Late cretaceous and tertiary rocks of broken river, Canterbury

Abstract

Most of Broken River watershed lies within a roughly triangular, complex, tectonic depression bounded by major faults at the foot of Torlesse, Craigieburn, and Broken Hill Ranges. Torlesse Group greywackes and other rocks (Triassic?) are overlain unconformably by the following sequence of Cretaceous and Tertiary formations now surviving as infaulted outliers between high-standing masses : Late Pleistocene glacial and glacifluvial deposits obscure a great deal of the pre- Pleistocene geology. In the western Castle Hill area the Cretaceous and Tertiary rocks are deformed by faults and folds mostly with a north-easterly trend, the folds plunging to the southwest and west. In the eastern Avoca area the structure is more complicated and unconformities within the sequence are more pronounced. Gravitational sloughing of covering beds from greywacke basement is suspected to have occurred during uplift of the Torlesse Range. The Cretaceous-Tertiary sequence records a transgression-regression interlude between the Rangitata-Kaikoura tectonic events, interrupted locally by uplift and tilting in the late Eocene and modified regionally by the effects of mid-Oligocene basaltic eruptions.