PSThe Influence of Basement Structures from Devonian Black Shale Thicknesses in the Northern Appalachian Foreland Basin*
Gerald J. Smith1, Robert D. Jacobi1, Jodi L. Seever2, and Stu Loewenstein1
Search and Discovery Article #50203 (2009)
Posted Posted September 25, 2009
* Adapted from poster presentation at AAPG Annual Convention and Exhibition, Denver, Colorado, USA, June 7-10, 2009.
1Nornew, Inc., Amherst, NY. (email@example.com)
2Department of Geology, University at Buffalo, Buffalo, NY.
Five thick black shales were deposited in western New York and northern Pennsylvania during the Middle and Late Devonian. Traditional models show the regional maximum black shale thickness successively steps farther west with the development of a gentle, structurally inactive clinoform. However, in the northern region of the Appalachian Foreland Basin, many of the areas of thickest black shale deposition coincide with areas of active faulting. From our outcrop studies in New York State and well-log analyses in New York and Pennsylvania we observed abrupt thickening of several of the black shales coincident with active faults that extend up from basement structures, primarily the Clarendon-Linden Fault System and Iapetan opening/Rome Trough structures. For example the regionally minor black shales the of Pipe Creek and the Hume formations are typically 1 meter or less thick and appear inconsequential as a reservoir/source rock. However, within the extent of the Clarendon-Linden Fault System, the Hume Formation averages 36m (120ft) thick, and the Pipe Creek Formation reaches 5.5m (18ft). More importantly for shale reservoirs, thick accumulations of the Geneseo (~45m/150ft) and Rhinestreet (91m/290 ft) formations coincide with basement structures of reactivated Clarendon-Linden, while greater thickness of the Marcellus (~56m/180ft) and Middlesex (~61m/200ft) correspond with the Iapetan-opening/Rome Trough structures.
We suggest that the combined stress of the Neo-Acadian collision and accompanying sediment loading reactivated the older basement structures, generating variable accommodation within the vicinity of the fault zones. In some cases, the thickening may result from thrusts that can be easily overlooked in the typical wireline logs if there are not distinctive marker units (as is typical in the black shales). However, such thrusts are recognizable in outcrop and FMI or similar logs. In addition to the increased localized accumulation of organic-rich shale, later fault reactivation would increase local fracturing, increasing the potential of these black shales as reservoirs and source rocks.
Recent influx of interest in black shales suggests a closer examination of the structural setting in which many of the black shale plays occur. (Figure 1) A simple, structureless foreland basin will contain clinoform sediments grading basinward to thick accumulations of organic rich clays and silt-sized material (Figure 2). In such a basin, the thickest black shales would occur in the deepest regions. With continued sediment influx and sea level cyclicity, subsequent black shales would form further basinward but still parallel the first black shale. However, structureless foreland basins are unlikely, particularly in regions that have experienced several orogenic events. (Figure 3, Figure 4 and Figure 5) The pre-existing basement structure in a foreland basin will contribute an additional level of complexity to the basin topography. The result is that in black shale, deposits may be structurally controlled within isolated areas, creating areas of enhanced preservation that follow structural trends oblique to the basin.
Many of the major black shale plays occur along the orogenic front. (Figure 6) If basement structures were reactivated in the Appalachian Basin, then it seems likely that other foreland basins will have also experienced structural reactivations of some level.
Structural cross sections were created across southwestern New York State incorporating outcrop measured at the centimeter scale with gamma ray logs. Cross-sections A-A’ (Figure 7), B-B’ (Figure 8) and C-C’ (Figure 9) are a series of east-west cross sections highlighting the thickness variations in the black shales. Each east-west cross section is also shown flattened along a distinct stratigraphic horizon. (Figure 10, Figure 11 and Figure 12) Where a cross section line crosses a basement structure a red-dashed line is superimposed on the cross section. Cross section D-D’ is a north-south cross section that ties A-A’, B-B’ and C-C’. (Figure 13) To utilize as many well logs as possible only wells drilled into the Upper Devonian were included using the Rushford Formation as a key correlation unit distinctive in outcrop and well logs.
Deeper structures generally coincide with preservation of the Rushford Formation. The broad anticline in E-E’ occurs where a thicker, coarsening upward sequence was deposited. (Figure 14) In F-F’, the thrust fault in the Tully coincides with the disappearance of the Rushford Formation. (Figure 15) G-G' is a north-south Upper Devonian cross-section showing units thickening towards the south. (Figure 16)
Isopach maps were created from well log data in New York and Pennsylvania (Figure 17) for four of the major Devonian black shales (Figure 18, Figure 19, Figure 20, Figure 21and Figure 22). Basement structures from Jacobi (2002) are overlain to show the general coincidence between the thicker deposition and basement faulting. It is important to note that for different black shales, different trending structures appear to have more control. As an example, the Tully Formation (Figure 23) and overlying Geneseo Formation both have a broad, north-south trend following the north-south Clarenden-Linden Fault System (CLF). The stratigraphically higher Middlesex Formation however follows northeast trending structures.
Syndepositional faulting or tectonic activity within foreland basins (Figure 24) has been shown to control/influence deposition and architecture in fluvial systems (Plint and Wadsworth, 2006), carbonate reefs (Dorobek, 1995), beaches (Hart and Plint, 1993; Smith and Jacobi, 2001) and offshore sand ridges (Nummedal and Riley, 1999). Just as relatively minor amounts of uplift (~1 m) may affect deposition and depositional pattern, so can minor amounts of subsidence (or relative subsidence) affect preservation through a localized increase in accommodation. (Martinsen, 2003) (Figure 25) Syndepositional faulting caused by the reactivation of basement structures would be expected within a foreland basin as forces from collisional tectonics, sediment load, development and migration of the peripheral bulge generate alternating periods of localized compression, extension and strike-slip stress. (Figure 26)
The effect of syndepositional faulting on black shale deposits is varied accumulation thicknesses following structural trends. In periods of tectonic quiescence, the black shale isopach will parallel the basin axis, but where cross faulting has occurred, the black shale isopach will trend obliquely to the basin axis. (Figure 27)
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