Torlesse stratigraphy and paleontology, Balmacaan Stream, mid Canterbury
Balmacaan Stream, Harper Range, central South Island, New Zealand is the first known Etalian (early Middle Triassic) locality in the Torlesse Supergroup. This study involved mapping of Torlesse rocks here, and collection of fossils for biostratigraphic, paleoecological and paleobiogeographical analysis. The Mt Taylor Group encompasses all Triassic-aged Torlesse Supergroup strata between the Rangitata and Rakaia Rivers. This area includes the Harper Range, which previously had been mapped as Balmacaan Formation - one of three Middle Triassic formations in the Mt Taylor Group. 50 field days spent reconnaissance mapping, measuring sections and collecting fossils, revealed three distinct sequences of strata on the northern slopes of Mt Harper, and southern tributaries to Balmacaan Stream. One sequence represents the previously described Balmacaan Formation which is amended, while the other two sequences represent the Mahaanui and Lake View Formations (both new). All are characteristically quartzofeldspathic. The Lake View Formation (basal) is dominated by thick-bedded flysch, consisting of alternating light grey sandstone and rarely fossiliferous (flora-bearing) dark grey siltstone. It also includes a conspicuous conglomerate lens near its top, and rare red and green siltstone. Graded beds indicate that the formation is overturned. Its base is unknown, while its top is disrupted by faulting, but this unit is essentially in conformable contact with overlying Balmacaan Formation, thus hinting at a Lower or pre-Etalian age. The overlying Balmacaan Formation (amended) consists of over 700 m of fossiliferous fine sandstone and siltstone, and subordinate medium and pebbly sandstone. Graded bedding and pressure expulsion structures indicate that the formation is south-dipping and northyounging (overturned), consistent with the underlying Lake View Formation. Marine macroinvertebrates (including ~13 cephalopod species) and woody plant remains occur at six or more levels throughout the formation. Defining the unit's base is a pebbly shellbed, containing abundant specimens of the bivalve Trigonia balmae (J36/f99). This bivalve, along with the molluscs Mellarium nodulosum, Parapopanoceras fraseri and Daonella jadii, are species characteristic of the Etalian Stage (New Zealand timescale), which correlates to the Anisian Stage (early Middle Triassic) on the global timescale. The Mahaanui Formation (?uppermost) constitutes over 500 m of thin-bedded flysch overlain by thick sheets of conglomerate interbedded with subordinate but conspicuous plant-bearing carbonaceous horizons. Above the folded basal strata, bedding dips and youngs steeply southeast. Vascular plant fossils are comparable to Permian forms from South Africa, but identifications are tentative, so a Permian age is not confirmed. The Mahaanui Formation is faulted against the Balmacaan Formation and possibly also against the Lake View Formation. Thus, stratigraphic relationships are unknown. If the Mahaanui Formation was considered to be uppermost, and involved in a near-continuous sequence, a broad shallowing trend would be evident, with Lake View Formation being distal, Balmacaan Formation middle and Mahaanui Formation proximal. 67 macroinvertebrates and one vertebrate are described, occurring at eight stratigraphic levels in the Balmacaan Formation. 55 are molluscs, of which 33 are bivalves, while 22 species of the total fauna are lmown only from float. The fauna is dominated by epifaunal suspension feeders, while free-swimming carnivores are also common. One fossil locality, J36/f94, possibly represents a life assemblage, with crinoids and brachiopod species apparently in life position, and is compositionally indistinguishable from an Etalian life assemblage in the Wairaki Hills (D44/f044). Direct comparison of the Balmacaan fauna and Etalian faunas of the Murihiku Supergroup (particularly the Wairaki Hills) has shown marked quantifiable similarity at species- and genuslevel. 33 out of 55 Balmacaan species and 33 out of 48 genera also occur in the Murihiku Supergroup, giving Simpson coefficient values of 0.60 and 0.69 respectively. Cephalopod and brachiopod faunas are particularly similar, giving species-level Simpson values of 0.88 and 0.81 respectively. These faunal-similarity numbers indicate physical closeness, and thus strong marine connections between the Torlesse and Murihiku depositional environments in the Middle Triassic, and throw doubt on previous ideas about Murihiku end
xi, 180 p, ill. , diagm, map (in pocket),; 30 cm.
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POLYGON ((171.011464167962515 -43.631639864562665,171.076894604479492 -43.631687696364381,171.096524685303621 -43.648811356709118,171.086423250277619 -43.687949911794107,171.00371709886673 -43.657590553397071,171.011464167962515 -43.631639864562665))
van Dusschoten, A, “Torlesse stratigraphy and paleontology, Balmacaan Stream, mid Canterbury,” Otago Geology Theses, accessed March 9, 2021, http://theses.otagogeology.org.nz/items/show/372.