Stephen E. Pierce, CPG, RG

SOME THOUGHTS ON THE ALAMO BRECCIA AND THE F -F BOUNDARY

It was a particularly difficult trip; the mini rotating black holes connecting the wormhole in the engine compartment were fluxuating out of sync. The word disaster or catastrophe comes to mind. For if the rotating black hole is not of just the right diameter to swallow the time machine and send it back into time through the wormhole it could hurtle the craft into the unknown void of Penrose space-time. However, the trip was made and now you step out of your craft on to a beautiful west coast tropical beach near or just south of the equator. The year is 376,000,000 B.C., hundreds of million years into the past known as the Devonian Period in geological time. You need to work fast because your mission is to examine the life forms on this idyllic beach before the asteroid would strike. And it would strike quickly, in a day or so (in the back of your mind you hope the astronomers calculations are accurate!) Because the moon was closer to the earth, forcing the earth to spin more rapidly the day would be a little shorter now, too. And the day has just begun as you gaze upon the ancient landscape. Of course there would be no palm trees; they hadn’t evolved yet. But there were other trees different types of ferns, simpler, but as you look into the foliage just as pleasing. Looking out to sea, you watch the ever-ceaseless waves breaking on the reef. Wading into the calm shallow back reef waters you take samples of trilobites and brachiopods long extinct from the world you came from in the 22nd century A.D. After a busy day of collecting specimens you site a labyrinthodont, the very first of animals to inhabit the land. Awkward looking, like something a committee would assemble, it used lobed fins for locomotion and the head and tail were more fishlike than you had first imagined. As it slowly shuffles into the foliage you sadly realize that within a short time it would be doomsday here.

Shortly, an asteroid will destroy this primitive Eden. It will violate this land, heating the atmosphere, and upon collision the shock wave would vaporize water and rock alike. The kinetic energy of the impacting bolide would excavate a giant crater, hurtling great masses of earth into the atmosphere. After being lofted into the sky, melted rock and rock debris would settle into the crater, some as breccia. With time though, life would again inhabit this area, and with more time the crater itself would disappear. And after 367 million years of erosion and the tectonic activity of a restless earth would there be any signs left of the event?

Just such a site has been described in Nevada. In a recent field trip Paul Harris and Jim Tobin the editors of Meteorite-Times.com, returned from Nevada with some interesting rocks from that collision, so long ago. Their trip can be seen on the Internet in Meteorite-Times-Jim’s Fragments. They describe the field trip along with photos. Upon their return they kindly provided me with two samples. These samples May record a catastrophic event that WIPed out some 21% of all the biological families (McGee, 1989). This extinction event occurred in the Late Devonian and is defined by a fossil zone (conodont) between the Fransnian and Famenian (F – F) Boundary some 367 million years ago.

The rock samples were of a breccia (Fig. 1) that was deposited during that catastrophe. Breccias, by their very nature are rocks that have undergone a violent destructive history. A breccia by definition is a coarse grained clastic rock made up of angular-subangular broken rock fragments held together by a cement or fine grain matrix. Up until the relatively recent there were four types of breccias generally recognized in the geologic community. These are (1) volcanic breccias, bred during violent volcanic eruptions, (2) collapse breccias resulting from unsupported roofs of rock, (3) fault and fold breccias, formed during an earthquake and/or during folding of rocks, and (4) intraformational breccia formed by fragmentation and deposition of rock strata, as during a debris flow.

Figure 1

However, during the past few years a new breccia has been recognized, the impact breccia. As can be easily imagined, an impact breccia is the result of a bolide that has collided with the earth. They are generally associated with interior crater fill or fallback and base surge deposits from the impact (King, 1976). They can usually be differentiated from other breccias by the association with craters and composition containing minerals that show shock damage.

