The Henley breccia

Chadwick, Peter Vinton.

The Henley Breccia is a Middle to Late Cretaceous alluvial conglomerate that rests unconformably on the Otago schist. At the base it is characterised by clast supported pebble conglomerates interbedded wi!h sandstones and siltstones. A general 'upfan' and north westerly increase in clast size and angularity is recognised along with a decrease in the proportion of matrix, number of sandstone I siltstone interbeds, channel structures and degree of sorting. The upper beds are essentially massive and often characterised by a small number of exceptionally large boulders (> lm diameter). The Henley is here subdivided into seven lithofacies including four varieties of conglomerate. The conglomerates are defined mainly on the basis of clast size and internal structure. The seven lithofacies are grouped into four associations representative ofa range ) · I I / of debris flow and stream flood deposits. The whole sequence ranges from 900 ~o )40?0m in thickness and has a consistent 15-25" W to NW dip. \ . The conglomerate clast fraction is dominated by psammitic greywacke and semi-schist (approx 80-85%) with subordinate quantities (5-10%) of higher grade (textural zone >2b) schist, argillite ( approx 5 %) and quartz ( approX: 5%). The proportion of higher grade schist increases northwards. The low grade greywacke clasts contain between 5-20% quartz, 10-15% feldspar and 70-80% rock fragments. The rock fragments are dominantly volcanic (60-70%) with a subordinate but strong meta-sedimentary component (30-40%). Sedimentological, including paleocurrent data indicate that the Henley was deposited by streams and debris flows travelling west to east. The predominantly coarse, angular nature of the clast fraction points towards a relatively proximal source. Th~-age of the Henley is poorly constrained. From pollen dates the base is thought to be / I between! SO and 94Ma (i.e. late TS or early PMl zone) roughly corresponding to R~u17inara Series. The a~e of the top is only constrained by the unconformably overl~ing T~atti Formation which is of latest Cretaceous (Haumurian) agy_~-) Studies of clay minerals from the Henley indicates that the majority of the clay is detrital in origin with some minor authigenic smectite, chlorite-smectite and illite. Small quantities of calcium carbonate cement containing traces of manganese and iron are also present. The calcite is typically pore filling with a variety of irregular, although predominantly equant, crystal shapes. The Henley Breccia accumulated in a fault angle depression formed in response to rapid i\ downthrow along the southeastern side of the Titrrfaplt. Movement along the fault was initiated in response to Middle Cretaceous extension and the Henley beds are interpreted to have experienced syn-depositional rotation in response to continued listric normal movement on the Titri and Akatore faults. Following peneplanation, deposition of the Taratu and of an overlying marine transgressive sequense, a late Middle Miocene change in !" . plate vectors caused reversal of movement on the Titri fa~lt resulting in further rotation and j consequent steepening of the Henley Beds.

1 v. (various pagings) : ill. (some col.), maps (some col.) ; 30 cm.

1995Chadwick

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