Submergent shorelines in the SW Pacific: evidence for an episodic post‐glacial transgression

Abstract

ABSTRACT High resolution seismic profiles, supported where possible by radiocarbon dates and regional stratigraphic data, indicate that the last post‐glacial transgression in the SW Pacific was episodic, comprising major stillstands punctuated by rapid rises in sea‐level.On the terrigenous continental shelf east of South Island, New Zealand, a succession of shorelines (S8‐S1) are recognized, as follows: S8 =c. −113 m, 18,000 yr BP; S7 =c. −88 m, 17,000 yr BP; S6 =c. −75 m, 15,000 yr BP; S5 =c. −56 m, 12,000 yr BP; S4 =c. −46 m, 11,000 yr BP; S3 =c. −28 m, 9,500 yr BP; S3a =c. −24 m, 9,000 yr BP; S2 =c. −9 m, 7,500 yr BP; S1 = 0 m, 6,500 yr BP.With the exception of S8, and possibly S2, the shorelines are associated with wedges of sediment, the size and presence of which imply that (1) sea‐level stabilized at some shorelines for a considerable period of time (up to 1‐2,000 yr); and (2) the intervening rises of sea‐level, estimated to have been at least 10‐12 m 103 yr−1, were too rapid to allow the reworking of the wedges into a transgressive sediment sheet, as favoured in some current models.On the Great Barrier Reef shelf, off Queensland Australia, shorelines S8‐S1 have also been recognized, with a further shoreline feature S4a occurring at c. ‐39 m. Shorelines S1a (0 m/0 yr BP), Sib (+ 2‐3 m/6,000 yr BP) and Sic (0 m/6,500yr BP) are recognized as discrete aspects of the post‐6,500 yr BP sea‐level behaviour in north‐eastern Australia. The rapid rise in sea‐level, at least between shorelines S5 (12,000 yr BP) and S3 (9,500 yr Bp), is known to have outpaced reef growth, causing in situ drowning of reefs located along the deeper shorelines. All modern reefs so far drilled and dated began their development at or above S3 (‐28 m, 9,500 yr BP).Some of the shorelines, particularly S5, appear to correlate between the northern and southern hemispheres on the basis of age, succession and general depth of occurrence, suggesting (1) that they may be global features controlled by the post‐glacial pattern of ice‐sheet decay; and (2) that hydro‐isostatic adjustment may exert only a minor control on the depth of particular shorelines, at least during the earlier parts of the post‐glacial transgression.