Alluvial sedimentology of the Upper Pleistocene Hinuera formation, Hamilton Basin, New Zealand

Abstract

The Upper Pleistocene HinueraFormation is an unconsolidated alluvial deposit, up to 90 m thick, underlying some 2000 km² of the plains of the Hamilton Basin and the southern half of the Hauraki Lowland. The sediments have been studied in detail at 19 widely-spaced sections in Hamilton Basin. The most striking lithological features are their coarseness, acid volcanic composition, and abundant cross-stratification.

Sediments are mainly gravelly or slightly gravelly sands, sandy gravels and silts; peat beds are developed locally. Gravel-sized material is dominated by fragments of rhyolitic breccia, rhyolite, pumice and ignimbrite; sand and silt fractions by volcanic quartz, oligoclase/andesine plagioclase, pumice, and glass shards. Heavy minerals are hypersthene, magnetite, hornblende, augite, epidote, and biotite. The minor amounts of clay minerals include halloysite, kaolinite, illite, and chlorite.

Sedimentary structures are dominated by thick cosets of cross-stratified gravelly sands and sandy gravels composed mainly of lithologically heterogeneous crossstrata. On the basis of height, Cross-strata fall naturally into small <5 cm), medium (5–15 cm) and large (>15 cm) scale sets. Four types of cross-stratification (Rho, Nu, Epsilon, and Sigma) are recorded. Rho and Sigma varieties are new and consist of grouped, medium- and large-scale, lithologically heterogeneous sets with erosional bases which in the case of Rho cross-stratification are scoop-shaped and overlain by discordant cross-strata but for Sigma cross-stratification are planar and overlain by concordant cross-strata. Horizontally stratified and massive units are locally common, particularly in pumice silts and sands. 'Sedimentary structures in sands and gravels are the product of specific bed forms developed on longitudinal and transverse channel bars under both lower- and upper-flow regime current characteristics. Liquefaction produced a variety of post-depositional deformation structures in gravelly sands and silts.

Grain size parameters are highly variable. 'Size frequency curves for sands and gravels portray a significant contact load population, a well-developed (up to 90%), exceptionally coarse and variably sorted saltation population, and a small, commonly coarse, suspension population. Complex grain size distributions result from mixing of up to four log-normally distributed size populations in polymodal sedinlents. Textural data indicate that the bulk .of the Hinuera Formation was deposited rapidly in an environment of fluctuating high turbulent energy.

From composition, texture and sedimentary structures seven lithofacies are erected for the Hinuera Formation which correspDndto deposits formed in a variety of subenvironments in a braided river system of the ancestral Waikato River. The geomorphological form of the formation in Hamilton Basin is ,that of a large, very low angle alluvial fan, formed from the debouching of sediment through the narrow Maungatautiri Gorge out into the basin. Aggradation occurred in two phases, both initiated by vDlcanism in the Central Volcanic Region, and sustained by increased rates of erosion in the Upper 'Waikato Basin under a cold, seasonally wet climate.