Although Late Cambrian microbial build-ups were recognized in the Point Peak Member of the Wilberns Formation in Central Texas (USA) nearly 70 years ago, only a few studies focused specifically on the build-ups themselves. This study focuses on the interpretation of the regional (15 measured sections described in literature representing an area of 8000 km 2) and local (field and drone photogrammetry studies in a 25 km 2 area from within south Mason County) microbial build-up occurrence, describes their growth phases and details their interactions with the surrounding inter-build-up sediments. The study establishes the occurrence of microbial build-ups in the lower and upper Point Peak members (the Point Peak Member is informally broken up into the lower Point Peak and the upper Point Peak members separated by Plectotrophia zone). The lower Point Peak Member consists of three <1 m thick microbial bioherms and biostrome units, in addition to heterolithic and skeletal/ooid grainstone and packstone beds. One, up to 14 m thick, microbial unit associated with inter-build-up skeletal and ooid grainstone and packstone beds, intercalated with mixed siliciclastic-carbonate silt beds, characterizes the upper Point Peak member. The microbial unit in the upper Point Peak member displays a three-phase growth evolution, from an initial colonization phase on flat based, rip-up clast lenses, to a second aggradation and lateral expansion phase, into a third well-defined capping phase. The ultimate demise of the microbial build-ups is interpreted to have been triggered by an increase of water turbidity caused by a sudden influx of fine siliciclastics. The lower Point Peak member represents inner ramp shallow subtidal and intertidal facies and the upper Point Peak member corresponds to mid-outer ramp subtidal facies. Understanding the morphological architecture and depositional context of these features is of importance for identifying signatures of early life on Earth.

The upper Cambrian Wilberns Formation in central Texas records deposition on a low-gradient shelf within a mixed carbonate-siliciclastic tidal-flat system that changes offshore to subtidal shelf and open-marine oolitic skeletal shoals with large microbial mounds. Siliciclastic sediment is interpreted to have been delivered to the tidal flat by aeolian processes because of the narrow range in grain size and paucity of clay. Tidal influence is dominant as evidenced by reversing currents and desiccation on the tidal flat, and megaripples with reversing current indicators in offshore shoals. Intraclastic conglomerates were deposited in broad channels on the tidal flats during storm surges. Microbialite deposition is interpreted to be controlled by accommodation favouring amalgamated thin biostromes developed in the tidal flat vs. larger mounds with greater synoptic relief in the offshore, and current energy resulting in preferential elongation of offshore mounds in a NE-SW orientation. Intertidal mounds and biostromes grew in the presence of significant siliciclastic flux and trapped it within their structure, whereas offshore large buildups incorporated little siliciclastic component. Oolite and skeletal grainstone formed in tide agitated shoals associated with large subtidal microbial mounds. Storms extensively recycled and redistributed skeletal and oolitic sands from the offshore shoals across the shelf as thin sand sheets. Spatial mixing of siliciclastic and carbonate sediment occurred across the tidal flat and shelf. Low-frequency and intermediate-frequency stratigraphic cycles were driven by shifts in the shoreline and changes in rate of siliciclastic flux in response to relative sea-level fluctuation. Random facies stacking and the lack of metre-scale cyclicity are interpreted to reflect stratigraphic incompleteness and an episodic signal introduced by storms.K E Y W O R D S Cambrian, carbonate, Laurentia, microbialite, oolite, siliciclastic 278 | LEHRMANN Et AL.microbialites, oolites and other deposits such as flat-pebble conglomerates, and the controls on cyclicity, in the Wilberns Formation through evaluation of the environmental constraints provided by sedimentary structures and sediment composition in detailed stratigraphic sections of a mixed carbonate-siliciclastic system.

We have developed and validated a new approach to upscale lithology and porosity-type fractions from thin sections to cores using dual energy and multiscale computed tomography (CT). A new rock-typing approach (genetic rock typing [GRT]) is proposed to upscale ⇋diagenetic mineral and diagenetic pore-type fractions, from thin sections to the core domain, eventually to create a diagenesis and porosity types logs. An extensive set of short cores from Mason County (Texas) provides a representative sample set of Late Cambrian microbial buildups and their interbuildup sediments to test the GRT approach. GRTs were defined by using a dolomite log as a proxy for diagenesis and the average percentage of dolomite from each observed depositional facies (buildup interior, buildup rind, and interbuildup sediment) as a cutoff. Dolomite, diagenetic calcite, and diagenetic porosity fractions are summed to form a diagenesis log, which captures depositional facies and the diagenetic overprint at a 0.5 mm resolution. The diagenesis log was subdivided based on the number of pore-throat size classes within each GRT and provided a framework to distribute porosity-type fractions from thin sections to log form. A high correlation coefficient is observed when the predicted extent of diagenetic alteration from the log is compared with that quantified for each thin section using image processing ([Formula: see text]). Multiscale CT imaging and dual-energy-derived logs could be directly linked to well-log photoelectric factor and bulk-density logs. This approach thus has the ability to span six orders of magnitude in resolution (500–0.0005 mm). The diagenesis log can be used to extrapolate porosity-type fractions from thin sections to logs, from which qualitative geologic interpretations can be generally translated into quantitative values.

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