Friday, December 06, 2013

Notes from a (former) beach

This summer I hiked to the rim of a canyon in central Arizona and lingered there, enjoying the warm day and spectacular scenery. The rimrock under my feet sparkled as the sun caught tiny grains of almost pure silicon dioxide, cemented into an attractive pink-white rock that had made the transition from soft sandstone to hard quartzite.

The grains had started as simple sand.

Though I was 7000 feet above sea level in a pine and juniper forest, I was standing on a beach.

Or at least it had been a beach 700 million years earlier.

Picture me there:

Two hundred feet below the cemented sand of my beach is a red-brown limestone which had once been sediments deposited in a marine basin. A large gulf on a continental shelf, perhaps? An ancient twin of the Gulf of Mexico?

Minute organisms died and fell to the bottom of that gulf for 300 million years to build up 1200 feet of finely layered material. For a time the gulf grew shallow and sported sticky mats of cyanobacteria which harvested the sun's energy in the warm waters and are now preserved as layered fossils, among Earth's oldest traces of life. Known to paleontologists as stromatolites, they break off their cliffs and fall into the canyon below:

The ancient gulf and its history ended 1.1 billion years ago.

Between the sea's carbonate remains and the cemented beach sand on which I stand is a 400-million-year gap during which some amount of material -- possibly a vast amount of material -- was removed by erosion. Complicating the structure is the presence of vast sheets of a dark, igneous rock which long ago shouldered its way into the existing strata in an act of titanic displacement.

If we could run a fast-motion film of Arizona over the last two billion years -- in which each span of a million years is reduced to one second to make a 33-minute-long geological documentary -- we'd see islands sail into view from the west (for Arizona was for a very long time on the ancient coast of North America) and be either worn away or pasted onto the continental margin. Basins off the Arizona coast would fill with sediments; long successions of beaches would develop and then be covered. Land would periodically heave up and then be eroded away. Trenches and fractures would form and crustal plates would shift. From time to time vast sheets of igneous material would emerge from deep in the Earth, shoving their way between existing sheets or erupting from the surface as lava flows. Our documentary film would most assuredly contain a lot of action.

But I like to think of it as a book with geological formations as its leaves, a book we have learned to read. It is a history text, albeit one with missing or shuffled pages.

So my summer sojourn on the top of the canyon becomes a deeply fascinating back-to-the-beach book, immensely long but fragmented, a story more textured and more mysterious than any other. It rewards minds willing to read closely.

It is the best book I know.

Figure from S.J. Skotnicki and L.P. Knauth, 
"The Middle Proterozoic Mescal Paleokarst, 
Central Arizona, U.S.A.: Karst Development, 
Silicification, and Cave Deposits," 
J. Sedimentary Research, 2007 (77), 1046-1062.

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