May 6, 2017 | David F. Coppedge

Archive Classic: The Mystery of the Ultra-Pure Sandstones

Reprinted from June 27, 2003, this mystery is worth considering still. What caused these global deposits that are not being formed today?

6/27/03 — R. H. Dott, Jr (Univ. of Wisconsin) has a problem.  He’s been trying to explain a geological puzzle for 50 years, and it is still unresolved.  All around the world, sandstones are found that are “remarkably pure” that “seem nonactualistic” (jargon for “They can’t really be there”).  These pure quartz arenites, as they are called, were considered a major puzzle half a century ago, when Dr. Dott was a student.  Some of them “extend laterally over vast areas encompassing one or several states,”  and they cover vast areas of Africa and Arabia, the Great Lakes region, South America, Australia, and more.  These “sheet sands” (as they are nicknamed) are part of a notorious gang: “Together with the origin of dolomite, red beds, black shale, and banded iron formation, they made up a group of seemingly intractable geological problems” (emphasis added in all quotes).

Dott tells autobiographically, “Having lived literally upon quartz-rich sandstones for almost 50 years, I have come to regard supermature quartz arenites as nature’s finest distillate—almost as remarkable as a pure single malt Scotch whiskey.”  In the July 2003 Journal of Geology, he has written a lengthy paper addressing the mystery of the quartz arenites, and the status of current hypotheses.  It amounts to a veritable State of the Century address to sandstone geologists.  He explains the puzzle in the Introduction:

What is the quartz arenite problem?  Foremost is the extreme compositional maturity of sandstones composed of more than 95% quartz.  Furthermore, the quartz consists almost exclusively of grains of unstrained, single-crystal units.  Very rare lithic [rock] fragments consist only of durable polycrystalline quartz types such as chert or vein quartz.  In addition, the extremely rare accessory mineral suite (generally <0.05% by weight) is dominated by durable zircon, tourmaline, ilmenite, and leucoxene.  Where present, associated conglomerates also consist only of durable clasts of vein quartz, quartzite, or chert.  How can we explain the complete disposal of at least 75% of any ultimate parent igneous or metamorphic rock to yield a residue that is at least 95% quartz sand? Extreme textural maturity is also characteristic of many, but not all, examples.  A high degree of sorting has always been emphasized, with high rounding being common but not universal.  Both properties imply much abrasion by one or more of nature’s most physically vigorous processes, such as surf and strong eolian [wind] or aqueous currents.

Dr. Dott mentions additional puzzles about these formations:

  1. Thin, tabular geometry: layers tens or hundreds of meters thick, very flat over vast regions, yet Paleozoic in age – i.e., prior to the emergence of land plants.
  2. A paucity of associated shale, in contrast to other sandstones.
  3. Interstratified with shallow marine carbonate strata.
  4. A lack of volumetrically significant analogues forming today (i.e., nothing on that scale can be seen forming now).  This implies weathering processes orders of magnitude greater in the past.
  5. Very rare body fossils, and some burrows.
  6. Frosting of the grains, making them rough on microscopic scales.
  7. Underlying mature shale high in kaolinite (clay) or illite.
  8. Even more pure quartz arenites, up to thousands of meters thick, in Precambrian strata.
  9. Many of them underlain by paleosols (ancient soils) that show a high degree of chemical maturation.

According to Dott, wind erosion is the most efficient, but not the only, agent for rounding of the sand grains.  Some geologists have resorted to theories of multicycling to explain the weathering and maturation of the grains, but theories of single cycles “under intense tropical weathering” also go back decades, and he cannot rule them out.  (Though there are small examples forming in isolated river deltas today, their grains are not nearly as rounded.)  The chemical maturation suggests that impurities were dissolved away, a process called diagenesis, but that is not possible in the presence of wind.

The paradox of the compositional maturation of the sand “seems to require some additional factor to reconcile geomorphic conditions that could have enhanced the transport and abrasion of enormous volumes of pure quartz sand, on the one hand, but could have allowed exceptional chemical maturation of soils on the other hand, as indicated by profiles beneath, and the composition of pelitic [mud, clay] strata interstratified within, many quartz arenites.”

Dott introduces his theory at this point.  To solve the paradox, he postulates thin microbial crusts or mats of cyanobacteria formed over the soils, similar to the stromatolites and cryptogamic soils seen forming in some regions today.  These might have protected the underlying paleosols while allowing wind transport of sand above.  The lack of trees and shrubs might have allowed much more energetic winds.  This assumes that the first land invaders were cyanobacteria, although “the fossil record has seemed mute” on this point.  In a sense, these crusts formed a cap that protected the lower strata while the high winds deposited the sand (although he does not propose sources for the sand).

He ends with one other paradox; without land plants, unless the landscape were perfectly flat, how could it be stable enough to allow the chemical weathering of both the sand and underlying paleosols?  “The abundance of medium-grained to coarse-grained sand and associated pebbles required streams with sufficient gradients to transport such materials, which in turn points to at least moderate topographic relief, which exacerbates the stabilization problem,” he says.  His best guess, in conclusion, is the microbial mat theory; this formed a crust enough to stabilize the landscapes for up to two billion years while these puzzling structures formed.

This was an interesting paper about an interesting puzzle that some readers may wish to investigate further.  Does his explanation satisfy you?  Notice how these formations are huge, and exist on every continent.  Notice how thick and flat they are.  Notice how they are interspersed with clays and soils, yet are exceptionally pure, “nature’s finest distillate.”  Notice how they give evidence of being deposited via nature’s most vigorous and energetic forces.  Doesn’t this sound like global cataclysm?  Since catastrophism is back in vogue, should we not follow the evidence where it leads?

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