Identification of a Waipawa Formation equivalent in the upper Te Uri Member of the Whangai Formation - implications for depositional history and age

Stable isotopes and biomarkers have identified a unit with similar organic geochemistry to the Waipawa Formation, in the upper Te Uri Member of the Wh angai Formation, exposed in the Akitio River, at Tawanui, southern Hawke's Bay, New Zealand. At Tawanui, the uppermost greensand of the Te Uri Member contains a large positive delta C-13 isotopic excursion from -27.0 parts pe r thousand to -20 parts per thousand and an increase in total organic carbo n from 0.1% to 1.0%. Biomarker analyses demonstrate a similar C-30 sterane fingerprint to other deposits of the Waipawa Formation. We propose that the uppermost greensand of the Te Uri Member at Tawanui is a condensed stratigraphic equivalent of the Waipawa Formation at nearby Ang ora Stream and other East Coast Basin localities. This correlation demonstr ates that Waipawa Formation is middle Teurian (middle Paleocene) and preced es the late Paleocene thermal maximum event by c. 5 m.y. The likely upwelli ng event that resulted in deposition of the Waipawa Formation was geographi cally widespread but probably restricted to the outer shelf/upper slope. In places, biogenic activity prevented the preservation of organic carbon in equivalent condensed stratigraphic intervals. Localised restriction of upwe lling and black shale deposition may be demonstrated by the occurrence of a thick black shale at Angora Stream only c. 10 kin from the coeval greensan ds at Tawanui. Alternatively, Oligocene-Miocene east-west shortening and st ructural reorganisation in the East Coast Basin may have juxtaposed facies that were originally many tens of kilometres apart. Our correlation also implies that the Te Uri Member is diachronous. It may have been on the outermost shelf to upper slope during lowstand conditions, where it is oldest, to higher on the shelf during transgression and highst and conditions.