The rocks in the upper half of the photo are black mudshales of the Upper Devonian Chattanooga Formation. The rocks below that are fine-grained dolostones of the Upper Ordovician Cumberland Formation. The intensely orangish-brown stains covering much of the Cumberland Formation formed by oxidation of pyrite at the formation contact. Considerable missing time is represented by this boundary - the entire Silurian, the Lower Devonian, and the Middle Devonian are all gone here - that's about 55 million years missing. Stratigraphic contacts with missing time are called unconformities - surfaces of erosion and/or non-deposition of sediments. This is a disconformity, with horizontal sedimentary rocks above & below the contact. The basal Chattanooga Shale is relatively rich in pyrite (FeS2 - iron sulfide). With oxidative weathering, the pyrite alters to various iron oxide minerals such as reddish-brown hematite and yellowish-brown limonite. These iron oxides end up staining underlying rocks as meteoric waters (rain & runoff) descend. The end result is a "bleeding unconformity".
This contact is a megasequence boundary - the Tippecanoe-Kaskaskia Megasequence boundary. Regionally recognizable packages of sedimentary rocks bounded by major unconformities are known as megasquences, or Sloss sequences (named after geologist L.L. Sloss, 1913-1996). The bounding unconformities were caused by erosion during regressions. Continental margins have more complete sedimentary successions, whereas craton interiors have more incomplete successions. The names for North America's megasequences are (from oldest to youngest): Sauk, Tippecanoe, Kaskaskia, Absaroka, Zuni, and Tejas.
Stratigraphy: Chattanooga Shale (Upper Devonian) over Cumberland Formation (Upper Ordovician)
Locality: roadcut on the northern side of Route 90, just west of Burkesville, central Cumberland County, southern Kentucky, USA (36° 47’ 51.17” North latitude, 85° 23’ 10.62” West longitude)
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