Prior to the middle of the nineteenth century, efforts to understand geologic features were usually based on belief in a massive worldwide catastrophe of brief duration, as described in Genesis, chapters 6-8. Then with the publication of Charles Lyell's Principles of Geology in 1830, the dominant view in the earth sciences for nearly 150 years has attributed geologic features to the cumulative effects of presently observable processes acting over long ages. How ever, an increasing number of geologists currently recognize a need to consider forces of enormous magnitude not now operating on the surface of the earth to explain phenomena in the geologic rec ord. Among these so-called "neocatastrophists" is Derek Ager, who has considered a catastrophic explanation for the origin, transportation, and deposition of megabreccias. 1
Megabreccias are sedimentary deposits in which angular fragments of rock (called clasts) greater than one meter in diameter occur as conspicuous components. These deposits may include many other clasts smaller than one meter, which may or may not be angular. A rock equivalent to one cubic meter in volume may weigh as much as three tons, and since most megabrecciaclasts are larger than this, transportation of megabreccias to the site of deposition becomes a formidable consideration. Thus most megabreccias are considered to have had an underwater origin be cause of the difficulty of surface transportation of large clasts by natural means over relatively long distances. Buoyancy supplied by clear water can reduce the weight by at least one-third and can also significantly decrease friction. Under appropriate conditions a mixture of water, mud, and small rocks can greatly modify buoyancy and other factors and move rocks of truly enormous dimensions.
We will consider three categories of underwater depositional processes that give rise to megabreccias: turbidity cur rents, debris flows, and slides or slumps. In each case we will define the process, describe its operation, outline the extent of such deposits, and discuss their significance.
Turbidity currents occur when soft sediment becomes resuspended in water, forming a fluid much heavier than water. Flow of such a suspension introduces turbulence which prevents the suspended material from settling out, thus perpetuating the existence of the high density fluid and prolonging its movement as a turbidity current. With sufficient momentum such a current can even flow uphill. As the velocity is decreased in the region behind the moving front, material in suspension is deposited, be ginning with the most coarse particles, so that the resulting deposit commonly exhibits normal grading with larger grains at the base and finer material at the top.
Turbidity currents of credible dimensions are capable of moving enormous clasts. It has been estimated that rocks weighing up to 100 tons could be moved in such flows. The flow of turbidity cur rents probably results most often from earthquakes, but other initiating mechanisms may be effective also. Rock fragments of all sizes involved in the initial disturbance can be transported by a turbidity current for great distances across gentle slopes of only a few degrees.
Turbidites, the deposits left by turbidity currents, occasionally are reported to contain megabreccias. Clasts exceeding a meter in diameter are known from breccia beds in Nevada, Arabia, New Hebrides, and elsewhere.2 Clasts up to five meters in diameter have been re ported in breccia beds interpreted as turbidity current deposits.
There is little question that turbidity currents capable of transporting large clasts represent catastrophic events. Earthquakes, for example, can trigger such currents. But it is more difficult to envision a process that can simultaneously produce, as well as transport, the brecciated clasts. These problems become even more complex as the clast sizes increase.
Debris flows include megabreccia deposits consisting of very large clasts that have been transported by a mass flow process, usually over a considerable distance. Like turbidity currents, debris flows do not require a steep slope for movement; but unlike turbidity currents they are less fluid and flow more slowly. There does not seem to be any limit to the size of clasts that can be moved in this manner. Clasts in a debris flow are generally supported by a matrix of mud or wet clay.
In Peru, exotic—different from the surrounding matrix blocks of up to 5,000 tons and 10 to 15 meters in diameter occur in Eocene strata far from the nearest possible site of origin.3 In Texas, slabs of exotic rock more than 30 meters long are found in Paleozoic mudstones, apparently derived from a source many kilometers distant.4 Similar phenomena are found in the Klamath Mountains of California, in the Pennsylvanian strata of eastern Oklahoma, in early tertiary strata of Venezuela, in Mexico, in the tertiary strata of Switzerland5 and in cretaceous radiolarites in Arabia. 6
Some authorities, in an attempt to develop a noncatastrophic explanation for the presence of exotic blocks in mega breccias, have posited glacial transport. Others have concluded that the rocks slid to their present position from distant highlands. Such attempts generally have failed to satisfy those who have carefully investigated the circumstances. For ex ample, the "glacial" boulders are located in strata that otherwise represent a warm temperate climate; the rocks that are presumed to have slid to their present positions give no indications of having done so. I have found no recorded instance of a tailing disturbance such as would have been left in the wake of a rock moving across a soft surface. On the contrary, the only disturbed strata occur immediately below the clast, indicating compaction below the clast fol lowing its movement. Since continuous, rapid movement would be required to prevent the clasts from settling during transit, these clasts must have been transported by mechanism of mass flow. But no contemporary model for such a process exists. Not only is it difficult to conceive of a transport mechanism, it is also difficult to imagine forces that could have produced clasts of such size.
The presence of these megabreccias represents catastrophes of extraordinary dimensions, as substantiated by both the clast size and by the requirement for rapid movement along gently dipping or flat terrain for many kilometers. One authority,7 assessing the magnitude of the problem, has suggested as an explanation "major disturbances originating outside the planetary system" that may have affected the speed of revolution of the earth and the earth's revolution about the sun. All things considered, such a statement may not be too far from truth!
