Recognition and significance of Upper Devonian fluvial, estuarine, and mixed siliciclastic-carbonate nearshore marine facies in the San Juan Mountains (southwestern Colorado, USA): Multiple incised valleys backfilled by lowstand and transgressive systems tracts

Abstract

The Upper Devonian Ignacio Formation (as stratigraphically revised) comprises a transgressive, tide-dominated estuarine depositional system in the San Juan Mountains (Colorado, USA). The unit backfills at least three bedrock paleovalleys (10–30 km wide and ≥42 m deep) with a consistent stratigraphy of tidally influenced fluvial, bayhead-delta, central estuarine-basin, mixed tidal-flat, and estuarine-mouth tidal sandbar deposits. Paleovalleys were oriented northwest while longshore transport was to the north. The deposits represent Upper Devonian lowstand and transgressive systems tracts. The overlying Upper Devonian Elbert Formation (upper member) consists of geographically extensive tidal-flat deposits and is interpreted as mixed siliciclastic-carbonate bay-fill facies that represents an early highstand systems tract. Stratigraphic revision of the Ignacio Formation includes reassigning the basal conglomerate to the East Lime Creek Conglomerate, recognizing an unconformity separating these two units, and incorporating strata previously mapped as the McCracken Sandstone Member (Elbert Formation) into the Ignacio Formation. The Ignacio Formation was previously interpreted as Cambrian, but evidence that it is Devonian includes reexamined fossil data and detrital zircon U-Pb geochronology. The Ignacio Formation has a stratigraphic trend of detrital zircon ages shifting from a single ca. 1.7 Ga age peak to bimodal ca. 1.4 Ga and ca. 1.7 Ga age peaks, which represents local source-area unroofing history. Specifically, the upper plate of a Proterozoic thrust system (ca. 1.7 Ga Twilight Gneiss) was eroded prior to exposure of the lower plate (ca. 1.4 Ga Uncompahgre Formation). These results are a significant alternative interpretation of the geologic history of the southern Rocky Mountains.

Early Pliocene Marine Transgression into the Lower Colorado River Valley, Southwestern USA, by Re-Flooding of a Former Tidal Strait

“…7A-C, 9B-C). This architecture is unlike other examples of backstepping bayhead deltas where riverderived pro-delta muds and silts overlie older sand-rich delta-plain deposits, thus defining a preserved transgressive systems tract located seaward of the retreating delta front (Aschoff et al, 2018;Simms et al, 2018;Evans et al, 2019;Rossi et al, 2021). In contrast to these published examples, the stratal architecture of the Bouse Formation in the southern Blythe basin (Figs.…”

CrowdedTrichophycusichnofabrics in the early Ordovician successions of central Iran: insight into the Ordovician radiation

“…However, its use is limited due to the narrow stratigraphical range (mostly Ordovician) and its restriction to the upper offshore and more rarely to distal lower shoreface (but see Fillion & Pickerill 1990 for examples in middle tidal flats, lagoon and sand bar facies, and Evans et al . 2019 for the occurrence of Trichophycus in Devonian estuarine firmgrounds). In the upper offshore mudstones, the increase in the cohesiveness can be inferred from the lack of burrow linings and by the preservation of horizontal, gutter‐shaped laminae with scratch marks.…”

“…2004) and the southwestern Colorado, USA (Evans et al . 2019), respectively, and in the Upper Carboniferous of Gondwana (Seilacher 1990).…”