cicimurri CRETACEOUS ELASMOBRA CHS OF THE GREE HOR FORMATIO (MIDDLE CE OMA IA - MIDDLE TURO IA ), WESTER SOUTH DAKOTA DAVID J. CICIMURRI Bob Campbell Geology Museum, Clemson, SC, 29634 ____________________ Abstract— Cooperation between the U.S. Forest Service, Bureau of Recreation, and the South Dakota School of Mines and Technology has resulted in the accumulation of a significant collection of elasmobranch fossils from the Greenhorn Formation of western South Dakota. Thirteen taxa were recovered from late Cenomanian rocks of the lower portion of the formation, whereas 14 taxa were collected from early middle Turonian rocks at the very top of the formation. Both of these assemblages are dominated by pelagic taxa. A few of the sharks and rays within the lower and middle portions of the formation are consistently found in rocks representing shallow-water environments, indicating that several regressive episodes occurred during the transgressive phase of the Greenhorn Cyclothem. The elasmobranch assemblages from the Greenhorn Formation of South Dakota are similar (or nearly identical) to those from temporally equivalent rocks of Texas, Kansas, and Arizona. The absence of these taxa in the Atlantic and West Coast regions is due to the lack of appropriate-aged strata in these areas. ____________________ I TRODUCTIO The Greenhorn Formation is exposed around the Precambrian core of the Black Hills of South Dakota and Wyoming. Despite this fact, the formation has largely been neglected with regard to its paleontological resources. Recently, the South Dakota School of Mines and Technology, in conjunction with the U.S.F.S. and U.S.B.R., has been conducting a systematic study of the formation within South Dakota. Museum personnel have discovered a diverse plesiosaur assemblage in the formation, including the first occurrence of the large pliosaur Polyptychodon interruptus in the Western Interior of North America (VonLoh and Bell 1998). Squamates from the Greenhorn include the enigmatic dolichosaur Coniasaurus crassidens , as well as a basal mosasauroid and another poorly understood taxon (Bell 1993; Bell and VonLoh 1998). Most unexpectedly, a pterosaur wing metacarpal was collected from the middle portion of the formation, and a neonatal chelonioid costal was found in the uppermost part of the formation (VonLoh and Bell 1998). Cobban (1951) noted the presence of teleosts in the base of the formation at Orman Lake, but stated that remains were too fragmentary for a generic identification. Later, Foell (1982) reported the large teleost Xiphactinus audax . In addition to this taxon, SDSM has collected Enchodus , Protosphyraena , Pachyrhizodus , and Micropycnodon . Prior to this report, elasmobranch distributions within the Greenhorn Formation of South Dakota were virtually unknown. Cobban (1951) reported Isurus appendiculata (= Cretolamna appendiculata ), Isurus desorii (= Cretoxyrhina mantelli ), Squalicorax falcatus , and Ptychodus whipplei , and Martin et al. (1996) noted the presence of odontaspidids. ABBREVIATIO S Institutional - AMM , Adams Memorial Museum, Deadwood, SD; SDSM , Museum of Geology, South Dakota School of Mines and Technology, Rapid City. Other - AR , Angostura Reservoir, Fall River County, SD; BFR , Belle Fourche Reservoir, Butte County, SD; E, Edgemont, Fall River County, SD; OLM , Orman Lake Limestone Member, Greenhorn Formation; PCM , Pool Creek Member, Carlile Shale. GEOLOGIC SETTI G The Late Cretaceous Greenhorn Formation was deposited in a north-south trending inland seaway between the cordilleran mobile belt to the west and the stable North American craton to the east (Macdonald and Byers 1988). Exposures of the formation are extensive in Colorado, Kansas, Wyoming, Nebraska, South Dakota, and northern New Mexico (Hattin 1975). In the Black Hills region of South Dakota, the Greenhorn Formation is exposed around the Precambrian core of the Black Hills Uplift (Figure 1). This area is also the site of a facies change within the formation from carbonate-rich facies to the south and east, and clay-rich facies to 1 cicimurri the west ( Hattin 1975; Macdonald and Byers 1988). Greenhorn carbonate rocks are thought to have been deposited far from the nearest shorelines in relatively deep, mostly quiet-water environments where pelagic sedimentation prevailed. There is considerable debate as to water depth of the Western Interior Seaway during deposition of the Greenhorn Formation. Using microinvertebrate and geological structure data, Hattin (1975) and Kauffman (1969) suggested that deposition occurred in middle to outer shelf depths (100- 200 m maximum) except during regression when the depth was shallower. Based on reconstructed slope measurements and microinvertebrates, Eicher (1967) suggested that water