Stratal patterns and sedimentology of lowstand deposits of mid-Canterbury, New Zealand

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Browne, G.H.; Thrasher, G.P. 1996 Stratal patterns and sedimentology of lowstand deposits of mid-Canterbury, New Zealand . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 96/13 43 p.

Abstract: The Canterbury Plains are a large and coarse-grained braidplain system, over 600 m thick, which formed during Plio-Pleistocene times along the eastern margin of the tectonically active Southern Alps, New Zealand. The plains comprise a series of alternating highstand and lowstand deposits that offer a unique opportunity to study contrasting styles of fluvial and marine sedimentation related to both highstand and lowstand deposition. Lowstand sediments of last glacial age are well exposed in coastal cliff outcrops up to 25 m high in mid-Canterbury. These show numerous, stacked coarse-grained gravel associations, interpreted as a variety of braided river and associated eolian and paleosol units. The most abundant lithology is massive gravel, with less abundant planar cross-bedded and trough cross-bedded gravel. The gravels are interpreted as within-channel gravel and bar deposits. Folding and brecciation of intercalated mud blocks, together with blocks of conglomerate with vertical clast fabrics, indicate that frozen bank material and ice-rafted blocks were a feature of these lowstand units. Trough and planar cross-bedded and horizontally laminated sands make up a minor lithologic component, and are interpreted as fluvial bedforms and eolian beds. Mud occurs as either mud blocks or as centimetre-thick layers, but not as matrix. The bedded mud is confined to stratification discontinuities such as at the base or along cross-bed laminae of cross-bedded units, or at set or bed contacts. Although mud-sized sediment was probably important when these units were deposited (based on the modern fluvial setting), the fines were very quickly lost by vertical and horizontal water movement through the gravel, and concentrated along permeability barriers. Fluvial incision occurs in the modern highstand and operates at two distinct positions along the river profiles - immediately downstream of the river gorges, and near the coast. During lowstand, however, the fluvial system prograded and aggraded, and filled the previously eroded incised valleys. The mid-Canterbury rivers therefore behave in a different manner from the traditional views of fluvial incision. During the last glacial lowstand, the shoreline would have been ten's of km eastward of its present position. A series of ancient lowstand beach ridges have been recognised in seismic reflection profiles across the area of the modern shelf. An estimate of the age of these beach ridges using the Pliocene-Recent subsidence curve suggests that these beach ridges relate to periods of lowstand during the Pleistocene. Several features of the lowstand outcrops are of significance to petroleum reservoirs in these types of coarse-grained depositional setting. The lack of fines in the resulting deposit is a consequence of post-flood, eolian activity in the exposed braidplain and water percolation through bar and channel deposits, both processes occurring immediately after flood events. There is therefore a significant de gree of immediately post-depositional modification in the fluvial sediments, which contributes to the reservoir character by 1) improving overall sorting, 2) removing fines and establishing distinct post-depositional permeability pathways where water has passed through the gravel strata and, 3) creating thin discontinuous clay banding in the gravels where the passage of fines (muds) has been impeded by permeability contrasts as lithologic boundaries. These create baffles to intra-reservoir fluid flow. Vertical stacking patterns within fluvial deposits will vary along the depositional profile. Based on the example of the Canterbury Plains, the lowstand stratal geometry will comprise a series of stacked, sheet-like gravels. During highstand however, an upstream portion of the depositional profile will be characterised by confined channels, filled with a relatively high sand and mud:gravel ratio, by heterolithic variability, and relatively thick sedimentary successions. In contrast, highstand gravels deposited in the downstream incised zone will be unconfined, with a low sand and mud:gravel ratio. (auth)