Sarah E. Tindall

Growth faults in the Kaiparowits Basin, Utah, pinpoint initial Laramide deformation in the western Colorado Plateau

Growth faults and synorogenic sedimentary strata preserved in Upper Cretaceous units on the margin of the Kaiparowits Basin in southern Utah pinpoint the timing of onset of the Laramide orogeny in this region between 80 and 76 Ma. The newly identified listric normal faults, exposed in the steep limb of the East Kaibab monocline, sole into shales and evaporites of the Jurassic Carmel Formation. Faults lose displacement up-section through the Cretaceous Wahweap Formation and are associated with numerous coseismic sedimentary features. Fault orientations and slip vectors yield strain directions consistent with fold-related extension parallel to the axis of the growing East Kaibab monocline, or with development of a pull-apart basin at a bend in the trend of the fold. The association of the faults with the steep limb of a major basement-cored structure links them to initial Laramide movement along the Kaibab Uplift. When combined with recent radiometric ages of rock units bracketing the fault-induced growth strata, these sedimentary and structural features narrowly define the onset of Laramide deformation in the western Colorado Plateau.

Predatory digging behavior by dinosaurs

Minimal direct evidence exists in the rock record of dinosaurs and mammals behaving as predators and prey, respectively. A newly discovered Late Cretaceous trace fossil association of digging traces of maniraptoran theropod dinosaurs and mammalian den complexes indicates a predator-prey relationship. Three distinct associated trace fossils occur within a floodplain siltstone-mudstone bed of the Upper Cretaceous Wahweap Formation in southern Utah, United States. One trace fossil morphology and its extramorphological variants record digging by a maniraptoran theropod dinosaur, possibly a dromeosaurid or troodontid. The other two are interpreted as mammalian den complexes. The proximal association of these trace fossils suggests that dinosaurs used excavation techniques to prey on mammals.

An Upper Cretaceous sag pond deposit: Implications for recognition of local seismicity and surface rupture along the Kaibab monocline, Utah

In the Bull Flat area of Grand Staircase (Escalante National Monument, Utah, United States), sedimentological analysis of the tip line area of a Late Cretaceous normal fault reveals that the fault was active during the deposition and preservation of a sag pond deposit. Sag ponds deposits are probably under-recognized in the rock record, and, as demonstrated in this study, they are helpful in chronicling activity of ancient faults. The sag pond sequence is located just below the contact between the upper and capping sandstone members of the Wahweap Formation, and it occurs within close proximity to other features of seismogenic origin. The sag pond deposit is a 2.2-m-thick succession of carbonaceous, interlaminated mudstones and siltstones intruded by sublithic sandstone seismogenic dikes and sills. At least two seismic events are required to produce the observed sequence. The initial event ruptured the surface, probably tilting the underlying strata, and produced the down-drop for the sag pond. After infilling of the sag pond, a second seismogenic event mobilized sediment in the underlying upper member and generated the dikes and sills that intrude the sag pond deposit. Subsequently, a minor channel scoured the sag pond deposit with flow parallel to the fault trace. Infilling of the sag pond ended before deposition of the capping sandstone member.