The geology of the Cobb Reservoir Area, North West Nelson
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
This thesis examines the geology of an 80 km2 area of Lower Paleozoic rocks centred on the Cobb Reservoir, N.W. Nelson. An extensive and detailed field mapping programme has identified three fault bounded slices, of markedly contrasting lithologic and deformational character, from west to east the Balloon, Junction and Mataki Fault Bounded Slices. The Balloon Slice comprises predominantly a pervasively foliated, east dipping "block in matrix" tectonite, with lenses of chert, limestone, conglomerate and andesite (collectively known as the Balloon Formation). The tectonite has a strong linear fabric plunging down dip parallel to a regional lineation. Hornblende andesites of calc-alkaline arc affinity are restricted to the Balloon Formation. Conglomerates outcropping north of the Reservoir (the Battey Conglomerate) are of uncertain affinity to the Balloon Formation The Mataki Slice is poorly exposed and understood. It contains three lithologic units of unknown interrelationship; the Salisbury Conglomerate, the M Creek Formation and the Mataki Basalts. The M Creek Formation comprises laminated siltstones and angular basalt rich mass flow conglomerates. The Mataki Basalts are distinctive high Ti and Mg tholeiitic basalts and have a probable seamount trace element signature. The Mataki Slice is bounded to the west by the Mataki Fault and to the east by the Junction Fault. The Junction Slice comprises well bedded quartzose sandstones and siltstones (the Junction Formation), folded into irregularly oriented but consistently east plunging asymmetric folds. Large lateral displacement between the Slices is indicated from the presence of igneous rocks of widely contrasting geotectonic origin. The timing of juxtaposition predates the imposition of a regional extensional deformation, interpreted to be related to the Devonian or Silurian Tuhuan event. It is apparent that the structural complexity of the study area is greater than can be fully resolved with the outcrop available. Previous attempts (Grindley 1971, 1980) to establish a regional stratigraphy are not applicable in this area.