|dc.description.abstract||The Opoiti, Whakapunake and Tahaenui Limestone formations (Opoitian to Waipipian; Pliocene) crop out extensively in northern Hawke’s Bay between Wairoa and Mahia Peninsula where they are encased within mudstone dominated Wairoa Formation. The limestones are shallow cool-water carbonates that formed about tectonically active antiform structures inboard from the convergent subduction plate margin along eastern North Island. Their coarse skeletal fraction is dominated by barnacle plates with common brachiopod, pectinid and oyster remains. The carbonate factory for prolific skeletal production was likely sited in shoal water (30-60 m deep), high energy conditions atop the antiforms, with deposition from carbonate shedding down the flanks of the antiforms and even into the bounding synforms.
The individual limestone units are laterally discontinuous and perceived as large lenses (up to c.3-4 km long by c.2 km wide). The enclosing thick mud-rich (M1-M4) to locally sandy (S1-S4) lithofacies, along with occasional volcaniclastic beds (V1), are here informally recorded as Wairoa Formation A, B, C or D, depending on their stratigraphic position with respect to the three limestone units.
Stratigraphic logging of sections at Nuhaka (Tahaenui/Clonkeen), Mt Moumoukai and on Mahia Peninsula established 12 sedimentary lithofacies based on field texture and composition, namely limestones (L1-L3), sandstones (S1-S4), mudstones (M1-M4), and a volcaniclastite (V1). The lithofacies discriminate well on triangular plots involving carbonate content and insoluble sand, silt and clay grain sizes and typically show vertical similarities between field sites for each limestone, suggestive of similar depositional patterns and controls operating at the different localities.
Opoiti Limestone occurs on Mahia Peninsula and Mt Moumoukai as a bedded, c.30 m thick, moderately dipping (25° W), siliciclastic sand-rich unit with common brachiopods and mudstone clasts (L1, L2, L3) that unconformably overlies late Miocene mudstone (M1). Petrography ranges from a sandy biomicrite to a variably sandy, poorly washed rounded biosparite or bioclastic arenite in which interskeletal space is occupied mainly by microbioclastic micrite with some isopachous sparite. Carbonate content is up to c.75%; siliciclastic grains range from very fine to fine sand size. The Opoiti Limestone is likely a transgressive event deposit (TST), fining upwards into sandstone and mudstone (HST) of Wairoa Formation B, and largely under tectonic control.
Whakapunake Limestone within the field area is restricted to the west coast of Mahia Peninsula where it is c.40 m thick and comprises limestone (L3) units (10- 100 cm thick) interbedded with unique mudclast-bearing shelly sandstone (S4) units (20-100 cm thick). These couplets may be related to storm emplacements although their origin, and that of the mudclasts, remains problematic. The elongated mudclasts possibly mark the positions of original Skolithos/Ophiomorpha-like burrows that have been later modified by seismic shaking. No lower contact was observed. Upwards, the limestone grades via interbeds into shelly sandstone (S2) and mudstone (M1). Petrography ranges from packed biomicrites to poorly washed biosparites, while the sandstone interbeds are typically muddy bioclastic arenites. Cements are isopachous sparite rims and microbioclastic micrite with common siliciclasts. Carbonate contents range up to c.70% with the siliciclastic grains being of medium silt and fine sand size. The Whakapunake Limestone is likely a transgressive deposit (TST) fining upwards into shelly sandstone (TST/HST) of Wairoa Formation C, again overall tectonically driven but with possible superimposed eustatic sea level changes.
Tahaenui Limestone (L1, L3) occurs in discrete outcrops (10-30 m thick) across the full area. At Nuhaka, it unconformably overlies late Miocene mudstone (M1), while the upper contact grades into shelly sandstone (S2). At Moumoukai, it unconformably onlaps the Opoiti Limestone. It is dominated by coarse barnacle plates with interparticle isopachous sparite rims and microbioclastic micrite. Petrographically the limestone ranges from rounded biosparite to packed biomicrite. Carbonate content is up to 90%, with the siliciclastic grains being of fine-medium silt and fine sand size. The Tahaenui Limestone is overall a transgressive deposit (TST) fining upwards into shelly sandstone and mudstone (HST) of Wairoa Formation D, largely under tectonic control.
The Pliocene limestones are economically important as potential subsurface petroleum reservoirs, as a lime resource for agricultural use and as a hard stone source for aggregate. Recommendations of the immediate economic potential for development include local aggregate and fertiliser sources from the Tahaenui Limestone at Nuhaka and Mahia.||