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The Chert Conundrum

“Elementary, my dear Watson.”

© Copyright 2016 James D. Henry

I was working for Mitchell Energy in the Dallas office when we drilled the Mitchell #1 Vera Henderson (API 42-237-33402) in October of 1980 a few miles northwest of Jacksboro, Jack County, Texas, in the northern Fort Worth Basin (Thompson Survey, A-612). (See E-log below.) Note the 35-foot (11-meter) zone with good SP, high resistivity, and clean gamma ray at 5255-90’ just below the base of the Atoka shale section. It’s in the right place to be the top of the Marble Falls—but that’s not what it is. 

Henderson Chart Log

The anomalous chert zone in the Mitchell Energy #1 Vera Henderson (Jack County, Texas, A-612) is at 5255'-90'. Unfortunately, this E-log is the only one I have been able to find for this well. Initially a small producer in the Ellenburger, the Henderson was recompleted in the chert zone (which Mitchell nominally and incorrectly listed as “Marble Falls”) and made a paltry 408 BO and 109 MMCFG, no doubt out of fractures, between late 1981 and 1988.

That interval is a tall three-storey building worth of pure, unadulterated  chert. Not a cherty limestone or dolomite, mind you—not a chert conglomerate with calcareous cement—no shale breaks—no sandstone: Just pure white chert, conchoidal fractures and all.  I know because I was the geologist in the dog house when we drilled through it and logged it.  (There was no mud logger.) I saturated the samples with hydrochloric acid, but I never saw one single bubble come off the cuttings. I brought the sample bags back to Dallas to prove that I wasn’t crazy. You couldn’t get purer chert cuttings if you drilled 35 feet of the Arkansas Novaculite. (More on that in just a moment.) It should not be possible to find a sedimentary anomaly like this in the Fort Worth Basin—but there it is; I saw it with my own eyes, and it’s not going away.

I was primed and ready to see more of this unbelieveable chert bed when we drilled the #2 Henderson (42-237-33625) less than half a mile south in the same survey in March of the following year. But it was gone—completely. There was not a single chert fragment to be found in any of the cuttings. It had disappeared altogether. Nor could I find anything that vaguely resembled it on the #2 logs nor any of the surrounding well logs. For that matter, I am not aware of any other such significant chert accumulations elsewhere in the entire Fort Worth Basin.

Where could twelve solid vertical yards of chert have originated? Chert nodules are not uncommon in sedimentary rocks, but very few of them grow larger than a grapefruit. A primary, in-place novaculite is out of the question; it had to be transported to where it was found. So, whence came the chert, and what was the transporting medium, and how did such a thickness of it get to that place without leaving even the slightest trace of itself in the nearest boreholes? The only other formation in the Fort Worth Basin known to contain a fair amount of chert nodules is the Ordovician Ellenburger about 400 feet (120 meters) on down the hole. Is it plausible that enough of those nodules could have been weathered out smooth and clean and then somehow raised up through the entire Barnett and Marble Falls Formations in one very small local area (where no faulting is known to have occurred) to where they have come to rest at the base of the Atoka shale section? Total rubbish, of course.

As Sherlock Holmes famously said to his faithful sidekick, “Watson, once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.” With that in mind, in my opinion, the only explanation that remains for this large and lonesome chunk o’ chert in the middle of nowhere in the northern Fort Worth Basin is . . . (wait for it) . . . ice rafting. Without question, all the necessary conditions were simultaneously in place at just the right time. To wit:

—The entire Pennsylvanian Period (aka Late Carboniferous) is known to have been one of the most extensively glaciated times in geologic history. 

—At the same time the Marble Falls sediments were being deposited in the foreland basin, the Atokan-age collision of North America with Gondwana/North Africa was raising the overthrust Ouachita Mountains to glacial heights less than a hundred miles to the east and northeast.

—As it is today in Arkansas and the Big Bend, the up-to-900-foot (275 meters) thick Arkansas Novaculite (Devonian and/or early Mississippian) was exposed at multiple locations in those lofty slopes.

It all comes together. A block or boulder of the Arkansas Novaculite was gouged out of the Ouachitas by, and incorporated into, a glacier which eventually broke off in the shallow ocean to form icebergs (i.e., ice rafts) which carried it a few dozen miles out into the Marble Falls sea before the melting ice dropped it to the sea floor where it was discovered by George Mitchell’s drill bit a few hundred million years later. Come to think of it, the occasional chert flakes we often see in Marble Falls drill cuttings could very easily be from small ice-rafted novaculite cobbles rather than indigenous, in situ chert nodules. Little or no chert is reported stratigraphically above the Marble Falls in the Atoka (Bend) shales, the Caddo Limestone, or the Strawn sands and shales. And it is rare to nonexistent in the underlying Forestburg and Barnett Formations. A brief and admittedly unscientific survey of my sample/mud log files suggests that most of the chert in the Marble Falls, like the exotic block in the Henderson well, occurs in the uppermost few dozen feet of that formation. Far from being conclusive, that might suggest that the ice rafting interval in the Fort Worth Basin, however widespread it might have been, was geologically relatively brief and/or that most of the novaculite debris was fairly small.

Ice Rafting

The lithology of the chert cuttings I saw was closely compatible with that of the same novaculite formation I have seen on the surface in Big Bend (where it is called the Caballos Novaculite). The uniformly white color of the drill cuttings suggests that it may have originated in the lowest of three unnamed zones of the novaculite formation. No doubt, there are more exotic blocks of novaculite and/or other Ouachita facies rocks lying in the Atokan sediments of the Fort Worth Basin where they have been ice rafted for no apparent reason other than to confound unsuspecting geologists and Sherlock Holmes fans like myself.

Ice rafting is neither incomprehensible nor uncommon in glacial areas. But the chances of accidentally drilling through an exotic ice-rafted boulder large enough to be recognized as such a mile or more down in the Earth are slim indeed. It is likely that we often drill through small novaculite chert cobbles and chunks in the Fort Worth Basin without recognizing them, or even noticing them, for what they are.

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