depth during maximum transgression was 915m. In western South Dakota the Greenhorn consists predominately of marl and calcareous shale, with abundant limestone concretions, thin to moderately thick limestone beds, and bentonites. West of this area the unit grades into non-calcareous shale of the Marias River, Frontier, and Mancos formations (Williamson et al. 1993). There is a lithologic difference in the Greenhorn Formation between the northern and southern limits of the Black Hills, which is probably the result of diachronous deposition within the basin, but could also be due to unconformities. Cobban (1951) described the Greenhorn Formation in the northern Black Hills using four informal lithologic units, but only the basal member was named. This basal unit was named the Orman Lake Member for exposures around Orman Lake (Belle Fourche Reservoir) near Belle Fourche, South Dakota. The formation reaches a maximum thickness of 110 m in Crook County, Wyoming, but in the study area the maximum measured thickness is 63 m (southern Black Hills at Angustora Reservoir). Maximum thickness in the northern Black Hills has not been determined because of a lack of continuous outcrops. The Greenhorn Formation overlies non-calcareous shale of the Belle Fourche Shale, and underlies black non-calcareous shale of the PCM. In the southern Black Hills, the Belle Fourche and Greenhorn are separated by a thick bentonite that Cobban (1951) has called the Figure 1. Exposures of the Greenhorn Formation around the "X" bentonite (Figure 2). This bentonite appears to be missing in the Black Hills. BFR (Belle Fourche Reservoir), AR (Angostura northern Black Hills, where basal beds of the Orman Lake Member Reservoir), and Edgemont (E) are the primary localities unconformably overlie the Belle Fourche Shale (Figure 3). The discussed in the text. SDSM localities for each location are Cenomanian-Turonian boundary is located near the middle of the indicated (i.e. V9536 is at BFR). pC = Precambrian core of Greenhorn Formation, not far above the "B" bentonite of Elder the Black Hills Uplift (1986). This distinctive bentonite is exposed both in the northern and southern portions of the Black Hills of South Dakota (Figure 4). There may be an unconformity at the top of the formation in the northern Black Hills, indicated by vertebrate lag beds and an abrupt change from light colored limestone and marl to dark non-calcareous shale (Figure 5). In the southern Black Hills, the top of the formation is placed at the contact between a gradational interval of 18 m of calcarenites and black non-calcareous shale of the PCM. 2 cicimurri Figure 2. Stratigraphic section of the entire OLM, Greenhorn Formation, as exposed at the southern portion of AR, Fall River Co., SD. The contact with the Belle Fourche Shale is just below the base of the "X" bentonite. Figure 3. Stratigraphic section of the Belle Fourche Shale/Greenhorn Formation contact as exposed at the southeast end of BFR, Butte Co., SD. OLM = Orman Lake Member. 3 cicimurri Figure 4. Stratigraphic section of the middle portion of the Greenhorn Formation found at the southern end of AR. The Cenomanian (Cen)-Turonian (Tur) boundary is extrapolated from the Hot Springs, Fall River Co., SD site of Elder (1986). This section is within the Sciponoceras gracile Biozone (late late Cenomanian age). The Greenhorn Formation of western South Dakota shares many ammonite biozones with the principal reference section in southern Colorado near Pueblo. In the northern Black Hills the basal bed of the formation is in the Dunveganoceras pondi Biozone that is early late Cenomaniain in age, whereas the uppermost beds are in the Collignoniceras woollgari Biozone of early middle Turonian age (Obradovich 1993). In the southern Black Hills the basal portion of the OLM is found in the Pleisiacanthoceras wyomingense Biozone, indicating a late middle Cenomanian age (Obradovich 1993). In the northern Black Hills, this ammonite is found in the Belle Fourche Shale, 5 m below the basal Greenhorn beds. The base of the Greenhorn Formation is therefore older in the southern Black Hills than it is to the north. In the southern Black Hills the top of the formation is also located in the Collignoniceras woollgari Biozone. Based on this ammonite zonation, the age of the Greenhorn Formation in Western South Dakota is considered to be late middle Cenomanian to early middle Turonian. Although elasmobranch fossils were collected from various locations in the Black Hills region of South Dakota and Wyoming, this report focuses on three particularly important areas (Figure 1). The Greenhorn Formation is exposed around nearly the entire perimeter of BFR in Butte County. Excellent exposures of the base of the OLM are found at the southern end , whereas