REWORKED LARGE, IRON-RICH CONCRETIONS AT THE BASAL CONTACT OF THE PALEOGENE HANNA FORMATION IN WYOMING’S CARBON BASIN CONTRADICT A YOUNGER-ON-OLDER KLIPPE INTERPRETATION

Reworked early-diagenetic concretions provide clues to the structural evolution of Wyoming’s Carbon Basin. Iron-rich concretions up to 1m x 2m are abundant in a ~3km-thick sequence of sandstones and siltstones in the Ferris and Hanna formations. Channels and reworked concretions are absent in strata hosting in situ concretions. Reworked concretions are present only in NW-SE-elongated exposures of lowermost Hanna Fm that lie above and just SE of in situ Ferris Fm concretions. The iron-rich clasts reach 1m in diameter and lie in a previously recognized, 60-m-thick paleovalley fill. The fill that bears the transported concretions is composed of fluvial, SE-dipping crossbeds. We argue that iron-rich clasts in basal Hanna fluvial deposits were derived from nearby, in situ siderite concretions in the Ferris Fm. Uplift caused the concretions to be reworked and transported southeastward.

A recent paper hypothesizes that the Hanna Fm in the Carbon Basin represents a 40-km-wide, ~1000-m-thick, gravity-driven, younger-on-older klippe that moved ~14 km from the northeast. This new hypothesis contradicts previous erosional-unconformity interpretations of the Ferris-Hanna contact. On the new map, the in situconcretions of this study lie within autochthonous strata; the stream-transported concretions lie at the base of the younger strata, newly mapped as a klippe.

Here we argue against the presence of a klippe and interpret the contact as a low-angle, erosional unconformity. If the Hanna Fm of the Carbon Basin were a klippe, no spatial or derivative relationships would be expected between concretions in the two blocks. We conclude that the four-dimensional linkage between: 1) the northwestward source (large, in situ concretions at the top of the underlying, older Ferris Fm), and 2) the southeastward sink (the paleovalley containing the large, transported concretions at the base of the overlying, younger Hanna Fm) are not coincidental. The Ferris Fm, with abundant in situ siderite concretions, was locally uplifted along a faulted syncline that was occupied by a SE-directed watercourse. Concretions eroded from this small uplifted area were transported a short distance to the SE and deposited in an incised paleovalley that occupied the same (autochthonous) structural block as the in situ concretions.