The samples that Paul and Jim have kindly given me provide an insight into the breccia. Both samples (one polished- one slabbed) were several inches in length and I measured them as having a specific gravity (sg) of 2.65 time that of water. Nothing unusual there, its about what one would expect in a rock that contained calcite (2.7 sg) and clays (~2.5 sg). The clasts within the breccia vary from .05” to 1.5” and the breccia is a very fossiliferous subangular to angular calcite cemented (impact) breccia. The brecciated rocks were observed to contain angular shale clasts. Fragments of fossil bryozoans and other fossil debris were also observed. The reason why the word impact is in parenthesis is because without other information the Alamo Breccia would be considered an intraformational breccia, resulting from a debris flow. However, the breccia (Warme, 2000) contains, (1) shocked quartz, (2) accretionary lapilli, and (3) is associated with an iridium (Ir) anomaly. Unfortunately, in the samples that I have been provided I could not see any lapilli, and of course, since the section was slabbed (not thin-sectioned) the presence of shocked quartz was not identified. However, these signs of impact can be seen on the Warme “Welcome to the Alamo Breccia research page” on the Internet. In general, many workers consider an Ir anomaly alone to conclude the presence of a bolide. However, it needs to be kept in mind that Ir anomalies can occur due to terrestrial causes. The marine plant Frutexites (Playford, et al., 1984) that lived during the Devonian concentrates among other elements, iridium. However, considering the combination of shocked quartz, accretionary lapilli, an Ir anomaly, and the geology of the area (see Warme’s website), the Alamo Breccia appears to be the site of an ancient asteroid impact.

Did this bolide cause the mass extinction in the Late Devonian? The Devonian Nevada impact site is only one of seven impact sites that are candidates for the bolide at the F – F boundary. Are they related? The craters are presently scattered across the Atlantic Ocean from the U.S. and Canada to Europe and the former USSR (Table 1). I say ‘presently’ because the locations that we visit today are not located in the same position as they were 367 million years ago. How did the earth look 367 million years in the past? The answer to that question is also the solution to another question. The Alamo Breccia is located on an ancient carbonate platform. Carbonate platforms are associated with the tropics, e.g., the Bahamas Platform. So, why is a tropical carbonate platform found in Nevada far from the present day tropics? Las Vegas (located south, relatively near the Alamo Breccia) is located at about 36º 10’ N, 115º 09’ W, almost 2,200 nautical miles from the present equator.

Plate tectonic theory provides the answer. The earth’s surface is composed of a dozen or so major thin plates and many minor ones that make up the earth’s crust. By convection within the underlying mantel, the plates move and interact continually, changing the appearance of the earth’s surface. For instance, the earth today (Fig. 2 a), has the Atlantic Ocean separating the U.S. from Europe and South America from Africa. However during the Devonian the U.S. was astride the equator and Europe was part of eastern North America (Euro-America). Africa, South America, and Antarctica formed a giant continent called Gondwanaland separating it and Euro-America by the Rheic Ocean (fig. 2b). As can be seen, the bolides did not impact over a wide area but in Euro-America only.

During the Devonian the bolides that struck the earth were much closer together because of plate movements than appear so today. But were they genetically related or just the result of a cosmic coincidence? This brings up another question, how often do bolides strike the earth? As shown in Figure 3 (modified after Grieve, 1999) is a histogram showing the frequency of impact craters through time from the Cambrian (600 MA) to the present. When reviewing the data however, it must be kept in mind that the dates are approximate only. Also, the apparent large frequency of cratering in the past 10 million years is probably not real. More likely, it is a function of the ease of finding craters that have not been disrupted by erosion (many small craters would fit here e.g. Meteor Crater, Arizona) and/or destroyed in time by tectonic activity. Some workers have speculated that the data suggests a cycle of greater and lesser intensity through time; others do not see any pattern except that the earth has been bombarded continually throughout geologic time.

The Devonian extinction event appears to me to have been associated with either one or more bolides that collided when North America and Europe were tectonically united as one continent. Whether the bolides were together and impacted at one time or spread out in time, the earth suffered from a major upheaval. Since the Devonian bolide(s) have been associated with mass extinctions (367 MA) and the K – T Chicxulub bolide is associated with mass extinctions (65 MA) is there a connection? The difference between the K – T and F –F events is about 302 million years. Does this suggest an orbital cycle of asteroids that would allow these monsters to return at regular intervals? Look again at Figure 3 do you see anything in the frequency pattern that others might have missed?

References

Dott, R.H. et al., 1971, Evolution of the Earth, McGraw-Hill.

King, E., 1979, Space Geology, John Wiley & Sons, Inc.

McGhee, G.R., 1989, Devonian, S.K., Mass Extinctions Extinction Processes and Evidences, Bellhaven, London.

Grieve, R.A., 1999, List of terrestrial impact structures, Worldwide Web Publication, www.spaceart.com/solar/eng/crater.htm

Playford, P. E., 1984, Iridium anomaly in the Upper Devonian of the Canning Basin, Western Australia, Science, 226.

Scotese, C., Paleomap Project, www.scotese.com

Warme, 2000, Alamo Breccia research page, Internet website, www.mines.edu/students/m/mmorgan/