NONMARINE SEQUENCE-STRATIGRAPHIC ARCHITECTURE OF THE UPPER CRETACEOUS HELL CREEK FORMATION, NEAR MARMARTH, NORTH DAKOTA

We apply principles of nonmarine sequence stratigraphy to the Hell Creek Formation near Marmarth, North Dakota to better understand how accommodation and sedimentation control spatial and temporal relationships in this fluvial setting. Here, analyses of facies and sequence-stratigraphic architecture were undertaken to develop a sequence-stratigraphic framework. Overall, the Hell Creek Formation near Marmarth, North Dakota is dominated by floodplain and poorly-drained gleyed paleosols, but it also contains two intervals that are more channel-dominated, one in the lower part of the formation and another in the upper part. The lower channel-dominated interval is typically two channels with tidally-influenced sedimentary structures like inclined heterolithic strata. These channels are commonly separated by 1 to 2 m of floodplain or paleosol deposits, but locally these channels are stacked in a multi-story unit. The upper channel-dominated interval typically contains two stacked channels that are collectively 10 m thick. The upper channel-dominated interval typically has a planar base, tidally-influenced sedimentary structures such as inclined heterolithic strata, reactivation surfaces, mud drapes, and bidirectional flow indicators. These channel-dominated units may have two origins. First, they may represent a low-accommodation systems tract (LAST), within an incised valley, and therefore overlying cryptic subaerial unconformities. Second, they may represent a LAST but not overlying a subaerial unconformity, that is, a period of low-accommodation within an overall high-accommodation setting. Both possibilities explain the preservation of floodplain and paleosols associated with the channel-dominated intervals of strata. Understanding the sequence-stratigraphic architecture of the Hell Creek Formation near Marmarth, North Dakota may allow the occurrence and distribution of fossils within the Hell Creek Formation to be better understood.