Interactions of A Paleocene River, A Rising Fold, and Early-Diagenetic Concretions

Abstract

The relative rates of sediment accumulation, erosion, and structural uplift determine whether a growing fold develops positive topographic relief, is beveled by antecedent streams, or is buried under thick growth strata. When folds rise in subsiding basins, upward, convergent flow of groundwater through the permeable growth strata that underlie antecedent streams enhances the flux of ions required for concretion growth. Early diagenetic concretions that grow in such alluvial strata may constitute the only clasts larger than sand size available for transport when antecedent streams become erosive. The first reworked concretions deposited by these streams should accurately mark the transition from aggradation to erosion as folds rise into the paths of streams. In this situation, the ability to differentiate reworked from in situ concretions is crucial. The west-vergent Simpson Ridge anticline, a N-S-trending, thickskinned Laramide structure in east-central Wyoming, separates the larger Hanna basin from the Carbon basin. Near the north nose of this anticline, in situ ironoxide-rich concretions are abundant in folded Paleocene strata (Ferris Formation) and, just to the east, large, reworked, iron-rich concretions are abundant in younger, more gently dipping conglomerates in the basal Hanna Formation of the backlimb. Smaller reworked concretions are also present near the base of the Hanna Formation at least 7 km south of the anticlinal nose and just east of the fold's axis. At the anticlinal nose, in situ (non-reworked) concretions up to 3 m 3 1 m 3 1 m are abundant at the top of an~7km-thick sequence of sandstones and siltstones that constitute the Late Cretaceous-early Paleocene Ferris Formation. Reworked concretions are absent in the strata hosting these in situ concretions, but reworked concretionary clasts up to 2 m in diameter are present in exposures of conglomerates in the lowermost Hanna Formation that lie just above the in situ Ferris concretions and southeast of the anticlinal nose. These early-diagenetic concretions were originally cemented by siderite (FeCO 3). Oxidation of some small, rinded siderite-cemented clasts took place after their fluvial transport into the Hanna Formation, but abundant angular, un-rinded, iron-oxide-cemented clasts indicate that many large, in situ siderite concretions had resided in the vadose zone before they were entrained. The distribution of reworked concretions and the orientations of crossbeds show that antecedent Hanna streams eroded a swath at least 5 km wide across the rising structure. These streams transported Ferris Formation concretions southeastward into the Carbon basin, and deposited them in a conglomeratic sandstone body in the Hanna Formation. Large calcitecemented concretions, many with a pipe-like morphology, then grew within Hanna crossbeds. In many cases, these in situ concretions enclose transported, iron-rich concretions, but there is no evidence any calcite-cemented concretions were reworked. The NW-SE alignment of the pipes record southeastward flow of...

A new stratigraphic framework and constraints for the position of the Paleocene–Eocene boundary in the rapidly subsiding Hanna Basin, Wyoming

“…These shallow lag deposits might represent fan rather than true fluvial deposits. The iron oxide-cemented sandstone clasts appear similar to ones described in the nearby Carbon Basin by Loope and Secord (2017), who proposed that these concretions originally formed as siderite nodules in the Ferris Formation. These nodules were then exposed by erosion from active uplift by underlying structures and resedimented in the Hanna Formation.…”

“…Initially, the Hanna Basin was part of the Greater Green River Basin, later isolated into smaller subbasins by localized uplifts of the Laramide orogeny during the mid to late Paleocene ( Fig. 1; Ryan, 1977;Blackstone, 1993;Perry and Flores, 1997;Secord;Kraatz, 2002;Wroblewski, 2003;Lillegraven, 2015;Smith et al, 2015b;Loope and Secord, 2017). The emergence of these uplifts influenced provenance and sedimentation patterns indicating predominantly local sources after this time (LeFebre, 1988;Blackstone 1993;Perry and Flores, 1997;Lillegraven et al, 2004;Peyton and Carrapa, 2013).…”

“…The emergence of these uplifts influenced provenance and sedimentation patterns indicating predominantly local sources after this time (LeFebre, 1988;Blackstone 1993;Perry and Flores, 1997;Lillegraven et al, 2004;Peyton and Carrapa, 2013). Synsedimentary deformation within the Hanna and underlying Ferris Formations is well documented, especially along the basin's edges (Secord, 1998;Kraatz, 2002;Wroblewski, 2003;Lillegraven et al, 2004;Loope and Secord, 2017…”

“…The base of the Hanna Formation is considered conformable with the Ferris Formation in the center of the Hanna Basin (Knight, 1951;Blackstone, 1993;Flores et al, 1999b). However, the basin margins show disconformable to angular contacts, because active underlying structures influenced accommodation and disrupted sedimentation (Knight, 1951;Love and Christiansen, 1985;Blackstone, 1993;Perry and Flores, 1997;Secord, 1998;Lillegraven et al, 2004;Loope and Secord, 2017). Hanna Formation reported thickness is therefore variable and greater in the center of the basin, and several maximum thickness values have been reported, ranging from ~3500 m (11,000 ft) to 2150 m (7000 ft) (Dobbin et al, 1929;Gill et al, 1970;LeFebre, 1988;Lillegraven and Snoke, 1996;Wroblewski, 2003;WOGCC, 2016).…”

“…(Fig. 13; Blackstone, 1993;LeFebre, 1988;Perry and Flores, 1997;Wroblewski, 2002;Flores, 2003;Loope and Secord, 2017). Emerging uplifts caused drainage reorganization and possibly climatic rain shadows that changed water balance and sedimentation patterns in the basin.…”