The Burgess Shale is among the most recognizable and studied geological sites in the world – and probably the world’s most significant fossil find.
Located high in the Canadian Rocky Mountains – in British Columbia, near Lake Louise and the Alberta border – Burgess contains an amazing record of Cambrian animal fossils, dating 545 to 525 million years ago.
Thousands and thousands of geoscientists have visited its lofty locale. Thousands more of the very best have studied it. Hundreds, maybe more, have written in-depth, comprehensive works on its very existence and profound meaning.
What more is there to tell?
“It’s true the Burgess Shale is well known and a UNESCO world heritage area,” says researcher and AAPG member Kimberley Johnston.
“Stephen Jay Gould (the late paleontologist and science writer) brought the Burgess Shale fossils to international attention, fueling natural curiosity of animal life and evolution so early in the fossil record,” Johnston continued, “but the fossils are only part of the story.
“What really intrigues us as scientists is not what was there, but why they were there.”
Why … and how.
Count Kimberley Johnston and her husband and colleague, Paul Johnston of Mount Royal University, also an AAPG member, among those who believe there is more to the story.
The Johnstons, along with AAPG member Stanley Keith, gave a special lecture on the area and its relation to mud volcanism and chemosynthetic communities during the recent AAPG International Conference and Exhibition in Calgary, Canada.
They believe their findings – “The new ideas,” Kimberley calls them – “link the sedimentary layers of the Burgess Shale to chemical mud volcanism; the disjunct animal fossil deposits to chemosynthetic communities. We have a hydrocarbon connection, too.”
Specifically, they believe that because recent studies show that meter-to-decameter-thick lithosomes, composed of nearly pure clinochlore, are relatively common in the shale – and since the composition of the clinochlore is a Mg-rich product of hydrothermal alteration of mafic minerals, that proves it could not have been deposited by normal basinal sedimentary processes.
It arrived, they believe, at the Cambrian seafloor via mud volcanoes.
Volcanic Shields
The Burgess Shale has a rich and long history, and no talk about its present can commence without at least a nod to its past.
“There was a 200-meter high submarine escarpment of the Cathedral Formation, with shallow-water animals living on the platform,” Kimberley said of the beginning. “Seasonal storms blew them over the edge to a dark watery grave, where they lay on the anoxic seafloor or were stuffed with sediment delivered by the storms or turbidity currents.”
Fast forward, if such a thing is possible, a couple of million years later when Burgess Shale layers were laid down.
“After its deposition, the Burgess Shale was exposed to regional green schist metamorphism,” she said. “The lead-zinc mines in the type area were supposedly emplaced during the building of the Rocky Mountains.”
Supposedly.
“Now we understand that there was not necessarily a tall escarpment at all,” she said, “but an active oblique thrust fault related to the movement of North America from the equator.”
As for the animals purported to be living during that time, Kimberley says their stories did not end well.
“They were living at the fringes of toxic environments, with these fluids and mud issuing from the fissures that probably built up shield volcanoes, probably very rapidly.”
She says their research indicates the fluids probably were responsible for the mass deaths of animals as they were bathed in toxic brines.
She continues, though, on the broader history.
“The regional differences in thicknesses of the Burgess Shale relate to the position of each volcano shield,” she said. “The sites of the calderas would have moved through time, according to the plate tectonic stresses. The hydrothermal dolomite, ore minerals and most of the sediment of the Burgess Shale package all are part of a system that generated both hydrocarbons and brines associated with serpentinization very deep in the crust.
“There are several recognizable pulses within the Burgess Shale,” she added, “and even into younger Middle Cambrian strata.”
The Never Ending Story
Maddeningly, she and her team were initially barred from studying in traditional areas of Burgess – something she believes was fortuitous.
“The blessing was to be forced to look at several Burgess Shale outcrops,” Kimberley said.
During that time of “forced” study it became obvious to them that the fossils were not distributed evenly, and that there was another explanation for the paleoenvironment other than deep, anoxic water.
So what happens now? Why, then, the excitement?
“The geochemical data we have are good, but incomplete,” she said. “We want to test our hydrocarbon results with more samples from the mud volcano conduits.
“The type area needs to be mapped with consideration for Middle Cambrian faults.”
So get ready. Another chapter will be added to the story that everyone knows so well.
Which probably will lead to another, and another, and another …