Graphic correlation method has been proved to be very useful in correlating sections in different places. In this study, we have selected five sections from the Western Interior Basin, USA and applied graphic correlation method to correlate them. The selected five sections are located from the eastern to the western margins of the Western Interior Basin, USA. The cross-basinal high-precision chronostratigraphic correlation provides geologists a better understanding about how facies and depositional environments evolved across the mid-Cretaceous Western Interior Basin during the mid-Cretaceous. In addition, our data fits well with the previous understandings about mid- Cretaceous Western Interior Seaway, which characterized by slow depositional rates and higher organic matter concentration.
Graphic correlation technique was first developed by Alan Shaw during the late 1950s, as he found that traditional biostratigraphic zonation techniques were often inadequate to solve some stratigraphic problems [
The Western Interior Basin (WIB) of North America was a marine depositional basin developed during mid- to Late Cretaceous (
Three geological formations were deposited in the Western Interior Basin during mid-Cretaceous. From the bottom to the top, they are the Dakota Sandstone, Graneros Shale and Greenhorn Limestone, respectively.
Dakota Formation
The Dakota Formation lies unconformably atop Lower Cretaceous, Paleozoic and Precambrian rocks. Deposition of the Dakota Formation began in latest Albian and continued into early Cenomanian [
and north-central part of the state [
Graneros Shale
The Graneros Shale was deposited during Middle to Late Cenomanian, and is a generally non-calcareous dark-grey shale formed after the Dakota Formation and before the Greenhorn Limestone. The Graneros Shale was formed non-uniformly in the Western Interior Basin. The contact between Dakota Formation and Graneros Shale is hard to define. From the upper part of the Dakota to the lower part of the Graneros, the lithology changes from complexly varied sandstone and shale interbeds to a more uniform shale formation. The change represents a transition of depositional environment from non-marine and marginal marine environment to full marine conditions [
Limestone is mostly comformable in the Western Interior Basin [
Greenhorn Limestone
The Greenhorn Limestone was formed during one of the most extensive transgressive episodes in the Western Interior Seaway during the mid-Cretaceous [
We have applied the graphic correlation technique to conduct high-precision chronostratigraphic correlation for the five stratigraphic sections we studied in Colorado, Kansa and Arizona (
Five outcrop sections and cores in the Western Interior were used. From east to west, they are: Bunker Hill section; Amoco No.1 Rebecca K. Bounds Core; Pueblo section; Mesa Verde section, and Black Mesa section (
The Bunker Hill section is located in the Russell County in the middle of Kansas. The section is 67 m in thick. From the bottom to the top, it contains the upper part of the Graneros Shale, Greenhorn Limestone and lower part of the Carlile Formation as described in the “Introduction”.
The Amoco No.1 Rebecca K. Bounds Core is located in the center of the Western Interior Basin. It is 305 m in length and cuts a complete Albian to Santonian section. The core is very important for it serves as a key reference section for Cretaceous strata in the middle of the WIS [
The Pueblo section is located in Lake Pueblo State Park, central Colorado. The section is about 20 m thick and mainly exposes the Bridge Creek Limestone. It is a GSSP section which defines the base of Turonian. The age of the base Turonian has been recently changed to 93.3 Ma [
associated bentonites [
The Mesa Verde section has been chosen as the key reference section for our study. The section has been studied and described with in detail through the 628 m formation. The Mesa Verde section is composed of seven units. In ascending order, they are Graneros, Bridge Creek Limestone, Fairport, Blue Hill, Juana Lopez, Montezuma Valley, and Smoky Hill members.
