Late Quaternary geology of Glenorchy district, Upper Lake Wakatipu

Abstract

The Late Quaternary Geology of the Glenorchy District, Upper Lake Wakatipu was investigated in terms of geomorphology, sedimentary facies distribution and characteristics and Quaternary deformation.

Terraces that are exposed are either due to a steplike lake level drop of Lake Wakatipu and reflect the paleohydrology of Lake Wakatipu or by alluvial plains and express morphology. A terrace-level correlation suggests the highest lake level of Lake Wakatipu of 358 to 363 m and drop of 50 to 55 m to the present level of 308 m. The indication of a zone (358 to 363 m) of a lake level rather than a line is due to a number of influencing factors such as: i) local control by deposition and erosion of a specific facies type, ii) syn - to postdepositional deformation, or iii) regional control (basinal characteristics, damming, uplift).

Quaternary sediments are mainly glaciofluvial and glaciodeltaic coarse-grained gravel deposits derived from the schistose hinterland (Caples terrane, Aspiring Lithologic Association).

Today the dominant modern facies is braided river facies whereas alluvial fan, floodplain, fan delta and aeolian facies largely dominated in Late Otiran/Early Aranuian times. Within the study area, alluvial fans developed at the flanks of the Richardson Mountains and the Earnslaw Massif and became lacustrine fan-deltas due to the larger extent and higher level of Lake Wakatipu. Alluvial fans are characterised by highly variable (grain size, layering, bedding, imbrication) debris flow to stream flood deposits. Fan-deltas are dominated by coarse-grained fluviodeltaic topsets and steeply dipping deltaic foresets deposited by gravity mass transport processes. Topsets and foresets are separated in most cases by an erosional surface, indicating rapid drop of lake level. Bottomsets have been not observed. Alluvial fan and fan-delta facies followed the retreating lake downvalley by incision in older deposits and development of new and lower level telescope-like fan-delta complexes to adjust to the changed base level. Smaller lakes as remnants of a larger Lake Wakatipu are evidenced by beach-pebble facies or still exist, such as Diamond Lake. Other lake deposits consist of laminated fine sand/silt layers. Alluvial and lacustrine facies distribution interfingered in many cases. Aeolian facies covers most of the inactive areas with loess deposits, usually 0.5 to 1 m thick, and causes considerable smoothing of longer exposed areas.

Deltaic slopes were affected by syn- to postdepositional slides. Sets of normal faults are of small scale (fault trace < 3 m) in the head region of slides, while local thrust faults are observed and interpreted as failure in the toe region of a slide. Large scale deformation (faults > 10 m) at the Buckler Burn juxtaposed massive gravel layers against lake sediments and caused rotation of layers. Gravitationally induced failure above an inclined slope is suggested as the cause of this deformation. Earthquake shaking, associated with the Moonlight Fault System, likely triggered the deformation of the Quaternary gravel deposits. Finally, facies and deformation are discussed within the framework of the structural setting and morphological formation of Wakatipu Basin and the late Quaternary paleoenvironment.