During the Cretaceous Idaho was finally above sea level. An excellent reconstruction of western North America (including Idaho) during the mid-Cretaceous by Ron Blakey is here. At this time active mountain building and volcanism was occurring in the central portions of the state, with narrow floodplains near the toe of the highlands grading into swampy lowlands near the Idaho/Wyoming border, where the Cretaceous Interior Seaway began.
Cretaceous rocks in Idaho are limited to the southeastern portions of the state. In Bingham, Bonneville, and Caribou counties the youngest formation is the Sage Junction Formation, followed by (in descending order), the Wayan Formation and the Gannett Group (Smoot, Draney, Bechler, Peterson, and Ephraim). In northeasternmost Bonneville, Madison, and Teton county counties Cretaceous rocks consist of the Aspen Shale and overlying Frontier Formation. In Clark County Cretaceous rocks consist of the latest Cretaceous to Paleogene Beaverhead Conglomerate.
FRONTIER AND ASPEN FORMATIONS
The paleontology of the Frontier and Aspen formations in Idaho has not received much attention. Freshwater and marine mollusks appear to be the most common fossils in the Frontier Formation, while the tree fern Tempskya (described below) has been reported from the Aspen Shale.
SAGE JUNCTION FORMATION
Based on the age of the underlying Wayan Formation, the Sage Junction Formation is at least middle Cenomanian in age if not younger. Rocks of the formation consist of somber colored calcareous siltstones and sandstone, conglomeratic grit, gray, tan, and red-gray mudstones, porcelanite, coal, along with rare limestones (Rubey, 1973). The lithologies and fossils indicate a humid, lowland environment of deposition.
The fossil flora of the Sage Junction Formation is well known and is dominated by the tree-like fern Tempskya, which produced a type of trunk from its roots. Other plants reported from the formation in Idaho include foliage from Nilssonia?, Platanus, Artocarpus, and Liriodendron; as well as dicotyledonous and coniferous wood and portions of a trunk from a cycadeoidea (Andrews and Kern, 1947). Bone fragments of probable dinosaurian origin have been noted by the author in the Sage Junction but these are extremely rare and so unidentifiable.
Cretaceous rocks in Idaho are limited to the southeastern portions of the state. In Bingham, Bonneville, and Caribou counties the youngest formation is the Sage Junction Formation, followed by (in descending order), the Wayan Formation and the Gannett Group (Smoot, Draney, Bechler, Peterson, and Ephraim). In northeasternmost Bonneville, Madison, and Teton county counties Cretaceous rocks consist of the Aspen Shale and overlying Frontier Formation. In Clark County Cretaceous rocks consist of the latest Cretaceous to Paleogene Beaverhead Conglomerate.
FRONTIER AND ASPEN FORMATIONS
The paleontology of the Frontier and Aspen formations in Idaho has not received much attention. Freshwater and marine mollusks appear to be the most common fossils in the Frontier Formation, while the tree fern Tempskya (described below) has been reported from the Aspen Shale.
SAGE JUNCTION FORMATION
Based on the age of the underlying Wayan Formation, the Sage Junction Formation is at least middle Cenomanian in age if not younger. Rocks of the formation consist of somber colored calcareous siltstones and sandstone, conglomeratic grit, gray, tan, and red-gray mudstones, porcelanite, coal, along with rare limestones (Rubey, 1973). The lithologies and fossils indicate a humid, lowland environment of deposition.
The fossil flora of the Sage Junction Formation is well known and is dominated by the tree-like fern Tempskya, which produced a type of trunk from its roots. Other plants reported from the formation in Idaho include foliage from Nilssonia?, Platanus, Artocarpus, and Liriodendron; as well as dicotyledonous and coniferous wood and portions of a trunk from a cycadeoidea (Andrews and Kern, 1947). Bone fragments of probable dinosaurian origin have been noted by the author in the Sage Junction but these are extremely rare and so unidentifiable.
Reconstruction of Tempskya from Andrews and Kern (1947).
WAYAN FORMATION
The Wayan Formation contains the lions share of fossils reported from the Cretaceous of Idaho. The age of the formation is latest Albian to Cenomanian, based on radiometric dating of detrital zircons (Krumenacker, 2010). The formation is dominated by colorful red, green, gray, and purple calcareous mudstones and siltstones, with silver-gray and brown sandstones (see Cretaceous header above). An abundance of calcareous nodules as well as fossil plants indicates deposition occurred in a seasonally arid environment with a distinct wet and dry season and a highly fluctuating watertable.
