Stratigraphy and Depositional History of the Morrow Formation, Southeast Colorado and Southwest Kansas

Abstract

The Morrow Formation of southeast Colorado and southwest Kansas unconformably overlies progressively older Mississippian units from south to north and is, in turn, disconformably overlain by the Atokan Series. The rocks between these bounding unconformities range from fluvial and marine sandstone to offshore marine shale and shallow marine carbonate. This wide range in lithology has resulted in numerous stratigraphic and combination traps and holds promise for continued exploration. The Morrow can be divided into two informal members. The lower Morrow in the area of the Hugoton Embayment consists primarily of offshore marine shale and shoreface sandstone. These units onlap the Mississippian unconformity northward and are older than the platform carbonates of the lower Morrow within the study area. The upper Morrow consists primarily of marine shale that encases transgressive valley-fill sequences. Seven valley-fill systems, each recording a cycle of relative sea level change, were identified. Each drop in sea level resulted in incisement of fluvial channels into the underlying strata. Sediment was transported southeastward, to the area of the Anadarko Basin, during these episodes of erosion. The valleys are generally less than 2 mi (3.2 km) wide and 90 ft (15.3 m) deep, indicating the magnitude and duration of the relative sea level drop was not great. The affect of the eustatic change may have been diminished by paleotectonic subsidence. In many areas the valleys developed in paleostructural lows that persisted through the subsequent depositional phase. Backfilling of each valley occurred during a relative sea level rise. The resultant sequences are comprised predominantly of fluvial sandstone at the base, the primary target for exploration in the study area, overlain by estuarine sandstone and mudstone. Offshore marine shale overlies the valley-fill sequences. These shales provide regional time-stratigraphic markers that can be used to define and map individual valley systems, thereby improving reservoir predictability.