SABKHA SEQUENCES OF THE BAJOCIAN-AGED GYPSUM SPRING FORMATION RECORD PALEO-SEA LEVEL FLUCTUATION WITHIN THE DEVELOPING CORDILLERAN FORELAND BASIN

The Gypsum Spring Formation of north-central Wyoming records initial syn-orogenic deposition within a portion of the developing medial-Jurassic Cordilleran retroarc foreland basin. Sedimentation during this time was influenced by transgression of the Sundance Sea from the north, forming a mixed carbonate and siliciclastic shallow and marginal marine record. Along the eastern, distal margin of the basin, carbonates, claystones, and varied evaporite-rich lithofacies, exhibiting characteristic fabric, indicate that an extensive coastal sabkha environment existed during Bajocian time. Facies cyclicity within these deposits suggests that the basin margin records at least three transgressive events during the Bajocian, prior to a more punctuated transgression during early Bathonian.

Each transgressive event is represented by a subtidal-to-supratidal facies sequence. Subtidal and certain intertidal facies represent a relatively low-energy carbonate dominated depositional system with some units containing tetrapod swim tracks, belonging to both crocodilian and apparent bipedal dinosaurian taxa. Intertidal and supratidal facies are represented by pale to dark reddish brown and greenish grey claystones containing scattered to tightly packed, spherical to “cow-patty” shaped gypsum nodules. Similar nodular gypsum is widespread within both modern and ancient sabkha environments and is thought to form during shallow burial of the sabkha surface. Red claystone beds likely indicate deposition in subaerial oxidizing conditions, while green and grey claystones were deposited in reducing conditions, possibly during the flooding of ephemeral ponds on the sabkha surface.

These low-energy, marginal marine facies are consistent with that expected above a distal, low-angle ramp subject to cyclic flooding events. While similar to modern coastal sabkhas of the southwestern Persian Gulf, these Bajocian-aged sabkhas were sensitive to hydraulic fluctuations of the Sundance Sea and are, therefore, a reliable indicator of paleo-sea level variation. This cyclicity coincides largely with transgressive/regressive sea level cycles defined from the Boreal record, with a more localized expression influenced by Cordilleran (Sevier) orogenesis thus defining a portion of the so-called J-unconformity record.