Recognition and interpretation of depositional units in a late Neogene progradational shelf margin complex, Taranaki Basin, New Zealand: foraminiferal data compared with seismic facies and wireline logs

摘要

The value of foraminiferal biofacies data as adjuncts to seismic reflection imagery and wireline logs for recognizing subsurface depositional units is assessed in the section at Taimana-1 through Giant Foresets Formation, a major latest Miocene-Quaternary progradational succession in Taranaki Basin. Seismic reflection imagery delineates three prograding lobes (#1-#3) in Giant Foresets Formation, overlain by topset facies. Data from a census of benthic foraminifera in 39 cuttings samples at 20-m intervals are analysed by ordinations, cluster analysis and summary assemblage signatures to identify biofacies units. The data are processed as presence/absences, relative abundances and as specimens/g> 150 μm fraction. Biofacies units closely correspond with seismic facies and log breaks but also identify possible intra-lobe subdivisions. They usefully characterize changes in the depositional environment and estimate sedimentary rates. A principal control on the benthic assemblages is the ca. 475 m decrease in water depth during clinoform deposition. The lowest lobe (#3) contains diverse, equitably distributed latest Miocene-Early Pliocene bathyal assemblages. Near-surface water masses intermediate between neritic and oceanic, indicated by percentages of planktonic specimens, suggest distal sedimentation. There is a strong abundance spike, particularly of benthic specimens, at the top of lobe #3, which may identify a condensed section. Microfaunal age datums indicate a long break in clinoform deposition at around this stratigraphic level. By the time deposition resumed in the late Pliocene, the site lay under sheltered neritic water (planktonic specimens < 10%). Rapid sedimentation (ca. 1 m/ky) in lobe #2, the principal clinoform unit, exceeded subsidence rate and the site shallowed by >250 m. Inequitably distributed assemblages dominated by Euuvigerina characterize its lower biofacies subunit and indicate weak dysoxia on the middle slope. Cyclic sedimentation, possibly in the Milankovitch precessional band, is recorded by the γ-ray log. This sedimentary pattern continued in lobe #1, which was deposited higher on the slope. Assemblages are species-poor. The change from progradation foresets to aggradational topsets forms a major boundary on reflection profiles. The composition of assemblages does not change at the boundary but species abundances are more evenly distributed in the topset faunas. Although seismic reflection imagery primarily defines depositional units in the Giant Foresets, the position and character of some bounding reflectors is equivocal and biofacies classifications and summary signatures provide useful, complementary, information. Foraminiferal biofacies data are multivariate and reflect characters of the bottom environment and overlying water masses. Diverse analyses are required to decode the environmental signals. Despite significant costs in their acquisition, biofacies data appear to be a rich source of information for studies of prograding sequences at shelf margins.