Diagenesis in Cool-Water Carbonate Systems: An Example from the Waitaki Region, New Zealand
Cool-water carbonate deposits and the mechanisms from which they develop have been the focus of significant work recently in what has traditionally been a tropical carbonate and siliciclastic dominated field. Here we look at how sequence boundaries within cool-water carbonate systems can be enhanced by diagenetic variability as a result of palaeotopographic influence on the sequence stratigraphic model.
This study focuses on Late Eocene to Early Miocene cool-water carbonate deposits within the Waitaki Basin in South Island, New Zealand, formed during a period of regional transgression. Detailed stratigraphic and thin section analyses of several outcrops were used to develop localised sea-level curves from palaeo-depth indicators, and to look at the nature of the carbonate cements within the various formations. From this we see significant depth variation over short distances within the individual limestone units, suggesting that there was variability in the palaeotopography and this had a considerable influence on the depositional environment. This then directly influenced the nature of sea-floor diagenesis within the units, but being most pronounced along sequence bounding surfaces. Results show that the lower limestone unit and syn-depositional volcanics form a transgressive systems tract which is capped by a regression-induced unconformity. Following this there is a deepening up sequence of greensand and limestone, capped by a second regressive karst or submarine erosion surface, after which deeper facies again predominate. The spectrum from subaerial to submarine surfaces atop the lower limestone indicates a westward deepening basin extending from the volcanically induced palaeo-highs in the east. The subsequent transgression produced a continuum from heavy karst through to deeply bored surfaces relative to the palaeo-highs, along which we see a related change in the nature and volume of cement, from significant spar through to deposits almost completely devoid of any cement. Along these sequence boundaries we see different diagenetic signatures forming during transgressive erosion surfaces than during regressions, such that this can be used to enhance the identification and interpretation of sequence bounding surfaces in cool-water carbonate sequence stratigraphy.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California