Permian-Jurassic Relations, Beaumont Area, western Southland.

Aslund, T

An area approximately 40km2, 10km north of Ohai, western Southland was mapped in detail. The area is roughly divided in two by the Tin Hut Fault, which is steeply dipping and strikes N-S. Rocks to the west of the fault comprise steeply eastward dipping, eastward facing volcaniclastic sediments of the Elbow Formation, Takitimu Group. To the east are rocks of the Caravan Formation, characterised by distinctive pyroxene-phyric basalts and two-pyroxene andesites intruding volcaniclastic sediments. In the southern part of the area, the Caravan strata have been overturned and now face south. To the north Caravan sediments are right way up and facing east. The genetic relationship between Takitimu Group and the Caravan Formation is uncertain. Productus Creek Group, an eastward dipping, richly fossiliferous Permian marine sequence, depositionally overlies the Caravan Formation. Together they comprise the Caravan 'subterrane'. Trapped between Takitimu and Caravan in the northern part of the area is a wedge of Lower Triassic Murihiku sediments and associated melange (Tin Hut Melange). The presence of this infaulted sliver places a lower limit on the timing of juxtaposition of the Caravan subterrane and Takitimu terrane. Emplacement of the Beaumont Intrusives (porphyritic hornblende-andesite) and an olivine monzonite dyke, both of which are found intruding Takitimu and Caravan rocks, post-dates any major strike-slip fault movement. The Caravan subterrane and intrusions are unconformably overlain by the Barretts Formation, a Jurassic conglomerate-sandstone sequence up to 200m thick, containing volcanic, plutonic and minor metamorphic detritus. Cretaceous Ohai Group sediments are faulted against Elbow Formation rocks in the NW. Ohai Group sediments are not found overlying Caravan subterrane, Murihiku Supergroup, Barretts Formation or melange. Murihiku Supergroup sediments have been thrust over Barretts Formation from the east along the Letham Ridge Thrust. Wairaki Melange crops out intermittently along the trace of this low angle fault. It comprises highly veined, deformed, fossiliferous blocks of Permian age in a sheared siltstone matrix. Triassic sediments also appear to have been incorporated in places.
QFL data from Barretts sandstones show a range of compositions, from relatively quartzofeldspathic (e.g. QFL- 34:43:23) to lithic-rich (e.g. 3:35:62) sandstones, which are remarkably similar to that of Jurassic, as well as Middle and Upper Triassic Murihiku sandstones. Heavy mineral assemblages are also similar: both contain clinopyroxene, biotite, hornblende, muscovite, epidote, opaques, and rare garnet and titanite. In contrast, Lower Triassic Murihiku sandstones plot in a confined QFL field (average QFL- 3:47:50), and heavy mineral assemblages are comprised dominantly of epidote and opaques. I suggest that the arc which supplied material to the Murihiku depositional basin also contributed detritus to the Jurassic Barretts Formation.

1988Aslund

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