The Wayan has produced all the identifiable skeletal remains of dinosaurs known from Idaho. The most common dinosaur from the Wayan is the small herbivorous, burrowing dinosaur Oryctodromeus; which is known from several partial skeletons and more fragmentary remains (Krumenacker, 2010). Skeletal remains from other dinosaurs are much more rare, perhaps reflecting either a general rarity of these animals in the original ecosystem or a bias towards the preservation of the burrowing Oryctodromeus. Other dinosaurs known from rare partial skeletons as well as more fragmentary bones and teeth (Krumenacker, 2010; Weishampel et al., 2002; Dorr, 1985) include nodosaurs, iguanodonts , possible neoceratopsians, dromaeosaurs, and large theropods. A recent and very importand discovery consists of a crushed but complete very large egg of the oogenus Macroelongatoolithus (Simon et al., 2012) that was associated with another partial egg as well as abundant eggshell fragments. Isolated fragments of Macroelongatoolithus eggshell are common in the Wayan Formation. The presence of this oogenus suggests the presence of a large oviraptorid in Idaho during Wayan times. Semionotid fish are represented by rare scales and numerous crushing teeth similar to Lepidotes, and turtle shell fragments are also known. A large crocodilian similar to Deinosuchus is also known from a partial skull (Krumenacker, 2010) and is undergoing further research. Rare mollusks such as clams and snails are known as well.
A moderate flora is known from the Wayan Formation (Krumenacker, 2010). Ferns such as Gleichenia and Anemia dominate the assemblage, with short-leaved conifers and magnolia-like angiosperms also being present.
A rare good outcrop of the Wayan Formation.
An Oryctodromeus family. Art by Mark Witton, used with permission.
Oryctodromeus partial skeleton (missing tail and head) recovered from lands of Caribou-Targhee National Forest in southeastern Idaho. Image used with permission of Caribou-Targhee National Forest.
Plants from Wayan Formation (from Krumenacker, 2010). A-B: Gleichenia, C: Anemia, D: Conifer foliage, E: Conifer cone in cross-section, F-G: Angiosperm leaves.
SMITHS FORMATION
The Smiths Formation underlies the Wayan and consists of a distinct upper unit dominated by tan quartzitic sandstones and a lower unit dominated by black shales (Rubey, 1973). Based on it's stratigraphic position it is Ablian in age. The Smiths has received little paleontological attention. With fossils such as the freshwater mollusks Viviparus, Goniobasis, Unio, leaf impressions, and fish bone fragments being reported (Dorr, 1985).
The upper sandstones of the Smiths Formation, with covered beds of the Wayan Formation to the left.
GANNETT GROUP
The Gannett Group underlies the Smiths Formation, in descending order it consists of the Smoot Formation, Draney Limestone, Bechler Formation, Peterson Limestone, and the Ephraim Formation. These formations consist of alternating floodplain (Smoot, Bechler, Ephraim) and lake ( Draney and Peterson) deposits. The extent of the Draney and Peterson limestones, which are coeval with the Kootenai Formation limestones of Montana, indicate a very large lake or lake system that spanned eastern Idaho and western Montana on a floodplain of very low relief. Fossil algal remains and freshwater mollusks (such as Unio and Goniobasis) similar to those reported from the Smiths have been reported from the Draney and Peterson limestones. In some layers of the Draney and Peterson the limestone is a coquina of mollusk shells.
DRANEY LIMESTONE
The Draney Limestone in Caribou County has produced a moderately diverse fossil assemblage representative of a lakeside community. Fossils from these locations consist of a dinosaur trackway, crocodilian teeth, turtle shell fragments, freshwater shark teeth of the genus Hybodus, fish teeth and bones similar to Lepidotes, as well as petrified wood and the more typical mollusk fauna. Research on these fossils is continuing.
A coquina of gastropod shells from the Draney Limestone.
General extent of limestone deposits representing 'Lake Draney' or 'Draney Lake System'.
Vertebrate fossils from the Draney Limestone (from Krumenacker, 2005). A: Hybodus tooth, B: Glyptops (turtle) shell fragment, C: Naomichelys (turtle) shell fragment, D: Crocodilian tooth. Fossils from lands of Caribou-Targhee National Forest in southeastern Idaho. Image used with permission of Caribou-Targhee National Forest.
Dinosaur trackway in the Draney Limestone on lands of Caribou-Targhee National Forest in southeastern Idaho. Image used
with permission of Caribou-Targhee National Forest.
with permission of Caribou-Targhee National Forest.