault zone is interpreted to have occurred approximately 13,500 to 12,800 years ago as a result of the weaker,  weathered rock having been squeezed between the adjacent harder bedrock masses. 

This squeezing produced either a forceful intrusion of highly weathered material into the till or an
arching of somewhat more competent but still relatively weaker rock in the fault zone. 

The squeezing and arching was a result of 1) lateral stress relief of the harder bedrock into the softer
fault zone materials during glacial unloading and/or

 2) a horizontal stress against the weaker fault zone rock through southeasterly directed base shear and stress distribution from the weight of a glacial lobe advance during the final overall glacial recession.

There is no evidence to indicate a tectonic origin of the till deformation.

END

John R. Rand, consulting geologist and Maine Certified Geologist #2, directed the investigation
and mapping of the bedrock geology and is a co-author of this report. Robert G. Gerber,
geologist for Central Maine Power Company and Maine Certified Geologist #110, directed the
investigation and mapping of the surficial geology and is a co-author of this report.