Microfacies and diagenesis of older Pleistocene (pre‐last glacial maximum) reef deposits, Great Barrier Reef, Australia (IODP Expedition 325): A quantitative approach

Abstract

During Integrated Ocean Drilling Program Expedition 325, 34 holes were drilled along five transects in front of the Great Barrier Reef of Australia, penetrating some 700 m of late Pleistocene reef deposits (post‐glacial; largely 20 to 10 kyr bp) in water depths of 42 to 127 m. In seven holes, drilled in water depths of 42 to 92 m on three transects, older Pleistocene (older than last glacial maximum, >20 kyr bp) reef deposits were recovered from lower core sections. In this study, facies, diagenetic features, mineralogy and stable isotope geochemistry of 100 samples from six of the latter holes were investigated and quantified. Lithologies are dominated by grain‐supported textures, and were to a large part deposited in high‐energy, reef or reef slope environments. Quantitative analyses allow 11 microfacies to be defined, including mixed skeletal packstone and grainstone, mudstone‐wackestone, coral packstone, coral grainstone, coralline algal grainstone, coral‐algal packstone, coralline algal packstone, Halimeda grainstone, microbialite and caliche. Microbialites, that are common in cavities of younger, post‐glacial deposits, are rare in pre‐last glacial maximum core sections, possibly due to a lack of open framework suitable for colonization by microbes. In pre‐last glacial maximum deposits of holes M0032A and M0033A (>20 kyr bp), marine diagenetic features are dominant; samples consist largely of aragonite and high‐magnesium calcite. Holes M0042A and M0057A, which contain the oldest rocks (>169 kyr bp), are characterized by meteoric diagenesis and samples mostly consist of low‐magnesium calcite. Holes M0042A, M0055A and M0056A (>30 kyr bp), and a horizon in the upper part of hole M0057A, contain both marine and meteoric diagenetic features. However, only one change from marine to meteoric pore water is recorded in contrast with the changes in diagenetic environment that might be inferred from the sea‐level history. Values of stable isotopes of oxygen and carbon are consistent with these findings. Samples from holes M0032A and M0033A reflect largely positive values (δ18O: −1 to +1‰ and δ13C: +1 to +4‰), whereas those from holes M0042A and M0057A are negative (δ18O: −4 to +2‰ and δ13C: −8 to +2‰). Holes M0055A and M0056A provide intermediate values, with slightly positive δ13C, and negative δ18O values. The type and intensity of meteroric diagenesis appears to have been controlled both by age and depth, i.e. the time available for diagenetic alteration, and reflects the relation between reef deposition and sea‐level change.

Late glacial to deglacial variation of coralgal assemblages in the Great Barrier Reef, Australia

“…The older Pleistocene basement (≥30 ka) was reached in 14 holes and consists mostly of reefal deposits below the barrier reef and inner terrace, and grey packstone and grainstone containing Halimeda segments and larger benthic foraminifers below the deeper terraces (Gischler et al, 2013;Yagioka et al, 2019). Diagenetic alteration of the older Pleistocene basement in holes from the barrier reef and inner terrace indicates subaerial exposure prior to postglacial sea-level rise (Gischler et al, 2013;. The MIS2 to last deglacial reef sequence penetrated by coring varies greatly in thickness, ranging from ca.…”

The evolution of the Great Barrier Reef during the Last Interglacial Period

“…were used for taxonomic identification of coralline algae and for microfacies examination. Point count analysis using two-hundred points per thin section was used to quantify skeletal and non-skeletal microfacies components (Gischler et al, 2013).…”

Coral reef response to Quaternary sea‐level and environmental changes: State of the science

“…()] and offshore the Australian Great Barrier Reef (IODP Expedition 325; Webster et al ., ; Yokoyama et al ., ). Furthermore, valuable information regarding sea‐level positions and insights into the climate system for periods prior to the last glaciation, such as MIS 3 and earlier, were obtained in the Tahiti and Great Barrier Reef records (Camoin et al ., ,b; Thomas et al ., ; Fujita et al ., ; Ménabréaz et al ., ; Webster et al ., ; Gischler et al ., ).…”