|dc.description.abstract||The Horwood Peninsula - Gander Bay area is located at NE Newfoundland
in the Botwood Zone (Williams et a1., 1974) or in the Dunnage Zone
(Williams, 1979) of the Central Mobile Belt of the Newfoundland Appalachians.
The area is underlain by Middle Ordovician to possible Lower Silurian
rocks of the Davidsville and Indian Islands Groups, respectively. Three
conformable formations named informally : the Mafic Volcanic Formation,
the Greywacke and Siltstone Formation and the Black Slate Formation, have
been recognized in the Davidsville Group. The Greywacke and the Black
Slate Formations pass locally into a Melange Formation. From consideration
of regional structure and abundant locally-derived mafic volcanic olisto-
1iths in the melange, it is considered to have originated by gravity
sliding rather than thrusting.
Four formations have been recognized in the Indian Islands Group.
They mainly contain silty slate and phyllite, grey cherty siltstone, green
to red micaceous siltstone and limestone horizons. Repetition of lithological
units by F1 folding are well-demonstrated in one of formations in
this Group. The major structure in this Group on the Horwood Peninsula is
interpreted to be a synclinal complex. The lithology of this Group is
different from the Botwood Group to the west and is probably Late Ordovician
and/or Early Silurian in age.
The effects of soft-sediment deformation can be seen from the lower
part of the Davidsville Group to the middle part of the Indian Islands Group
indicating continuous and/or episodic slumping and sliding activities
throughout the whole area. However, no siginificant depOSitional and
tectonic break that could be assigned to the Taconian Orogeny has been recognized in this study.
Three periods of tectonic deformation were produced by the Acadian
Orogeny. Double boudinage in thin dikes indicates a southeast-northwest
sub-horizontal compression and main northeast-southwest sub-horizontal
extension during the D1 deformation. A penetrative, axial planar slaty
cleavage (Sl) and tight to isocJ.ina1 F1 folds are products of this deformation.
The D2 and D3 deformations formed S2 and S3 fabrics associated
with crenulations and kink bands which are well-shown in the slates and
phyllites of the Indian Islands Group. The D2 and D3 deformations are
the products of vertical and northeast-southwest horizontal shortening
The inferred fault between the Ordovician slates (Davidsville Group)
and the siltstones (Indian Islands Group) suggested by Williams (1963,
1964b, 1972, 1978) is absent. Formations can be followed without displacement
across this inferred fault.
Chemically, the pillow lavas, mafic agglomerates, tuff beds and diabase
dikes are subdivided into three rock suites : (a) basaltic komatiite
(Beaver Cove Assemblage), (b) tholeiitic basalt (diabase dikes), (c)
alkaline basalt (Shoal Bay Assemblage). The high Ti02 , MgO, Ni contents
and bimodal characteristic of the basaltic komatiite in the area are
comparable to the Svartenhuk Peninsula at Baffin Bay and are interpreted
to be the result of an abortive volcano-tectonic rift-zone in a rear-arc
Modal and chemical analyses of greywackes and siltstones show the
trend of maturity of these rocks increasing from poorly sorted Ordovician
greywackes to fairly well-sorted Silurian siltstones. Rock fragments in
greywackes indicate source areas consisting of plagiogranite, low grade
metamorphic rocks and ultramafic rocks. Rare sedimentary structures in both Groups indicate a southeasterly provenance. Trace element analyses
of greywackes also reveal a possible island-arc affinity.||en_US