PETROGRAPHY AND CHARACTER OF THE BEDROCK SURFACE BENEATH WESTERN CAPE COD, MASSACHUSETTS

Cores collected during recent drilling on western Cape Cod, Massachusetts provide insight into the topography and petrology of the underlying bedrock. Sixty-two drill sites spread over a ~140 km² study area produced, from depths of ~70-115 m, cores of granitoids (31), orthogneisses (20), basalts/diabases (4), amphibolites (3), felsic mylonites (2), and dolomitic rock (2).

Granitoid cores range in composition from granite to tonalite to quartz diorite, but are dominated by biotite granites. Alteration is common in nearly all cores examined in this study, and is evidenced by the secondary growth of chlorite and epidote. The granitoids resemble rocks of the Dedham and Fall River plutons. Gneisses from the study area generally contain the mineral assemblage K-feldspar+plagioclase+quartz±biotite±chlorite±muscovite±sphene±epidote+oxides. Amphibolite cores contain the assemblage hornblende+plagioclase+quartz+biotite+epidote±chlorite±sphene±K-feldspar±sericite+oxides. Based on mineral assemblages, we estimate peak metamorphic grade to be of lower amphibolite facies. X-ray powder diffraction of unmetamorphosed dolomitic cores shows the presence of clays, plagioclase, and possible magnesite.

The bedrock surface elevation was determined from land surface elevation and bedrock depth data collected during drilling. Contours of the bedrock surface show locally irregular topography (~20 m depressions) suggesting erosion by glacial scour. No distinct correlation is recognized between bedrock lithology and bedrock surface topography.

Basalts/diabases are interpreted to be post-metamorphic dikes associated with Mesozoic rifting. Dolomitic rocks may represent a lithified fault gouge material at the eastern terminus of a gneissic zone. The distribution of these lithologies suggests a possible continuation of the New Bedford gneissic terrane that outcrops 25 km to the west.