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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"
Many geologic phenomena of the past do not appear to be adequately accounted for in terms of the processes now occurring on the earth’s surface. In some cases, it is difficult to conceive of any mechanism capable of explaining them other than through an agency such as the global deluge described in Genesis. Among these problem areas in geology, the explanation of the origin, transportation, and deposition of megabreccias deserves a prominent place.
Megabreccias are sedimentary deposits in which angular fragments of rock in excess of one meter in diameter occur as conspicuous components. A rock equivalent to one cubic meter in volume may weigh three metric tons, and most megabreccia clasts are larger than this. Consequently, transportation of megabreccias to the site of deposition becomes a formidable consideration. Buoyancy supplied by clear water can reduce the weight by one-third or more and can significantly decrease friction as well. As we shall see, conditions in the past enabled rocks of truly enormous dimensions to be moved, sometimes over great distances.
Submarine mass transport is regarded as the most common depositional process that can give rise to megabreccias consisting of very large clasts transported over an often considerable distance. Such mass wasting processes, like submarine slides, slumps, and debris flows, do not necessarily require a steep slope for movement, and there does not appear to be a set limit to the size of clasts that can be moved. The clasts are commonly exotic (blocks derived from a source different from that of the matrix) and are generally supported in a matrix of mud or clay.
A few examples follow from many that could be cited of the results of such catastrophic processes. In Peru, a rock formation covering an area of about 80,000 km2 contains locally derived blocks of Cretaceous limestone up to one kilometer by several kilometers and up to 500 m thick, and numerous other examples of megaclasts.[1] While it is difficult to pin down a transport distance, at least several kilometers are envisioned. In southern Iran, slabs of exotic rock over a kilometer in size are found in Miocene mudstones, apparently derived from a source many kilometers distant.[2] In the Apennines of central Italy, bodies of transported sediment exceeding 400 m in thickness and covering hundreds of square kilometers of areal extent are known.[3] Many other examples could be added, but perhaps one more will suffice. Wilson reported exotic blocks of Jurassic limestone in Cretaceous radiolarites in Arabia.[4] The largest such block covers an area of 1,600 km2 and is 1,000 m thick. This and other similar mountainous clasts are postulated to have moved a distance of many tens of kilometers to their present position.
Attempts have been made to develop a non-catastrophic explanation for the presence of exotic blocks in megabreccias. The most common alternative interpretations involve slow tectonic emplacement of allochthonous blocks along thrusts in subduction or collision settings.[5] However, a tectonic origin can be easily discarded for many megabreccia deposits because of the absence of tectonic contacts, presence of wet-sediment deformation structures, stratigraphic attributes indicative of minimal overburden at the time of formation, and spatial changes consistent with depositional trends expected in a basin. An active tectonic regime can still be important as a possible trigger for catastrophic mass wasting, and it has even been suggested that mélanges of tectonic origin can be source units for megabreccias and sedimentary mélanges.[6]
Advances in oceanographic and subsurface imaging are increasingly affirming the importance of catastrophic submarine mass-wasting processes along continental margins and have revealed deposits of a few massive slides containing rafted blocks several hundred meters high and several kilometers in size.[7] However, very little is still known about specific transport mechanisms and triggering forces responsible for the origination and accumulation of megabreccia clasts.[8] It is difficult to imagine forces operative under uniformitarian constraints that would have produced clasts of the size we find in megabreccia deposits.
In conclusion, the presence of various kinds of megabreccias in the geologic record, showing the rapid transport of extremely large clasts over gently dipping or flat substrates for many kilometers, indicates energy levels on a scale that staggers our imagination. Their common occurrence indicates significant catastrophic activity in the past not readily explained in terms of contemporary processes, but consistent with an extended catastrophic episode such as the one described in Genesis.
NOTES
[1] P Callot, T Sempere, F Odonne, E Robert. Giant submarine collapse of a carbonate platform at the Turonian-Coniacian transition: the Ayabacas Formation, Southern Peru. Basin Research 2008; 20(3):333–357.
[2] JP Burg, D Bernoulli, J Smit, A Dolati, A Bahroudi. A giant catastrophic mud-and-debris flow in the Miocene Makran. Terra Nova 2008; 20(3):188– 193.
[3] CC Lucente, GA Pini. Basin-wide mass-wasting complexes as markers of the Oligo-Miocene foredeep-accretionary wedge evolution in the Northern Apennines, Italy. Basin Research 2008; 20(1):49–71.
[4] HH Wilson. Late cretaceous eugeosynclinal sedimentation, gravity tectonics, and ophiolite emplacement in Oman Mountains, Southeast Arabia. American Association of Petroleum Geologists Bulletin 1969; 53:626–671.
[5] MP Searle, GM Graham. “Oman Exotics”—oceanic carbonate build-ups associated with the early stages of continental rifting. Geology 1982; 10(1):43–49.
[6] W Cavazza, M Barone. Large-scale sedimentary recycling of tectonic mélange in a forearc setting: the Ionian basin (Oligocene–Quaternary, southern Italy). Geological Society of America Bulletin 2010; 122(11– 12):1932–1949.
[7] M Vanneste, J Mienert, S Bünz. The Hinlopen Slide: a giant, submarine slope failure on the northern Svalbard margin. Arctic Ocean. Earth and Planetary Science Letters 2006; 245(1–2):373–388.
[8] DG Masson, RB Wynn, PJ Talling. Large landslides on passive continental margins: processes, hypotheses and outstanding questions. Mosher DC, Shipp RC, Moscardelli L, Chaytor JD, Baxter CDP, Lee HJ, Urgeles R, editors. Submarine mass movements and their consequences. Advances in Natural and Technological Hazards Research 28, New York: Springer, 2010; pp. 153–165.
Art Chadwick is a research professor of biology and geology and the director of the Dinosaur Science Museum and Research Center at Southwestern Adventist University. He earned his PhD from the University of Miami, followed by additional work in geology at the University of California, Riverside. His current research efforts include systematic analysis of the geologic column in the Colorado Basin and directing a taphonomic study at one of the largest dinosaur bone beds in the world. He has conducted geological research in Grand Canyon and Yellowstone National Parks, in Peru on fossil whales, and on global circulation patterns through time. In addition to numerous scientific papers, he has recently co-authored a science textbook titled Faith, Reason, and Earth History.