An outcrop study of bed thickness and continuity in thin-bedded facies of the Whakataki Formation at Whakataki Beach, east Wairarapa

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Edbrooke, S.W.; Browne, G.H. 1996 An outcrop study of bed thickness and continuity in thin-bedded facies of the Whakataki Formation at Whakataki Beach, east Wairarapa . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 96/34 18 p.

Abstract: A reservoir architecture study was made of Middle Miocene thin-bedded sandstone and mudstone rocks at Whakataki, Wairarapa. The steeply dipping Whakataki Formation exposed in the shore platform at Whakataki Beach is characterised by overtly uniform bed thickness across the exposure. A ~3.5 m thick stratigraphic interval comprising up to 34 sandstone and 33 mudstone beds was traced laterally for almost 500 m, using a number of thick sandstone beds as markers. The orientation of the study area was approximately parallel to depositional dip. Beds were correlated at 10 reference stations, where sandstone and mudstone bed thicknesses, nature of contacts and sedimentary features were recorded. Gamma-ray readings were taken at 20 cm vertical spacings through the beds at each station. Sandstones comprise between 66% and 80% of the total sediment thickness at individual stations. Four sandstone lithofacies and a single mudstone lithofacies are recognised. Sandstone lithofacies comprise horizontally laminated sandstone, climbing ripple laminated sandstone, ripple laminated sandstone and convolute bedded sandstone. Sandstone correlation and bed thickness measurements at the 10 reference stations show that most (80%) have maximum thicknesses less than 12 cm, but beds with maximum thicknesses of as little as 5 cm are continuous throughout the horizontal distance of the section. Several beds are truncated by erosion at the base of a low-angle channel. Sandstone bed correlation gradually decreases with increasing horizontal separation. Over a distance of 200 m about 90% of beds can be correlated, and over 400-500 m about 80% of beds can be correlated. The major control on lateral bed continuity is depositional pinch-out; beds that pinch-out within the transect have maximum thicknesses usually less than 5 cm. Erosional cut-out of beds by low angle channels is comparatively rare. Gamma-ray readings gave low counts (140-190 cps) in sandstone beds and higher counts (190-250 cps) in mudstones. Gamma-ray profiles have a typically serrate appearance, reflecting the interbedding of sandstone and mudstone. The thickest sandstone beds produced prominent low count values (140-165 cps) in many profiles. The outcrop data obtained during this study suggest that these beds were deposited by north-flowing, low density turbidity currents on a relatively flat, deep-water basin such as a slope fan or basin floor fan depositional setting. The lateral continuity of beds is likely to be enhanced by the down-depositional dip orientation of the study section. (auth)