Styles, origins and implications of syndepositional deformation structures in Ediacaran microbial carbonates (Nama Basin, Namibia)

Abstract

Outstanding exposures of Ediacaran-aged thrombolite-stromatolite bioherms and biostromes crop out in the Nama Basin, SW Namibia. Fieldwork, dovetailed with remote sensing and a terrestrial laser scanning (LiDAR) survey, allow the fracture network of this succession to be characterized, and the relative age of fracture sets and families to be determined. The results show that the microbial carbonates were affected by intense syndepositional brittle and ductile deformation. Early brittle fracturing was favoured where early lithification of microbialites took place upon deposition. Such deposits were prone to gravitational collapse due to internal weaknesses during early lithification. Timing of syndepositional fracturing of bioherms and biostromes is demonstrated by contemporaneous microbial overgrowth over brecciated material in open-mode fractures. Ductile deformation occurs preferentially around massive thrombolite domes and columns, represented by folding of mud-dominated sediments in inter-column fill. Secondary fractures developed during the long-lived structural history of the Nama Basin, resulting in a complex fracture network of syndepositional fractures overprinted by secondary fractures. These findings have important implications for carbonate reservoir characterization in microbial reservoirs and subsurface fluid-flow estimations. The observed syndepositional fractures form due to body forces that are intrinsic to the microbial system and thus do not require an external tectonic driver.

Microbial carbonates in space and time: introduction

“…These microbial systems were able to form homoclinal ramps, distally steepened ramps with fringing microbial reefs and ooids shoals, and rimmed carbonate shelves with accretionary and escarpment margins (Grotzinger 1989;Amthor 2013b). Winterleitner et al (2015) present a 3D digital reservoir model of mid ramp microbialite buildups from the Neoproterozoic Nama Group of Namibia. Digital outcrop models are used to capture relationships between buildups and intervening packgrainstones as well as the distribution of fractures.…”

Questioning the microbial origin of automicrite in Ordovician calathid–demosponge carbonate mounds

“…By process, these two characters distinguish physical sedimentation in combination with particle binding (Awramik & Grey, ) from the localized adherence, agglutination and final enclosure of sedimentary particles (Hoffmann et al ., ; Neuweiler et al ., ). The parameter consolidation separates early indurated fabrics (AM‐1 to AM‐4; Neuweiler & Reitner, ) from ductile–brittle deformation (AM‐5) in response to gravitational effects and mechanical compaction (microgliding, microfolding, repacking and rupture; Awramik & Grey, ; Winterleitner et al ., ). The parameter contemporaneity distinguishes contemporaneity or diversity in space from a time‐controlled stratigraphic sequence (Hepburn et al ., ).…”

The role of siliciclastics in carbonate fabric diversity and preservation: A case study from the Neoproterozoic carbonate − siliciclastic Horse Thief Springs Formation, Death Valley

“…Although stromatolites re‐occur throughout Units 2 and 3 in both localities, they do not form massive metre‐scale reefal facies with complex morphological thrombolitic to stromatolitic varieties as observed in other Proterozoic shallow water carbonate platforms (Bekker & Eriksson, 2003; Corkeron & George, 2003; Kah et al ., 2009; Winterleitner et al ., 2015). The global scale of shallow marine carbonate precipitation has varied little over the course of Earth history (Grotzinger & James, 2000), suggesting a constantly productive shallow water carbonate factory despite dramatic changes in sources for carbonate sediment.…”