Sedimentology, petrology, and structure of Mangapokia Formation (Pahaoa Group) near Te Awaiti, southeast Wairarapa, New Zealand

Abstract

Near Te Awaiti, southeast Wairarapa, middle Cretaceous Mangapokia Formation (Pahaoa Group) consists of indurated, poorly fossiliferous, alternating sandstone and argillite, minor conglomerate, grit, pebbly-sandstone, and pebbly-mudstone (sedimentary association), and minor basalt, coloured argillite, chert, and micritic limestone (volcanic association). Sedimentary structures indicate deposition by turbidity currents and related mass flow mechanisms. Lithofacies analysis indicates deposition in inner-mid and mid submarine fan environments. Channel, interchannel, lobe, and possibly levee deposits are present. Two large-scale progradational suites of facies reflecting large-scale migration of the inner-fan channel complex are superimposed upon numerous small-scale depositional cycles resulting from lateral migration of numerous braided channels within the inner and mid-fan environments. Sandstone compositions indicate that the sand and finer-grained detritus were derived mainly from a partly dissected continental margin magmatic arc. Conglomerate compositions are consistent with such a provenance and also indicate a significant contribution from older sedimentary rocks and reworking of Mangapokia-type sandstones. A model for sediment dispersal is presented involving derivation of magmatic arc and sedimentary detritus from the Antartic region of the eastern continental margin of Gondwana and reworking of previously uplifted Mangapokia-type sediments from the trench slope break, followed by longitudinal northward sedimentary transport within a trench. The volumetrically minor but widespread volcanic association is restricted to zones of melange. Chemically the basalts are tholeiitic, having trace element concentrations similar to modern ocean-floor basalts. Radiolarian-bearing cherts and coloured argillites arehighly oxidised pelagic sediments, chemically significantly different from the 'normal' black argillites of the sedimentary association. Chemical indices distinguish two main geochemical types of coloured argillite and indicate that the cherts are mixtures of coloured argillite and silica which is mainly biogenic. Chemical comparison with modern marine sediments indicates that the cherts and most of the coloured argillites were deposited with pelagic sedimentation rates of less than 10mm/ky, and at least 800-1000km away from terrigenous sources. Tectonic deformation is complex, involving four generations of folding, multiple periodsof faulting, and local development of melange fabric. The following sequence of deformational events is inferred: i) intense shearing and disruption of strata on an uncertain scale, ii) F1 isoclinal to gentle mesoscopic folding, iii) extension to produce complex fracture patterns and veins, iv) movement on bedding-parallel shears, low-angle to bedding thrusts and normal faults, and high-angle to bedding reverse and normal faults, disrupting strata on a regional scale and producing weakly to strongly attenuated beds,boudins, 'pinch and swell'-type structure, a lozenge (transposition) fabric, and localdevelopment of melange, v) F2 isoclinal to tight, NE and SW plunging mesoscopic folding,vi) F3isoclinal to gentle, NE and SW plunging mesoscopic and macroscopic folding, vii) F4 open to gentle, NW and SE plunging mesoscopic and macroscopic folding, viii) mesoscopic faulting at a high angle to bedding, ix) recent uplift and seaward tilting associated withthe growing Mt Adams Anticline. The rocks are metamorphosed to at least the zeolite grade. Similarities in depositional processes and setting, composition, structural style and inferred sequence of deformational events of the Mangapokia Formation and Torlesse rockselsewhere, suggests that Mangapokia Formation represents the youngest part of the Torlesse terrane. The juxtaposition of relatively coherent, strongly folded and tectonically imbricated trench-fill submarine fan sediments (sedimentary association) and minor ocean-floor material (volcanic association), and the localised occurrence of melange, are consistent with a model of concurrent sedimentation and accretion at the base of the inner trench slope in a convergent plate setting, possibly coupled with a slow rate of subduction. In southern North Island Torlesse deposition and concurrent subduction tectonism were probably continuous at least until the middle Cretaceous. Rare highly alkalic basaltic rocks that locally intrude the sedimentary deposits are considered to be related to the post-Torlesse phase of igneous activity centred on the Kaikoura Ranges of the South Island.