The Lower Taylor Group: Taylor and Wright Valleys, southern Victoria Land, Antarctica; Paleoenvironmental Interpretations and Sequence Stratigraphy
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The Devonian Taylor Group (the lower Beacon Supergroup), in the Taylor and Wright Valleys, southern Victoria Land (SVL), Antarctica, is separated from basement by a regional nonconformity, the Kukri Erosion Surface. Thereafter the Taylor Group sediments, observed in this thesis, are affected by two localized unconformities; the Windy Gully Erosion Surface, separating the Terra Cotta Siltstone Formation (TCzst) and older units from the younger overlying New Mountain Sandstone; and the Heimdall Erosion Surface (HES), separating the New Mountain Sandstone Formation (NMSst) and older units from the overlying Altar Mountain Formation. The depositional environments of the Windy Gully Sandstone, New Mountain Sandstone and Altar Mountain Formations have long been under debate. The Kukri Erosion Surface (KES) truncates the crystalline basement and separates the basement rock from the overlying Beacon Supergroup. Interpretation of the erosion surface characteristics and the directly overlying basal conglomerate lithofacies (WG-BCL) suggest a high relief rocky shore platform environment during a sustained and significant relative sea level fall. The environment has been suggested to be similar to what is currently seen on the West Coast, New Zealand today. The Windy Gully Sandstone Formation directly overlies the KES and consists of a basal conglomerate (WG-BCL) followed by moderately to well sorted tabular and trough cross bedded felds- to subfeldsarenites. At one location an interbedded siltstone and cross bedded sandstone lithofacies was observed and interpreted as a tidal flat. Overall interpretation of the WGSst suggests continued progradation from a rocky shore platform (WG-BCL) to a series low angle beach, to shallow marine, and back to low angle beach environments. This occurred during a relative sea level rise. Shallowing of the water column produced a gradational relationship with the Terra Cotta Siltstone Formation (TCzst).. The fine to very fine sandy mottled, well laminated siltstones moving to very fine fissile dark siltstones suggest a progression from sandy estuarine to a mud flat environment. The Terra Cotta Siltsone is truncated by the Windy Gully Erosion Surface The Windy Gully Erosion Surface is observed in the Handsley Valley by the presence of TCzst rip-up clasts in the directly overlying New Mountain Sandstone Formation. Elsewhere the horizon is either very sharp or has desiccation cracks present suggesting a cessation of deposition and subaerial exposure respectively. This suggests a small relative fall in sea level with only localized erosion. The New Mountain Sandstone Formation (NMSst) predominantly consists of a series of low angle tabular and higher angle trough cross beds. It has a subfeldsarenite base that progressively becomes a pure quartz arenites. Interpretation suggests an initial beach environment with rejuvenated sediments moving to quartzose shallow marine and back to beach environments. This represents a relative sea level rise with continued progradation The NMSst is truncated in the north by the HES forming a characteristic saw tooth pattern in the cross bedded sandstones; elsewhere the HES is represented by a feldspathic influx moving into the Altar Mountain Formation. The HES was formed due to a significant relative sea level fall leading to exposure and erosion of lithified NMSst cross beds in the north but continuation of deposition in the south. The Altar Mountain Formation consists of tabular and trough cross bedded subfields- to feldsarenites. The Odin Arkose Member directly overlying the HES is a granule to cobble conglomerate in the north where the HES is erosional and very coarse sand to granule feldsarenite in the south where the HES is conformable. This has been interpreted as a pebbly shore platform to coarse sandy to granular beach environment. The following Altar Mountain Formation is interpreted as a continuation of medium to coarse sandy beach environments with influxes of coarser sediments and possibly moving into shallow marine in places. Sequence stratigraphy identifies three stratigraphic sequences: S1, the Windy Gully Sandstone and Terra Cotta Siltstone Formations; S2, the New Mountain Sandstone Formation; and S3, the Altar Mountain Formation. The first two sequences (S1&S2) show a clear progression through transgression to a high stand systems tract through regression to a low stand systems tract. The Altar Mountain Formation follows a very similar trend but due to the lack of time and data the above measures have been speculated. Zircon age dating suggests the source of the sediments in the area come from the Neoproterozic Skelton Group and the DV2a Granite Harbour Intrusives, both directly underlying the sandstones but are exposed elsewhere in SVL. Laminated sandstone clasts within the New Mountain Basal Conglomerate Lithofacies (NM-BCL) are suggested to be sourced from recycled sediments directly below. Other exotic clasts are also observed in the lithofacies are of unknown origin.