FOSSIL ANIMAL BURROWS

Last week I was driving up an unnamed cinder cone near O'Leary Peak north of Flagstaff.  A road cut in the hill had exposed a number of fossil animal burrows.  They stood out against the black cinders because they had filled with white material.  

Trace fossil of an animal burrow.
A trace fossil is the evidence or imprint of an animal but not the animal itself.

I took a piece of a borrow that had fallen out of the hill home and looked at it with a 10x loupe. It was a conglomerate of pyroclastic (fire broken) rocks and small cinders surrounded by a white matrix. I immersed a piece of it in vinegar. It started to fizz and within an hour the matrix had pretty much disintegrated into a brown ooze leaving just rock pebbles and cinders behind. This indicated the matrix to be calcium carbonate (limestone.)

When I collected this trace fossil burrow it was a single piece
but it was very fragile and broke in two on the drive home.

Ash funneled down into an animal burrow.

At least three separate cinder eruptions (1-3) took place in a relatively short period of time.  (There is no soil accumulation between layers 1, 2 or 3.)  Layer 2 was preceded by a small ash eruption visible as the white seam just above the number 1.  The ash boulders at the bottom fell from the top pyroclastic layer as they were undercut by the faster eroding cinders beneath them.   

Layer A of soil accumulated.

As the volcano lay dormant for many years about 6 inches of eolian  red/orange soil (layer A) accumulated on top of the black cinders.  Grasses and small plants started to grow then critters moved in and dug the burrows down through the soil and into the cinder layers below.

Where did the limestone come from?  

Underneath all the volcanoes and lava flows around Flagstaff is a 350' thick layer of the Kaibab Limestone.   All volcanoes and vents must penetrate through this limestone to reach the surface.  Since this volcano had been dormant a hard cap had formed at the base of the volcano and sealed the vent.  As the volcano reawakened pressure built up under the plug until it violently exploded enlarging the original vent and pulverizing more of the limestone.  It was a short lived pyroclastic eruption as there is only a thin layer of ash 3 or 4 inches thick on top of soil A. 

After the limestone rich ash cloud descended the volcano erupted a layer several feet thick of cobble size cinders before it died.  Over the following millennium soil blew in and covered the cobbles (layer B), plants returned and so did the burrowing creatures.