Grand Canyon: Paleobotanical Research on the Bright Angel Shale

Introduction

The Bright Angel Shale is a Middle Cambrian sequence composed of shales, siltstones and finer to medium sandstones, sometimes ferruginous. We have been investigating the palynology of the Bright Angel Shale since 1996 when we received funding from the National Geographic Society. Todate we have completed 3 research trips to the Bright Angel Shale:

• In May of 1996, an investigative team of 5 descended into the northern part of the canyon in the Deer Creek and Tapeats Creek areas.
• October 1997. A measured section at Sumner Butte was partially completed.
• April 1998. Completion of the Sumner Butte Section.

• April 2000. Participants Schedule.

We have now sampled through the uppermost Tapeats Sandstone through a good part of the lower and middle Bright Angel Shale. Many samples have revealed microscopic cells, most probably the resistant cysts of previously undescribed algae, but some decidedly like cryptospores of terrestrial originseen in younger strata.

This finding is particularly significant because this assemblage has qualities in between those of younger terrestrial spores attributed to true plants (bryophytes) and some Cambrian tetrads attributed to algae incertae sedis. These microfossils from the Bright Angel are decidedly unlike acritarchs assemblages typically found in (marine) Cambrian strata. If fact, we have yet to record any typical acritarch forms from the samples we have processed to date. This leads to the conclusion that we are looking at a non-marine assemblage of microscopic organic remains. Freshwater remains of Cambrian age are literally unknown in the fossil record and this find could represent our first glimpse into the terrestrial half of the Cambrian world.

Sample site at Deer Creek showing the interbedded sandstone and shale lithology typical of this part of the section.

Fossil spore polyads and a dyad from the Bright Angel Shale (Middle Cambrian age). Similar problematic spores are abundant in samples from the middle to upper sections of the Bright Angel Shale. These spores are unlike marine acritarchs typical of this time period and lead us to believe that this deposit was unfluenced by freshwater influx into a very nearshore depositional site. The individual cells represented here range from about 20 to 40 microns in size.

The table below displays thumbnails of SEM images of cryptospores and spore clusters obtained from a sample of the Bright Angel Shale collected near Thunder River Falls. Click on an image to see a more detailed version.