Foraminiferal Ecostratigraphy of Late Oligocene Sequences, Southeastern Australia: Patterns and Inferred Sea Levels at Third-Order and Milankovitch Scales
Qianyu Li, Peter J. Davies, Brian Mcgowran, Therese Van Der Linden, 2003. "Foraminiferal Ecostratigraphy of Late Oligocene Sequences, Southeastern Australia: Patterns and Inferred Sea Levels at Third-Order and Milankovitch Scales", Micropaleontologic Proxies for Sea-Level Change and Stratigraphic Discontinuities, Hilary Clement Olson, R. Mark Leckie
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The physical packaging into unconformity-bounded units of the upper Oligocene and lower Miocene neritic strata in southeastern Australia is chronologically consistent with third-order putative global sequences and glaciations. Foraminiferal biofacies data show both recurrence and progression. Species used as proxies for inner-neritic and outer-neritic environments display recurring fluctuations in close harmony with stratal packaging. In contrast to this recurrence or cycling, biofacies cluster groups are strongly sequential or “progressive” at the third-order, 106-year scale, with very little overlap between the successional assemblages named Angahook, Jan Juc-1, Jan Juc-2, and Puebla. The Angahook–Jan Juc-1 and Jan Juc-2–Puebla biofacies boundaries, implying some turnover in communities, fall respectively at sequence boundaries Ru4–Ch1 and Ch4–Aq1 and glacioeustatic perturbations OCi-1 and MAi-1 (= Mi1), but the Jan Juc-1–Jan Juc-2 boundary within the Jan Juc Formation falls close to the flooding surface of sequence TB1.2.
These third-order patterns of recurrence and sequential change were largely sustained at higher frequencies in the study of an interval approaching a glacial within the Jan Juc-1. Samples at the centimeter scale (about 2–4 cm spacing) over one meter of alternating soft and hard (lithified) layers yield four biofacies groups mainly on abundance variations of individual species and species groups. The clusters are cleanly separated superpositionally (thus, strongly successional), reflecting environmental cycles at 104-year scale, perhaps in the Milankovitch band of 41,000 years. Shallower-water species dominate clusters B and D from hard layers, whereas deeper-water species are more abundant in clusters A and C from soft layers. The differences suggest paleodepth change of 50–70 m, with maxima in the soft layers and minima at the tops of hard layers. The high abundance of infauna and a stronger mixing between shallower-water and deeper-water species indicates an oxygen-poor environment coupled with bioturbation.
Similarities between faunas of third-order and Milankovitch scales include: (i) coincidence of biofacies with lithofacies or lithostratigraphy is due largely to abundance variations of the prominent species, (ii) recurring biofacies signals of sea-level change are chronologically consistent with other published proxies of glacioeustasy, and (iii) clustered assemblages of benthic foraminifera are distinct and strongly successional.