Mudstones and embedded concretions show differences in lithology-related, but not source-related biomarker distributions

Abstract

The mudstones of the Western Australian Gogo Formation harbour numerous carbonate concretions which often contain preserved fossils of Lagerstätte-like quality. These are especially notable in places where the mudstone has eroded, giving way to nodule fields, which afford valuable paleobiological records. It is, however, a challenge to determine their paleoenvironmental context. Here, we analysed concretions from a core drilled in the Canning Basin Gogo Formation. At two different depths, concretions were compared to the surrounding mudstone found at the same depth. Electron microscopy and X-ray spectroscopy showed that the concretions were carbonate-rich and contained detrital fragments. Biomarker data showed that mudstone and concretions had very similar distributions and presented marine biosignatures including indicators of anoxic depositional conditions, a stratified water column, and photic zone euxinia. The concretions contained higher amounts of C27 steranes, indicating that more labile organic matter such as animal remains could have triggered concretion formation. Statistical analyses of the results showed that concretions and shales differed (p<0.1) in indices diagnostic for lithologies (and often related to maturity), particularly in the sample recovered from the younger section of the core: the diasterane/sterane indices at each of the depths were 0.34 in the shale vs 0.12 in the concretion in the younger set of samples, and 0.21vs 0.19 in the older set. The homohopane isomerisation indices of the mudstone and concretion were 0.66vs 0.58 and 0.62vs 0.60 (47 and 54m depth, respectively). Furthermore, shales contained higher relative amounts of moretanes. Concretions also differed from the mudstone in their methylhopane distributions, with shales showing higher amounts of 3-methylhopanes. Conversely, when biomarker composition was compared at the two depths, they only differed significantly (p<0.1) in the 2-methylhopane index,%C31 homohopanes, and hopane/sterane ratios (p<0.1). Our results show that the biomarker composition of the mudstone at the time of deposition is largely preserved in the concretions with only a minor overprint from diagenesis, bacterial communities involved in concretion formation, or the organic matter nucleus. It is therefore possible to use biomarker analysis on carbonate concretions to determine their provenance if found outside their immediate geological context.