Three Scales of Sequence Stratigraphy in the Middle Devonian Marcellus Shale and Associated Strata
Kohl, Daniel; Slingerland, Rudy; Arthur, Mike; Engelder, Terry
Mudrock sequence stratigraphy of the Middle Devonian Marcellus Fm. in the Central Appalachian Basin, Eastern United States occurs at three scales: 2nd order depositional sequences (approximately 10's of million years duration) that make up thousands of feet of strata in this basin, 3rd order sequences (1-10 million years) with up to several hundred feet of strata, and parasequences, that comprise tens of feet of strata. The Marcellus Fm. was deposited within a second-order (8 ± 3 my) depositional sequence containing the Onondaga, Marcellus, and Mahantango Fms. This 2nd order sequences comprises five, 3rd order (approximately 3 - 0.5 my duration) depositional sequences. Each 3rd order sequence begins with basal limestone beds that represent the lowstand to early transgressive systems tract. The limestone beds are overlain by high total organic carbon (TOC) mudrocks representing the transgressive and highstand systems tract. Finally, the upper portion of each sequence contains relatively organic carbon-lean and clay-rich mudrocks representing the falling stage systems tract. These regional parasequences can be identified by gamma ray (GR) log patterns that decrease systematically upwards from high-TOC, massive to faintly laminated mudrocks at the base to coarser, well-laminated, organic-carbon-lean, occationally sandy mudrocks near the top. Tops of the parasequences commonly contain a calcareous interval, commonly containing shell debris, overlain by a sharp transition into the high TOC mudrocks of the next overlying parasequence. GR-correlation of the parasequences across Pennsylvania reveals a high TOC fairway. This fairway occurs distal from both the Mahantango delta complex of eastern PA and a 75 m high carbonate bank/pinnacle reef complex in northwestern PA. Based on bulk rock geochemistry and pinnacle reef heights this fairway was likely the deepest portions of the basin with paleowater depths greater than 120 m.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013