Palaeoecology of the Bright Angel Shale in the eastern Grand Canyon, Arizona, USA, incorporating sedimentological, ichnological and palynological data

Abstract

The Middle Cambrian Bright Angel Shale in the eastern Grand Canyon contains a depauperate normal marine fauna, but trace fossils and palynomorphs are abundant throughout the formation. Conventional interpretations place the depositional setting of this shale below wavebase as the distal component of a shelfal transgression, but the palynological signature in the mudstones of the Bright Angel Shale indicates a freshwater source to these muds. Examination of several sections in the vicinity of Proterozoic monadnocks and the integration of sedimentological, ichnological and palynological observations yield a more robust model for the palaeoecology of the Bright Angel Shale. Initial correspondence between organic matter content in mudstones and feeding type and intensity (as indicated by traces) is consistent with an estuarine setting for this deposit. The level of organic activity preserved in these sediments indicates that the carbon flux into shallow marine settings due to terrestrial runoff was substantial by middle Cambrian (Glossopleura biozone) time.

Were all trilobites fully marine? Trilobite expansion into brackish water during the early Palaeozoic

“…Trilobite nests in early to middle Cambrian open-marine tidal flats reveal that some groups were able to tolerate stressors in extremely shallow water, albeit not necessarily reduced salinity [65][66][67]. Trilobite trace and body fossils have been recorded in middle Cambrian marginal-marine deposits [79,80], although doubts persist regarding the taxonomy and precise stratigraphic position of the latter [80]. The trilobite ichnofossils occur in deposits interpreted as formed in unincised estuaries [79] or the so-called expansive epicratonic estuaries [80].…”

AeroterrestrialColeochaete(Streptophyta, Coleochaetales) models early plant adaptation to land

“…Cambrian deposits of estuarine and near‐shore marine provenance associated with the Laurentian Inner Clastic Belt (Lochman‐Balk, 1970) contain a vast suite of cryptospores ranging in age from the uppermost lower Cambrian (Rome Formation, Tennessee) to the latest Cambrian strata in Wisconsin (Taylor and Strother, 2009). These deposits are in direct proximity to paleolandscapes in the middle Cambrian Bright Angel Shale in the eastern Grand Canyon (Baldwin et al, 2004). The deposits lack typical marine elements in the palynomorph associations (Strother and Beck, 2000).…”

“…Organic‐walled cryptospores (as defined by Strother and Beck, 2000) of Middle Cambrian age are common in near‐shore marine and estuarine deposits marginal to the ancient Laurentian continent (Strother et al, 2004; Baldwin et al, 2004). These deposits contain a diverse mixture of spore‐like morphologies, however, they all lack populations of tetrahedral tetrads, a key feature used to associate Ordovician spores with embryophytic sources.…”

The terrestrial biota prior to the origin of land plants (embryophytes): a review of the evidence

“…These are from the following: (1) middle Cambrian estuarine deposits of the Bright Angel Shale from the Grand Canyon, Arizona, USA (Strother ; Strother and Beck ; Baldwin et al . ; Strother et al . ; Taylor and Strother ); (2) middle Cambrian shallow marine deposits of the Rogersville Shale, Conasauga Group from Tennessee, USA (Strother ; Strother et al .…”