Creatures from the Bakken Lagoons: Palynology Confirms the Brackish Depositional Environment with Implications for Basin Evolution

P.T. DOUGHTY; G.W. GRADER; RICE B.J.; ISAACSON, P.; DI PASQUO, M.M,

salt lake city

Conferencia; AAPG meeting; 2018

AAPG

Rice, B.J., di Pasquo, M., Doughty P.T., Grader G.W., Isaacson P. 2018. Creatures from the Bakken Lagoons: Palynology Confirms the Brackish Depositional Environment with Implications for Basin Evolution. AAPG meeting (Salt Lake City, may 2018). CONTROL ID: 2857447 https://www.aapg.org/Abstract Body: The Middle Bakken Formation and correlatives in western Montana and Alberta have brackish-waterlagoonal deposits of non-fossiliferous mud and silt that cover large areas of each basin and may record basin-wideevents. In the Bakken of Saskatchewan and North Dakota, they occur in the upper Middle Bakken (Unit B in SK orFacies E or 4 in the U.S.) where they have been well documented with sedimentology, impoverished ichnofacies, andlow abundance of marine fossils. They overlie the clean marine sands and underlie the capping marine carbonates ofthe Middle Bakken. A transgressive, bioclastic lag is locally present at the upper boundary.The Sappington Formation in western Montana is temporally and stratigraphically correlative with the Bakken andcontains a similar, yet thicker, brackish water lagoonal facies of non-fossiliferous mud and silt with an impoverishedichnofacies. It overlies offshore marine siltstone of the lower Middle Sappington and underlies shoreface siltstone andsandstone of the upper Middle Sappington. A transgressive crinoid limestone lag is locally present at the uppercontact of these lagoonal facies.New palynology from these strata in outcrops in Western Montana and from core in the Williston Basin of SheridanCounty Montana independently confirms the restricted lagoon environment as inferred from physical attributes. Sporesin these dark green non-fossiliferous shales are dominated by terrestrial species (80%) including Retisporalepidophyta with much fewer marine phytoplankton (acritarchs, prasinophytes). The high ratio of terrestrial to marinepalynomorphs indicates deposition in a marginal marine environment consistent with impoverished ichnofacies and alack of marine macrofauna.The large areal extent of these lagoonal deposits suggests that they record fundamental basin reorganizationdominated by allocyclic controls rather than local autocyclic facies controls. Tectonically driven paleohighs such as theNesson Anticline and Swift Current Arch in the Williston Basin and the Lemhi Arch in the Sappington Basin may haveformed the temporary barriers that separated these lagoons from the open ocean. The incursion of marine watersacross these barriers is related to relative sea level rise and subsidence of these paleohighs, and is marked by thethin bioclastic, transgressive lag that represents a return to marine deposition. Whether lagoonal deposits in differentareas are the same age remains uncertain.