The Late Jurassic ammonite fauna of New Zealand

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Stevens, G.R. 1997 The Late Jurassic ammonite fauna of New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences monograph 18; New Zealand Geological Survey paleontological bulletin 74 217 p.

Abstract: Late Jurassic ammonite assemblages are found primarily in strata of the Murihiku Terrane (=Hokonui facies) of the New Zealand sedimentary sequence. Three ammonite localities are known from South Island strata of the Younger Torlesse (or Pahau Subterrane). The Younger Torlesse rocks are interpreted as representing the deformed contents of an accretionary prism that was developed along a trench-transform system flanking the Gondwana coastline. New Zealand Late Jurassic (Kimmieridgian and Tithonian) macrofaunas are dominated by siltstone/mudstone bivalve assemblages, with ammonites being usually in the minority. The general paucity of the New Zealand ammonite record is in marked contrast to the richly abundant populations that lived in the tropical and sub-tropical shallow-water shelf seas of the Central Tethys. Climatic and facies conditions in New Zealand were substantially different from those of the Central Tethys. First, New Zealand was situated in mid-latitudes, probably between 40 degrees S and 50 degrees S lat., and sea water temperatures were probably equivalent to those of the modern warm-temperate zone. Second, New Zealand straddled a tectonically active plate margin, and the Murihiku sediments, in which almost all of the ammonites are preserved, were laid down in an arc-trench situation, adjacent to a major subduction zone developed along the southeastern edge of the Gondwana continent. Phylloceratidae are more richly represented in the Late Jurassic of New Zealand than might be deduced from the published records. Oppeliidae are represented. The following ammonite range zones can be recognised in the Heterian-Puaroan succession of New Zealand: (i) Idoceras zone, correlated with Early and Late Kimmieridgian; (ii) Paraboliceras zone: Middle Tithonian; (iii) Aulacosphinctoides zone: Middle Tithonian; (iv) Subplanites zone: Late Tithonian. The Early Tithonian is either absent or poorly represented in the New Zealand sequence. The Late Jurassic ammonite assemblages of New Zealand show well-defined variations in the proportions of Leicostraca and Trachyostraca and assessed values for faunal turnover, diversity and abundance. In general, there is a good match between biotic and sedimentological changes and published eustatic models. Strong Tethyan provincial affinities are shown by the ammonite assemblages of the Late Jurassic of New Zealand. (auth/EB)

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