Figure 8: Lithostratigraphy of the Late Cretaceous for north-central Nuevo Leon (Mexico), southwestern Texas, New Mexico, Colorado, Montana, and southern Alberta (Canada).
The Cenomanian/Turonian boundary is characterized by several different geological units from Alberta to Mexico. The Bridge Creek Interval of the central part of the seaway and it’s equivalents to the north and south were sampled during this study and are described below.
Blackstone formation - Vimy Member
The Vimy member of the Blackstone formation is exposed in the southern Alberta/Canada foothills region. The Blackstone formation is part of the late Cretaceous Colorado/Alberta Group, which consists predominantly of mudstone interspersed with relatively thin sandstone and conglomerate beds and deposited in the southwesternmost part of the Canadian foredeep. The Blackstone Formation is thought to record the initial transgression. The cycle began during the middle to late Cenomanian in southern Alberta and northeastern British Columbia, and reached its peak during the Turonian (STOTT, 1993). The calcareous, non-concretionary shale making up the Vimy Member of the Blackstone in Alberta and British Columbia is, in part, correlative with the
widespread coccolithic Second White Speckled Shale which marks the peak transgression. It is the
5. Sedimentology 28
northern correlative of part of the Greenhorn Formation of the United States (WILLIAMS and BURK, 1964; STELCK and WALL, 1954).
Marias River Shale - Cone Member
The calcareous Cone Member (equivalent to the Greenhorn Fm. of Colorado and New Mexico) is part of the Marias River Shale (upper unit of the Colorado group) on the Sweetgrass arch in
northwestern Montana. The name Marias River Shale was given by COBBAN et al., 1959 to a 275-365 m thick sequence of dark gray Upper Cretaceous shale that lies between the Blackleaf and Telegraph Creek Formation. The Cone Member (also called Greenhorn Limestone of the Sweetgrass Arch) has been divided into four members. The Cone Member is defined as the calcareous beds between the non-calcareous Flowree Member, below, and the non-calcareous Ferdig Shale Member, above. At the location sampled for this study the total thickness is about 16 m (COBBAN et al., 1959), but the lower part is mostly covered by vegetation and small landslides.
The Cone Member is a thin unit of latest Cenomanian and Turonian age. Most of the Cone Member is dark-gray highly calcareous shale. Other minor but easily recognized lithologic types include a persistent bed of large concretions of dark gray limestone at the base, and several thin, ledge-forming, shales. At the top fossiliferous shales are present. Several bentonite beds are also present.
Greenhorn Formation - Bridge Creek Limestone Member
The Bridge Creek Limestone Member was named by BASS (1926a) for a series of limestone beds at the top of the Greenhorn Limestone on Bridge Creek in Hamilton County, Kansas. It is part of the Greenhorn Formation and can be divided into three informal subdivisions (Lower, Middle, Upper) based on overall lithologic characteristics and slope forming section. The development and preservation of cyclic sedimentation during deposition of the Bridge Creek Member is largely the result of slow sedimentation rates of 0.5 to 1.0 cm/ka in the central Western Interior Seaway.
Sedimentation rates characteristic of the fine-grained shelf and basinal facies range from 0.5-4.0 cm/ka before compaction (KAUFFMAN, 1988). Sedimentation rates of 1 to 2cm/ka are typical for modern calcareous pelagic deposits above the CCD (Carbonate Compensation Depth).
Sedimentation of the Bridge Creek Limestone was characterized by oscillations between mud-rich and carbonate-rich facies that ultimately formed limestone/marlstone bedding couplets. These couplets are characterized by fluctuations in biofacies and organic carbon content, and interpreted by many (e. g. BARRON et al., 1985) to reflect changes in bottom-water oxygen content.
The outcrops are characterized by 10 to 40 cm thick micritic to chalky limestones or marlstones interbedded with 30 to 100 cm thick marly to chalky shale beds. The base is defined by a thick 20
to 50 cm limestone bed (PBC 1). PBC is an expression from KAUFFMAN, meaning Pueblo Bridge Creek. He introduced this terminology to describe the prominent marker beds in the Bridge Creek limestone Member.
The type section for the Cenomanian/Turonian Stage boundary is at Rock Canyon Anticline at Pueblo, Colorado and occurs in the Bridge Creek Limestone Member of the Greenhorn Formation.
Boquillas Formation – Ernst Member
The Boquillas Formation consists of almost one hundred meters of impure flaggy, chalky limestone beds interbedded with gray platy marls and calcareous shales. The Boquillas Formation contains ammonites, bivalves, ophiuroid and echinoid fragments, an abundance of foraminifera, and some ostracods, burrows and borings. Several genera of algae are present. It is underlain by the Buda Limestone and overlain by sediments of middle Turonian age, called the San Vicente Member of the Boquillas Formation. The Ernst Member is the lower part of the Boquillas Formation and about 150 meters thick. This limestone consists of strata from 1 to 10 cm thick and has a platy character.
The common weathered color of both the limestone and the shales is light yellowish gray, but on fresh surfaces they appear bluish gray.
Agua Nueva Formation
The Agua Nueva Formation is characterized by rhythmic bedding consisting of thin- to medium- to thick-bedded clayey limestone and marl couplets with continuous parallel stratification and thin- to medium-bedded bentonite beds. Locally the limestone beds are rich in organic matter giving them a black coloration on fresh surfaces. This unit weathers to brownish, yellowish colors and forms synclinal valleys and slopes (LONGORIA, 1998).
5. Sedimentology 30