Current Research

Taphonomy of fossil turtles, sedimentology, and geological mapping of the Eocene Bridger Formation, SW Wyoming

Leonard Brand, Ph.D., Prof of Biology and Paleontology, Loma Linda University

 

Taphonomy of turtles

This research focused on the taphonomy of the most abundant fossils in unit B of the Bridger Formation - turtles, and the sedimentological context of the turtles. The goal of the research was to understand the paleoecology and sedimentary processes that produced the fossil assemblage.

Brand, L. R.  2007.  Lacustrine deposition in the Bridger Formation: Lake Gosiute extended.  The Mountain Geologist, 44:69-77. (download pdf here)

Brand, L. R., H. T. Goodwin, P. G. Ambrose, and H. P. Buchheim.   2000.  Taphonomy of  turtles in the Middle Eocene Bridger Formation, SW Wyoming.  Palaeogeography, Palaeoclimatology, Palaeoecology, 162:171-189. (download pdf here)

Buchheim, H. P., L. R. Brand, and H. T. Goodwin.  2000.  Lacustrine to fluvial flood-plain deposition in the Eocene Bridger Formation.  Palaeogeography, Palaeoclimatology, Palaeoecology, 162:191-209. (download pdf here)

Brand, L. R. 1995. An improved high-precision Jacob's staff design. Jour. Sedim. Res., A65:561. (download pdf here)

 

Geological mapping of Bridger Unit B

During the process of this research, geological mapping of Bridger Unit B was accomplished. For information on the geological maps, and the stratigraphy of Bridger B, go to the lower part of this page.

 

 

Taphonomy research

Students excavating turtle

 

 

 

Two students, Mark Loewen and Judy Holbert, collecting a turtle from a sandstone.

 

 

 

 

 

 

Relative abundance of turtle bones

 

 

 

 

 

Relative abundance of turtle bones in mudstones in the upper part of Bridger B, above the Lower turtle layer limestone. Data from a 30 meter-wide transect in the Devils Playground area. Turtles are abundant at specific levels, above limestones, and taphonomic evidence indicate these are mass mortality assemblages. Ltl = Lower turtle layer (mudstone above the limestone). Gbl = Golden bench limestone. BMtl = Black Mountain turtle layer (mudstone above the limestone). Utl = Upper turtle layer (mudstone above the limestone).

 

 

 

 

 

 

 

 

bridger turtle isopach map

Geographic distribution of turtle bones in Bridger B. At each study site (e.g. NR-10) the number of turtle bones per hectare is given, and isopach lines are drawn through areas with the same number of bones per hectare. Turtle distribution shows a basin-wide pattern of the turtle mass mortalities, rather than local concentrations.

 

Black Mountain turtle layer

 

 

 

Student George Kim prospecting for vertebrate fossils in the Black Mountain turtle layer.

 

 

 

 

 

Model for turtle and sediment accumulation

 

 

Model for accumulation of turtles and sediments in Bridger B. Time 1: A shallow lake fills the basin. Turtles are abundant in the lake. Time 2: An episode of volcanism begins to the north, in the Absarokas. Ash or volcanic gases kill many turtles, and clay to sand sized volcanic sediments begin to accumulate over the basin, delivered by air fall and/or prograding fluvial-deltaic systems.

 

 

 

 

Model for turtle and sediment accumulation

 

Time 3: The sedimentary sequence becomes more fluvial in character, in a well-established fluvial floodplain environment with large fluvial channels representing meandering rivers. Time 4: The volcanic episode ends, and a lacustrine system again begins to fill the basin, forming another limestone. This sequence accounts for the repeating pattern of turtles concentrated in large, mass mortality assemblages right above limestones. Turtles are much less common outside of these concentrations.

 

 

Chipmunks

 

 

 

 

 

 

 

 

Local chipmunks at our Cedar Mountain camp site make use of the wastewater bucket at our cooking trailer.

 

 

 

Mapping of Bridger Unit B

bridger B cross section

Cross section through Bridger Formation, Unit B, showing the mapped marker beds. (download pdf here)

bridger geological map summary

Summary map of Bridger Formation geological map, showing the location of measured sections. (download pdf here)

 

bridger geological map showing quads

Summary map of Bridger Formation geological map, showing boundaries of topographic quads. (download pdf of the above summary map here)

Quads shaded in blue were published elsewhere (Murphey, P. C., and E. Evanoff 2007. Stratigraphy, fossil distribution, and depositional environments of the Upper Bridger Formation (middle Eocene), Southwestern Wyoming. Wyoming State Geological Survey Report of Investigations 57.).

The other mapped quads are available for download below.

 

bridger B measured sections

 

 

 

 

 

 

 

Measured sections of Bridger Formation Unit B. Locations for these sections are shown on the summary map above. (download pdf here)

 

 

 

 

 

 

 

Geological mapping of marker beds

All mapping of Bridger B was done in the field. The marker beds were limestones, tuffs, and other prominent beds. Some limestones transitioned into whitish limey mudstones (sometimes within a few meters), but the limestones/ limey mudstones and the beds above and below them were continuous, mappable units. Some marker beds generally consisted of a set of two or three limestones, a few meters apart. The upper of these was the mapped bed. This was especially true of the Lyman limestone, Golden bench limestone, and the Black Mountain turtle layer (a turtle-rich mudstone immediately above a limestone/limey mudstone). There were some exceptions. South of the Devils Playground quad the limestone immediately below the upper Golden bench limestone became the prominent, mapped unit. Also in the Devils Playground area there was a discontinuous limestone about a meter above the Black Mountain turtle layer.

Evanoff, E., L. R. Brand, and P. C. Murphey.  1998.  The Bridger Formation (Middle Eocene) of southwest Wyoming: widespread marker units and subdivisions of Bridger B through D.  Dakoterra, 5:115-122.

 

Access to the geological maps

The quads shaded in blue, above, have been published elsewhere (see info above). Pdf's of the other geological map quads can be downloaded by clicking on the individual links below.

Antelope Knoll; Antelope Knoll NE; Butcherknife Draw; Church Butte; Church Butte NW; Devils Playground; Granger; Halfway Hollow West; Haystack Buttes North; Little America; Massacre Hill; Moxa; Needle Reservoir; Roberson Creek; Sevenmile Gulch; Verne; Wildcat Butte; Winter Fat Reservoir

A georeferenced ArcGIS map of the entire Bridger B is also available. Download the zipped file by clicking this link (Download ArcGIS map). If you have trouble with this download contact the author at lbrand@llu.edu