Print this pageSlope Fan Turbidites from the Mt. Messenger Formation, Taranaki, New Zealand with Applications to Thin-Bedded Reservoirs
BROWNE, GREG H., ROGER M. SLATT and EUGENE T. WILLIAMS
Late Miocene slope fan sediments are well exposed in a 5 km long and up to 200 m high coastal section in north Taranaki, New Zealand. Sediments are dominated by very well sorted, very fine- to fine-grained sandstone (Bouma T[b] and T[c] and interbedded sandy mudstone. Climbing ripple laminated sandstone is particularly abundant. Net:gross in the outcrop section is 0.85. Sandstone porosity is in the range 30-35 percent with up to 750 mD permeability. Mudstones have <1 mD permeability.
Photomosaics of the outcrop section have been computer-superimposed directly onto slope fan seismic reflection intervals for improved stratigraphic interpretation of the seismic. Truncation surfaces are the most striking stratal feature, these varying in scale from steep-sided cuts ten's of meters deep, to broad low-angle scours. They are filled by onlapping and draping mudstone, sandstone and conglomerate. Thinning-upward cycles are also common.
Cut-and-fill was the dominant depositional process and has a much greater control on lateral bed continuity than depositional pinch-out. Scours were zones of sediment bypass as most are lined by mudstone. A depositional setting dominated by high energy, episodic, unconfined flows is envisaged, such as would occur in a channel-lobe transition zone immediately downslope of a larger feeder channel(s), or in an upper slope fan setting where the slope gradient is relatively steep. Recognition of cut-and-fill processes can have important application to development of analog thin-bedded turbidite reservoirs in the Gulf of Mexico and elsewhere.