Microfacies Characteristics of a Tropical, Mangrove-Fringed Shoreline, Cleveland Bay, Queensland, Australia: Sedimentary and Taphonomic Controls on Mangrove Facies Development

Abstract

Mangrove ecosystems strongly influence the sedimentology and geomorphology of contemporary intertidal environments and are known to have occupied similar ecological niches at least as far back as the early Cenozoic. Given this it is remarkable how few records of mangrove occurrence have been reported in the geologic record. Here a microfacies approach is adopted to examine the characteristics of a contemporary mangrove-colonized intertidal succession along an active prograding shoreline in north Queensland, Australia, and compared with a proximal mid–late Holocene preserved intertidal succession. This enables a comparison of microfacies development and preservation potential in like-for-like depositional environments over short temporal and spatial scales. In the contemporary intertidal system, active shoreline progradation has produced a highly gradational shallowing-upward succession. Mid- to low-intertidal substrates, characterized by an abundant and diverse bioclast (especially molluscan) fauna and a rich calcareous foraminiferal assemblage, are overlain by organic-rich (upper-intertidal) mangrove facies, characterized by abundant organic material (especially wood and bark fragments) and a distinct fauna of agglutinated foraminifera. The major sedimentary effects of mangrove progradation are to introduce substantial amounts of organic matter into the substrate (effectively overprinting the mudflat facies), to mix and re-texture (due to root network development and infaunal bioturbation) the sediment substrate, and to modify the depth ranges over which infaunal molluscs and foraminifera occur. Variations in the relative abundance of constituents permit, however, the clear delineation of mangrove and intertidal mudflat facies. Whilst an essentially similar facies succession is identifiable in the sub-Recent, many of the diagnostic facies constituents are either not preserved or are present in far lower abundances than in the modern. In particular, the inferred mangrove units contain far less organic material (most wood and root material is absent), are essentially devoid of bioclastic material and contain a depauperate agglutinated foraminiferal fauna. The primary drivers of this are (1) loss of calcareous foraminifera and bioclasts due to carbonate dissolution, (2) loss of agglutinated foraminifera due primarily to biological (fiddler crab) test degradation and mineralization of their organic binding agents, and (3) aerobic and anaerobic degradation of organic material. Consequently, actual criteria for identifying the presence of a former mangrove in this succession are limited. Such poor preservation, even within sub-Recent mangrove deposits, suggest that the longer-term preservation potential of key diagnostic criteria is likely to be extremely poor within similar terrigenoclastic-dominated, organically enriched successions. Thus, whilst mangroves significantly modify and re-texture intertidal substrates, use may often need to be made of sparsely preserved faunal or floral remains or traces, interpreted in the context of the broader intertidal depositional succession, to infer the presence of mangrove-colonized substrates in the rock record.