Since the original release of the Ohio seismic reflection profile by the Consortium for Continental Reflection Profiling (COCORP) in 1988, several interpretations have been presented for the structure and orogenesis of Precambrian rocks underlying this part of the eastern Midcontinent.
The present study restricts its focus to a portion of a reprocessed version of the COCORP seismic line (Fig. 1) and concentrates on seismic depths to 3 sec. This focus is sufficient to present a detailed interpretation of shallow Precambrian structure and stratigraphy that outline possible Precambrian hydrocarbon traps. Precambrian rocks worldwide have significant oil and gas production, although not in the great quantities as in Phanerozoic strata.
Regional structural geology
Two Precambrian provinces lie beneath thin Paleozoic sedimentary cover in Ohio: the Eastern Granite-Rhyolite Province (EGR) (≈ 1.3-1.4 billion years)1 and the Grenville Province (GP) (≈ 1.0 to 1.2 billion years)2 (Fig. 1). A north-south-trending line through western Ohio, referred to as the Grenville Front (GF),3 separates older continental rocks of the EGR from younger, largely metamorphic rocks of the GP (Fig. 1).
Grenville Province rocks of high magnetic susceptibility show pronounced positive anomalies against less magnetic rocks of the EGR at the GF.4 Within the EGR, a series of Precambrian rift basins termed the Fort Wayne Rift (FW)5 and East Continent Rift Basin (ECRB)6 are present throughout western Ohio, and adjacent Indiana and Kentucky (Fig. 1).
The discovery of the Middle Run formation7 8 in the Ohio Department of Natural Resources, Division of Geological Survey well in Warren County, Ohio, (well 1 on Fig. 1) led to regional interpretation of the ECRB, which may be associated with the Midcontinent Rift System6 (MR) (≈ 1.1 billion years).9 An upper age date of the Middle Run formation of the ECRB is 1.04 billion years.10 Metamorphism and thrust emplacement of GP rocks represents the most extensive mountain-building event (i.e., Grenville Orogeny) to affect the eastern US.
East Continent Rift basin
Although the presence of a Precambrian basin in western Ohio has been suspected since the 1930s,11 only gravity and magnetic data were available to support this interpretation.12 13
An earlier interpretation of the COCORP OH-1 seismic line supported Precambrian rifting in western Ohio.14 Following the discovery of the Middle Run formation in Ohio, excess of 20 wells in Ohio, Indiana, and Kentucky were reinterpreted6 15 or newly drilled into red sandstone.16
Mapping of the Middle Run using well and seismic data indicated that rifting segmented the regional basin into a series of complex component sub-basins, postdating the emplacement of rocks of the EGR and predating Grenville thrusting.6 The widespread occurrence of the Middle Run formation in faulted sub-basins of western Ohio and adjacent states was also recognized by others.17 18
Detailed seismic correlation of COCORP OH-1 for this study using sequence stratigraphy clearly shows that the ECRB had a long and complex history. Fig. 2 shows the seismic interpretation, with schematic diagram, across the ECRB and to the east into the Grenville Province. Fig. 3 illustrates the proposed evolution of the ECRB based on isolation of seismic sequences.
Subsidence began at the eastern side of the ECRB prior to Grenville thrusting. Basin development progressed westward, with the development of a series of half grabens, each bounded on the west by step-faulted scarps. The ECRB "depo-axis" remained toward the western end of the basin through deposition of the Middle Run formation. The depo-axis then shifted eastward as a thin veneer of foreland basin sediments was laid down in response to subsidence, erosion, and sediment deposition from advancing Grenville Orogeny thrust complexes.
Erosion of foreland basin-fill was the last event in the ECRB and was followed by deposition of Paleozoic sediments. Pre-Grenville ECRB geometry is simplified when Grenville thrusting is removed. For example, an inverted sub-basin (i.e., anticline atop a syncline) occurs over the easternmost central sub-basin (Fig. 2, box B). The anticline is eliminated and this sub-basin assumes the correct asymmetry of sediment thickening westward toward the fault-block boundary when Grenville faulting is removed. Precambrian sedimentary strata, interpreted as ECRB reflectors on the COCORP line, may have source rock and reservoir potential.