The Graneros Member is a dark-grey sandy mudstone and silty shale in the lower 10 m, and a slightly silty calcareous shale in the upper 14.1 m. The Graneros Member of the Mancos Shale at Mesa Verde ranges from upper Cenomanian Metoicoceras mosbyensis zone to the Cenomanian/Turonian boundary. Accordingly, the Graneros Member correlates to the middle and upper parts of the Hartland Shale Member and lower part of the Bridge Creek Limestone Member of the Greenhorn Limestone along the Colorado Front Range [
The lower 3.7 m of the Bridge Creek Limestone Member is composed of interbedded limestone and calcareous shale, and its upper 10.1 m changes to interbeded calcarenite, shaly limestone and calcareous shale. The faunal data and marker-bed correlation indicates that the Bridge Creek Limestone Member contains a relatively continuous stratigraphic record extending from the basal Turonian Pseudaspidoceras flexuosum Zone to the Woollgari woollgari-Mytiloides hercynicus Zone [
The Fairport Member is composed of fossiliferous calcareous shale which contains numerous limonite seams, bentonite seams, and bentonite beds. A regional disconformity at the Fairport-Blue Hill boundary is suggested by abrupt lithologic change from calcareous to non-calcareous shale [
The Blue Hill Member is a dark grey, non-calcareous, poorly fossiliferous shale to silty shale that contains widely scattered concretions. The lower 15.5 m of the member mainly consists of shale and scattered concretions. The upper 56 m of the member consists of silty shale with occasional thin siltstone beds. Two thin bentonite beds at 73.8 m and 79.3 m have the characteristics that also observed at the type locality of the Blue Hill Member of the Carlile Shale in Kansas [
The Juana Lopez Member is a dark, non-calcareous, slightly silty to silty shale that contains numerous beds of orange-weathering calcarenite. The member at Mesa Verde extends from the lower upper Turonian Prionocyclus macombi Zone into the upper Turonian Scaphites whitfieldi Zone [
The Montezuma Valley Member is silty calcareous shale that contains numerous septarian concretions. The member ranges from within the upper Turonian Scaphites whitfieldi Zone upward into the uppermost Turonian Prionocyclus quadratus Zone. The uppermost Turonian and lower Coniacian rocks are missing in the sharp shale-on-shale disconformity between the Montezuma Valley Member and the overlying Smoky Hill Member [
The Smoky Hill Member is composed of dark-grey, well-laminated calcareous shale, mudstone, and marlstone. The lower contact of the Smoky Hill Member corresponds with a wide-spread disconformity separating Carlile rocks below and Niobrara rocks above. The amount of the time represented by the disconformity can be estimated based on fossil records in the underlying Montezuma Valley Member and the lowermost age-determination fossils within the Smoky Hill [
The biostratigraphic data is based mainly on molluscan fossil record of the central and northern parts of the Western Interior of the United States [
The chronostratigraphic database we used came from the Standard Composite of “CRET1 Chronostratigraphic Database” [
The line of correlation (LOC) needs to be established before conducting graphic correlation. In this study, we generated the LOC using the chronostratigraphic data from Bunker Hill Section. As shown in
1) The age of the base of Lincoln Limestone Member is 93.7 Ma;
2) The age of the base of Hartland Shale Member is 93.4 Ma;
3) The age of the base of Jetmore Member is 92.9 Ma;
4) The age of the base of Pfeifer Member is 92.5 Ma;
5) The age of the top of Pfeifer Member is 92.2 Ma.
Rock accumulation rates for each Greenhorn member can be calculated based on these data: Lincoln Limestone Member (21.64 m/Ma); Hartland Shale Member
(17.43 m/Ma); Jetmore Member (15.85 m/Ma); Pfeifer Member (20.73 m/Ma). The calculated results show that the rock accumulation rates became slower during the deposition of Hartland and Jetmore members. If we assume there are no significant erosional unconformities present within the Bunker Hill Section, then our data fits well with the previous understandings about Cenomanian/ Turonian Western Interior Seaway, which characterized by slow depositional rates and higher organic matter concentration [
The Upper Mancos Shale Member at Black Mesa correlates with the upper Blue Hill Member at Mesa Verde, and then it changes to Codell Sandstone in the eastern part of the basin. The Juana Lopez Member at Mesa Verde Section thinning to the east. The thickness of the member changes from 40 m in Mesa Verde Section to 0.5 m in Pueblo Section, and disappears in Bounds Core. The Toreva Sandstone in Black Mesa Section changes to Montezuma Valley Shale in Mesa
Verde Section, and then the Montezuma Valley Shale changes to Fort Hays Limestone in the central and eastern parts of the basin. The Graneros Shale in the central part of the basin is older than the Graneros formed in the eastern margin of the basin. The Thatcher Limestone found in Bounds Core and Pueblo Section can be correlated together successfully. They were formed at about the same age.
There is a regional unconformity that can be traced around the WI basin. The unconformity represents about 0.6 ~ 1.0 million years time of non-deposition, and is generally found below the Blue Hill Member and Codell Sandstone.
It is a pleasure to thank the many people who made the study possible, including Dr. Peter Michael, Dr. Bryan Tapp, Dr. Jingyi Chen, Dr. Michael Formolo, Dr. Dennis Kerr, and Dr. Kumar Ramachandran for the basic knowledge and insightful comments they provided.
Shang, F., Chen, R.Q., Zhao, Z.H., Scott, R.W. and Song, L. (2018) High-Precision Chronostratigraphic Correlation of Mid-Cretaceous Strata in Western Interior Basin, USA through Graphic Correlation Technique. Journal of Geoscience and Environment Protection, 6, 266-277. https://doi.org/10.4236/gep.2018.65023