A TWIST ON THE SEQUENCE STRATIGRAPHIC MODEL: CLIMATE INDUCED INCISION WITH A SEA LEVEL ASSIST

The sequence stratigraphy of Middle to Upper Pennsylvanian strata in the Appalachian Basin is complex, partly owing to the icehouse co-response to climate and sea level change during the late Paleozoic. The Breathitt Group resembles a typical marine-to-terrestrial stratigraphic model. The overlying Conemaugh Group exhibits more fluvial-dominated sequences. Sequence stratigraphy largely presumes sea-level drive for sequences and accommodation. We present a model driven by both sea level and climate. We hypothesize that once the land surface builds up high enough above the water table, sea level drop is not required to induce valley incision, and in fact no evidence exists for a shelf slope break that would promote incision. Rather, we offer that climate change may be the main driver of valley incision.

This model is tested using strata in the Breathitt and Conemaugh Groups in the Appalachian Basin. Measured sections along a basin cross section in outcrop and 3D models built from UAV photographs help reveal this past environment to address the potential of climate change as a sequence driver.

The Breathitt to Conemaugh Group shift records a composite of sequences that are a progradational basin-fill and define a switch from a mixed marine and fluvial to fluvial fill. The Conemaugh sequences record upward shifts from a low-accommodation, valley-incised tributive to a high-accommodation, un-incised distributive systems tract. As a marine transgression tops the low-accommodation valleys below, it lays a basal peat, flooding the tributive system. Next, the rivers in the distributive fluvial system prograde and push out the shore, and build a slope above sea level, creating an elevated coastal prism. Continued progradation creates the elevation needed for incision, but incision is not necessary, even if sea level falls. A climate change will eventually spur water table decline owing to a locally drier climate, or an upstream water-sediment ratio change. Valley incision begins at that time, possibly with no sea level change. In this model, regression with or without sea level drop sets up the conditions needed for valley incision, but does not cause incision itself. Incision waits for adequate climate change to make buffer valleys. The valleys record regression but are climate driven and do not have to define sea-level change.