Interval velocities are shown on Fig. 4, with correlated zones to EGR interface, base of Middle Run formation, base of foreland basin, and Precambrian-Paleozoic unconformity indicated. Interval velocities within ECRB fill above the depicted base of the Middle Run are consistent with those of well-indurated sedimentary rocks (sandstones at 17,000 to 21,000 ft/sec) and are generally lower than those in the underlying ECRB fill.
Below the Middle Run, interval velocities are generally higher (18,000 to 24,000 ft/sec), but variable, consistent with a variety of different rock types, ranging from intertonguing sediments derived from fault blocks, volcaniclastics, and felsic intrusive and extrusive rocks. Extensive mafic rocks in the ECRB fill are not volumetrically significant because of the overall low magnetic signature of the area.
Foreland basins
East of the GF in Ohio, all Precambrian rocks have previously been considered to be GP metamorphic and igneous rocks.3 4 19 20 However, similar to the ECRB, detailed sequence stratigraphy along the COCORP OH-1 line has revealed several foreland basins within the overall framework of Grenville folded and thrusted rocks in Ohio.
Possible Grenville structures and foreland basin sedimentary rocks have previously been interpreted in western Ohio.20 The original configuration of these basins and accompanying deposits cannot be determined in the overall context of the Grenville Orogeny because of the pronounced erosion at the Precambrian-Cambrian unconformity. These foreland basins do not appear to be present at the eastern end of the COCORP OH-2 line near West Virginia.
Virtually nothing is known about deeply buried GP rocks beneath the Appalachian basin until they emerge at the Blue Ridge Mountains of Maryland, West Virginia, and Virginia.
The presence of GP foreland basins along the COCORP OH-1 line is marked by pronounced seismic contrasts between the interpreted sedimentary basin-fill and basal Paleozoic sediments, as well as thrusted GP basement rocks that underlie and separate the basins. In general the seismic signature of GP foreland basins reveals a marked lower reflectivity in foreland basin-fill than in the Paleozoic strata and GP basement rocks that bound them. The foreland basin-fill also lacks continuity of reflectors, implying intertonguing, discontinuous, clastic sediments derived from nearby or moderately distant highland areas.
Other possibilities that may account for an absence of continuous reflectors in the foreland basin-fill are unconformities and thrust faults into basin-fill deposits that indicate uplift and deformation after sediment deposition. Transposition of foreland basin sediments is commonplace in such basins worldwide and has been well documented, for example, in Precambrian sedimentary strata in northwestern Canada, where early-formed basin deposits have been transported in "piggyback" fashion by later thrust faulting.21
Velocity analyses (Fig. 4) show a marked correspondence between interval velocities and reflectivity contrasts in the Precambrian section. The comparison is striking between basin geometry and interval velocities. Foreland basins typically consist of lower velocity material (14,000 to 21,000 ft/sec), consistent with clastic sedimentary rock fill. Surrounding and subjacent GP metamorphic and igneous rocks have higher values (18,000 to 23,000 ft/sec).
A most interesting seismic aspect for comparison is the bilateral symmetry of velocities centered about the Upper to Middle Cambrian Knox-Rome carbonate rock section. Velocities fall off markedly from the 20,000-22,000 ft/sec range in these Upper to Middle Cambrian carbonates just above the Precambrian unconformity to much lower values in stratigraphically younger Paleozoic clastic sedimentary strata (Fig. 4).
Below the Cambrian carbonates, this same decrease in velocity occurs across the Precambrian-Paleozoic interface, indicating lower velocity clastic sedimentary rocks as foreland basin-fill. This bilateral velocity distribution exists in large part across Ohio, including the ECRB, where the latest GP foreland basin-fill overlies ECRB deposits.
Next: Oil and gas potential of the Ohio basement.
