This study presents a subsurface facies analysis of the Cambrian Conasauga Formation and Kerbel Formation using well core and geophysical logs. Well- 2580, drilled in Seneca County (Ohio), was used for facies analysis, and the correlation of facies was based on the gamma- ray (GR) log for three wells from adjacent counties in Ohio (Well-20154 in Erie County, Well-20233 in Huron County, and Well-20148 in Marion County).
In Well-2580, the Conasauga Formation is 37- m thick and the Kerbel Formation is 23-m thick. Analysis of the core identified 18 lithofacies. Some of the lithofacies are siliciclastic rocks, including: massive, planar laminated, cross-bedded, and hummocky stratified sandstone with burrows; massive and planar- laminated siltstone; massive mudstone; heterolithic sandstone and silty mudstone with tidal rhythmites showing double mud drapes, flaser-, lenticular-, and wavy- beddings; and heterogeneous siltstone and silty mudstone with rhythmic planar- lamination. Other lithofacies are dolomitized carbonate rocks that originally were massive, oolitic, intraclastic, and fossiliferous limestones. In general, the Conasauga Formation is a mixed siliciclastic-carbonate depositional unit with abundant tidal sedimentary structures consistent with a shallow- marine depositional setting and the Kerbel Formation is a siliciclastic depositional unit consistent with a marginal-marine depositional setting.
The lower part of the Conasauga Formation consists of sandstone beds organized into a coarsening- and thickening- upward sequence of massive bedded, planar laminated, cross- bedded, and hummocky stratified sandstone. These beds are interbedded with beds having heterolithic tidal features such as flaser- bedding and wavy- bedding of heterogeneous sandstone and silty mudstone. This section in the GR- log shows the irregular shaped pattern with no trend, and is interpreted as bay- head delta deposits.
This section is followed by a coarsening upward sequence of sandstone beds with planar- bedding, cross- bedding, and hummocky cross- stratification interbedded with minor tidalite beds. In the GR- log, this section is represented by a funnel- shaped pattern with abrupt tops suggesting a coarsening-upward trend. This sequence is interpreted as deposited in offshore transition zone from subtidal to lower intertidal flat deposits.
The overlying section, middle part of the Conasauga Formation, consists of dolomitized limestone, sandstone, and siltstone with planar- lamination, cross- bedding, hummocky- stratification alternating with beds rich in tidally- influenced structures such as flaser-, lenticular-, and wavy-beddings, and planar rhythmites of heterogeneous sandstone and siltymudstone. In GR- log this section is represented by an irregular pattern with no trend and is interpreted to be the part of intertidal deposits.
This section is followed by a funnel- shaped gamma- ray log pattern in upper part of the Conasauga Formation and whole section of the Kerbel Formation, suggesting an overall coarsening- upward trend. The upper part of the Conasauga Formation consists of very fine- grained sandstone with minor interbedded beds of thin mudstone. The Kerbel Formation consists of a coarsening-upward sequence, changing from fine- grained sandstone at the base to coarse- grained sandstone at top, that presents massive, parallel- laminated, and cross- bedded sandstone with carbonate intraclasts consisting of micrite, peloids, ooids, and bioclasts. These sandstone beds are interpreted as a barrier and/or overwash deposit.
A similar sequence of GR- log pattern is also found from logs of Well- 20154 in Erie County, Well- 20233 in Huron County, and Well- 20148 in Marion County. The only difference between wells is the thickness of the various sections. The Kerbel Formation is thickest in the Seneca County with thickness of 23-m and the Conasauga Formation is thickest in the Marion County with thickness of 49-m. The matching of the general succession of lithofacies signature of the logs in four wells suggests that the depositional environment interpreted for Well- 2580 is at least regional in character.
In summary, this study found that the Conasauga Formation sandstone beds of a bay- head delta system. These sandstones are reworked and overlain by tempestites, tidalites, and carbonates deposited in an offshore transition zone from subtidal to sand flat environments. These deposits are overlain by tidalite beds representing intertidal deposits in an estuarine or lagoonal depositional environment. The coarsening- upward trend of the strandplain (beach- barrier- overwash) of the Kerbel Formation above estuarine or lagoonal deposits suggests landward migration of the strandplain during transgression. As a whole, the sequences of the two units represent the late Cambrian marine transgression: landward migration of estuary, lagoonal, or tidal flat environments over a bay- head delta environment, and then landward migration of a barrier and/or overwash environment over estuarine, lagoonal, or tidal flat environments on a microtidal to mesotidal coast.