Geology of the Tapui glauconitic sandstone

Sharp, Jonathan Richard.

The Tapui Glauconitic Sandstone is a marine transgressive unit which was deposited in a mid-shelf environment on the east coast of the New Zealand landmass during the mid-Eocene. Sedimentary structures in the form of storm generated hummocky cross-stratification and the presence of exotic macrofauna, indicate deposition below fair-weather wave base and above storm-wave base. Strong concentrations of glaucony near the base of the sandstone indicate it was forming insitu in the early stages of the transgression. Up section terrigenous input increases and the amount of glaucony decreases. The glaucony is disseminated throughout the upper part of the formation and is allocthonous, evident by the degree of mechanical abrasion the grains have undergone.
The Tapui Glauconitic Sandstone contains approximately one percent heavy detrital minerals. The heavy minerals are predominantly derived from a granitic terrane, 300 km to the south of the depositional environment. Heavy minerals with a granitic derivation include spessartine rich garnet, schorl tourmaline, zinc spinel (gahnite), zircon and rutile (type A). High rank metamorphic minerals sillimanite, corundum?, kyanite and staurolite, indicate a southern ultimate source for some heavy minerals in the sandstone. There is an Otago Schist and underlying Papakaio Formation proximate source for dravitic tourmaline, gold, zircon, anatase? and magnetite in the sandstone.
Longshore and bottom currents flowing in a northerly direction up the east coast of the New Zealand land mass in the Bortonian, deposited the heavy minerals in the Tapui Glauconitic Sandstone. Contemporaneously rivers on the adjacent landmass, deposited locally derived heavy minerals in the shelf environment. Abundant clays derived from thorough weathering of the basement, and the presence of warm water taxa in the sandstone, indicate a sub-tropical to tropical climate during the Bortonian.
Magma of tholeiitic composition fed by vents in the ocean floor, intruded the Tapui Glauconitic Sandstone between the mid-Eocene and late-Eocene. Where Large volumes of magma intruded, it cooled slowly forming columnar jointed sills. Less voluminous amounts of magma resulted in glassy pillow lavas which are high strontium, indicating intrusion in to sediment saturated in sea water.

ix, 161 leaves. : col. ill., maps ; 30 cm.

1993Sharp