Cross‐platform architecture of a sequence boundary in mixed siliciclastic‐carbonate lithofacies, Middle Cambrian, southern Great Basin, USA

Abstract

Stratigraphic analysis of mixed siliciclastic‐carbonate lithofacies within the Middle Cambrian Bonanza King Formation of the southern Great Basin reveals three distinct facies associations that record a range of depositional environments from semi‐arid tidal flats to deeper subtidal, restricted lagoons. Stratigraphic trends, cross‐platform facies variations and correlation of individual surfaces across 250 km of the study area suggest that these mixed lithofacies were deposited in three temporally distinct phases. (1) Extensive progradation of mixed peritidal environments culminated in a prolonged episode of subaerial exposure marked by an areally extensive intraclast breccia (0·5–1·2 m thick) that we interpret to be a major Type 1 sequence‐bounding disconformity. (2) Abrupt flooding of the exposed platform resulted in the deposition of mixed deeper subtidal lithofacies, including a condensed interval of fissile, fossiliferous shale. (3) Progressive shallowing and aggradational accumulation was accompanied by a decrease in siliciclastics and a shift to pure carbonate deposition. Deep‐water siliciclastics and megabreccias record deposition along the base‐of‐slope off the Middle Cambrian shelf‐edge, and are interpreted to represent lowstand deposits emplaced during the prolonged episode of subaerial exposure of the shallow shelf. The presence of fine siliciclastics in both peritidal facies and sharply overlying deeper subtidal facies of the study interval within the Bonanza King suggests a variable, but relatively continuous, influx of terrigenous material throughout an extended period of accommodation change, apparently asynchronous with respect to the predictive model of reciprocal sedimentation. We suggest that the primary siliciclastic source changed with relative sea‐level position. During lowered sea level, aeolian processes acting upon the unvegetated Cambrian craton transported fine siliciclastics onto peritidal and shallow‐subtidal environments. During higher sea level, coastal siliciclastic reservoirs supplied sediment that was transported for long distances by geostrophic currents flowing along the submerged platform. As opposed to many Cambro‐Ordovician grand cycles that are commonly interpreted to consist of a transgressive shaly half‐cycle grading upward into a regressive carbonate half‐cycle, the sequence boundary within this Middle Cambrian succession occurs within siliciclastic‐rich, mixed lithofacies rather than in adjoining purer carbonates, implying that some ‘grand cycles’ should not be considered synonymous with ‘sequences’. Interbasinal correlations of the Type 1 sequence boundary within the mixed unit are speculative, primarily because of the inherent imprecision of available trilobite biostratigraphy. However, there is evidence that an extended episode of subaerial exposure may have been continent‐wide during the Ehmaniella trilobite biochron.

Carbonates in Skeleton-Poor Seas: New Insights From Cambrian and Ordovician Strata of Laurentia

“…1). Extensive carbonate platforms developed on its northern, southern, and western margins; in some areas, Cambrian carbonate strata alone attain thicknesses near 2.5 km (e.g., Ginsberg, 1982;Montañ ez and Droser, 1991;Osleger and Montañ ez, 1996). Our samples come from two regions.…”

Argentine Precordillera: Sedimentary and Plate Tectonic History of a Laurentian Crustal Fragment in South America

“…The sediments were deposited under marine conditions below fair-weather wave base, where thick successions of carbonate mud with well-preserved fossils accumulated. As pointed out by Osleger and Montañez (1996), these deeper or quieter water conditions do not necessarily imply truly deep water. Instead, those sediments may also form in a deep shelf lagoon that is protected from the open ocean by a well-developed shelf-margin rim (Fig.…”

Modern non-tropical mixed carbonate-siliciclastic sediments and environments of the southwestern Gulf of California, Mexico

“…Dullo and Hecht, 1990;Ferro et al, 1999;Piller and Mansour, 1994;Best and Kidwell, 2000;Dunbar et al, 2000;Kronen and Glenn, 2000;Testa and Bosence, 1998; whereas ancient mixed systems were investigated by, e.g. Sieverding and Harris, 1991;Ginsburg and Cunningham, 1996;Osleger and Montanez, 1996;Warzeski et al, 1996;Guertin et al, 2000;Sanders and Hoefling, 2000. Most publications dealing with recent mixed systems, however, are concerned with the interactions of carbonates and siliciclastics in tropical environments, thereby neglecting the less common cool and temperate or so-called heterozoan carbonate depositional systems. Exceptions to this are studies by Scoffin (1988) (Scotland); Nelson et al (1988) (New Zealand); James et al (1992) (Lacepede Shelf, southern Australia); Carey et al (1995) (western Canada); Gillespie and Nelson (1997) (New Zealand); Light and Wilson (1998) (Shetland Shelf).…”