Palynostratigraphic analysis of a measured section through the Karekare, Tahora and Whangai formations (Upper Cretaceous) at Koranga Stream (Raukumara Peninsula)

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SR_2001-027-pdf
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Schioler, P.; Crampton, J.S.; King, P.R. 2001 Palynostratigraphic analysis of a measured section through the Karekare, Tahora and Whangai formations (Upper Cretaceous) at Koranga Stream (Raukumara Peninsula). Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2001/27 15 p.

 Abstract: A palynostratigraphic study of 20 samples from a measured section through the marine Karekare, Tahora and Whangai formations (Upper Cretaceous) exposed in the Koranga Stream at Koranga Bridge, has revealed the presence of a moderately diverse and well preserved dinoflagellate cyst assemblage. Based on key dinoflagellates, the section can be dated and correlated with zonation schemes from the Marlborough area. The samples from the Karekare Formation are tentatively assigned to the mid-Teratan (middle-upper Coniacian) Cymososphaeridium benmorense Zone. Samples from the overlying Tahora Formation are assigned to the zonal succession of Isabelidinium cretaceum Zone, Nelsoniella aceras Zone and Trithyrodinium suspectum Subzone, covering the uppermost Piripauan-lowermost Haumurian interval (middle Santonian-lower Campanian). Thus, the boundary between the Karekare and Tahora formations represents a hiatus that spans at least two dinoflagellate zones and eliminates the Teratan-Piripauan stage boundary. Samples from the Whangai Formation are assigned to the zonal succession of T. suspectum Subzone, Vozzhennikovia spinulosa Subzone and Isabelidinium korojonese Zone, covering most of the remainder of the lower Haumurian interval (lower Campanian-lowermost upper Campanian). The boundary between the Tahora and Whangai formations is gradational and represents continuous sedimentation. A preliminary, semi-quantitative study reveals a change in palynofacies from the very fine sandstone of the Tahora Formation to the muddy siltstone of the Whangai Formation. The palynofacies of the Tahora Formation is dominated by black phytoclasts with rare, brown phytoclasts, rare palynomorphs and very rare amorphous organic matter (AOM). In the Whangai Formation, phytoclasts are less dominant and palynomorphs and granular AOM increase in relative abundance. The change is interpreted as reflecting a shift from a high energy, nearshore, oxic depositional environment in the lower part of the section to a less turbulent, offshore setting under oxygen deficiency in the higher part of the section. (auth)