Paleoecology of an early Miocene, rapidly submerging rocky shore, Motuketekete Island, Hauraki Gulf, New Zealand

More than 70 macrofossil taxa (including 14 bivalves, 6 gastropods, 8 corals, 4 echinoderms, and 10 barnacles) are recorded from early Miocene (Otaian) Kawau Subgroup strata (Cape Rodney Formation and Motuketekete Limestone, lower Waitemata Group) at Motuketekete Island, Hauraki Gulf, north of Auckland City. Both in situ and transported fossils occur in deposits of greywacke boulder conglomerate, cobble to pebble conglomerate/sandstone, bioclastic calcareous grainstone, and an allochthonous breccia debris event unit, which correspond to lithofacies A, C, D, and E, respectively, of Ricketts et al. Greywacke boulders accumulated at the base of a greywacke paleocliff or sea stack that was planed off at its top to form a shore platform during the Miocene. A >2 m long, 16 cm thick coral colony grew atop a mixed substrate of boulders, pebbles, and sand, and exhibits two successional regrowth phases following debris‐influx events. Boulders and cobbles bored by pholadid bivalves (Parapholas aucklandicum Powell) are common in these basal bouldery talus deposits. A diverse suite of macrofossils, including chaliciform corals, occurs in somewhat finer grained deposits that buried the greywacke basement and boulder talus pile, and indicates either slightly deeper or more turbid conditions in the shallow photic zone. The cross‐bedded, bioclastic Motuketekete Limestone overlies these coarse‐grained Cape Rodney Formation units. Its fauna indicates deepening, with replacement of shallow by offshore taxa (e.g., Crenostrea with Bathylasma), as the Waitemata Basin underwent rapid tectonic subsidence and redeposition of sediments with the onset of subduction along the Hikurangi convergent margin. A newly identified lens of “upper breccia” (lithofacies E) in the Motuketekete Limestone contains rounded blocks of colonial coral and Tertiary siltstone. The Motuketekete occurrence of lithofacies E extends the known geographic range of this geologically instantaneous deposit; it records a regional tectonic event that is interpreted as an avalanche/debris flow triggered by faulting. The breccia appears to be a reliable marker for local lithostratigraphic correlation in lower Waitemata strata exposed north of Auckland.

Appendices provide a systematic analysis of the barnacle fauna and the description of a new gastropod species, Bolma (Bolma) ballancei. The overall biotic content of the Kawau Subgroup, especially those taxa associated with hard substrate community development, indicate warm subtropical conditions similar to those found elsewhere in northern New Zealand during the early Miocene.