Slides or slump deposits
If a mass of sediment is deposited on a sloping surface or is uplifted unevenly so that a slope is formed, the sediment will tend to move downslope. This tendency is counteracted by internal friction that is much greater in cemented or compacted sediment. Once movement is initiated, by either external or internal forces, the sediment will move downslope more or less as a body, forming a slide or slump deposit. Unconsolidated (soft) sediments moving in this manner tend to form folds. If the moving mass contains sediments that differ in resistance to flow or internal shear, then the more resistant members will tend to fragment and form a megabreccia within a matrix of the less resistant members.
Slide deposits of immense dimensions with associated megabreccias are en countered in many parts of the world. The tertiary strata of the Apennines in Italy contain megaclasts ranging up to many cubic kilometers. These blocks have in some cases traveled up to 100 kilometers from their source area. One inverted slab of limestone covers an area of over 200 kilometers!8 Nearby in Greece are similar late tertiary sediments containing blocks ranging from several hundred meters to several kilometers in length, many of which are overturned. These sediments are believed to have traveled 100 to 500 kilometers from their sources to the point of deposition. 9 Numerous other examples of gravity-induced slides and slumps have been re ported.
A catastrophic interpretation for these deposits depends somewhat upon the time frame involved. If the movement of a mountainous clast over a distance of 100 kilometers occurs at the rate of a millimeter per year, it can hardly be considered a catastrophic event. If the clast moves the same distance in a matter of hours or days, it represents a catastrophe of earthshaking dimensions. How fast do slides move? The authors of most scientific studies either do not con front this question directly or merely assume very slow rates of movement.
The rate of slide movement depends in some degree upon the slope of the underlying surface. A number of authors have cited a figure of about 3 degrees for the slope over which slide deposits have traveled.10 This minimal figure is chosen because a lower slope probably would not support movement and a steeper slope would require a source area that was many kilometers distant to be unreasonably high.
Several reports of recent offshore slumps and slides are available for comparison with the tertiary deposits. In two examples the slides moved across slopes of approximately 3 degrees for several kilometers, and the movement is either known or inferred to have been catastrophic. 11 Because we cannot be certain that this was the case in the fossil examples, under similar circumstances it is difficult to conceive of such movement as having been slow.
The presence of various kinds of megabreccias in the geologic column, showing in some cases the transport of extremely large clasts, indicate energy levels on a scale that staggers our imagination. Their common occurrence in major portions of the geologic column of some localities indicates significant catastrophic activity in the past not readily explainable in terms of contemporary processes, but readily accountable within the events described in Genesis, chapters 6-8.
1 The Nature of the Stratigraphical Record (New York: John Wiley and Sons, 1973).
2 T. G. Morgan, "Lithostratigraphy and paleontology of the Red Hill area, Eureka County, Nevada," University of California, Riverside, unpublished M.A. thesis (1974); H. H. Wilson, "Late Cretaceous eugeosynclinal sedimentation, gravity tectonics, and ophiolite emplacement in Oman Mountains, southeast Arabia," American Association of Petroleum Geologists Bulletin 53:626-671 (1969); J. G. Jones, "Clastic rocks of Espiritu Santo
Island, New Hebrides," Geological Society of America Bulletin 78:1281-1288 (1967); R. H. Dott, Jr., "Dynamics of subaqueous gravity depositional processes," American Association of Petroleum Geologists Bulletin 47:104-128 (1963).
3 J. M. Dorreen, "Rubble bedding and graded bedding in Talara Formation of northwestern Peru," American Association of Petroleum Geologists Bulletin 35:1829-1849 (1951).
4 W. E. Hall, "Genesis of 'Raymond Boulder Beds,' Marathon Basin, West Texas," American Association of Petroleum Geologists Bulletin 41:1633-1641 (1957); P. B. King, "Problems of boulder beds of Raymond Formation, Marathon Basin, Texas," American Association of Petroleum Geologists Bulletin 42:1731-1735 (1958); E. F. McBride, "Characteristics of the Pennsylvania lower-middle Raymond delta-front sandstones,
Marathon Basin, West Texas: discussion," Geological Society of America Bulletin 86:264-266 (1975).
5 D. P. Cox and W. P. Pratt, "Submarine chertargellite slide-breccia of Paleozoic age in the southern Klamath Mountains, California," Geological Society of America Bulletin 84:1423-1438 (1973); E. E. L. Dixon, "The Ouachita Basin of Oklahoma vis-a-vis the Craven Lowlands of Yorkshire," The Geological Magazine 68:337-344 (1931); W. A. vander Gracht and J. M. van Waterschoot, "The pre-Carboniferous exotic boulders in the so-called 'Caney Shale' in the northwestern front of the Ouachita Mountains of Oklahoma," Journal of Geology 30:697-714 (1931); R. C. Moore, "The origin and age of the boulder-bearing Johns Valley shale in the Ouachita Mountains of Arkansas and Oklahoma, American Journal of Science 27:432-453 (1934); O. Renz, R. Lakeman, and E. van der Meulen, "Submarine sliding in western Venezuela," American Association of Petroleum Geologists Bulletin 39:2053-2067(1955); N. D. Newell, "Supposed Permian tillites in northern Mexico are submarine slide deposits," Geological Society of America Bulletin 68:1569-1576 (1957); E. C. Quereau, "On the cliffs and exotic blocks of north Switzerland," Journal of Geology 3:723-739 (1895).
6 H. H. Wilson, "Late Cretaceous eugeosyn clinal sedimentation, gravity tectonics, and ophiolite emplacement in Oman Mountains, southeast Arabia," American Association of Petroleum Geologists Bulletin 53:626-671 (1969).
8 J. C. Maxwell, "Review of: Geology of the northern Apennines," by Giovanni Merla; "Composite wedges in orogenesis," by Carlo I. Migliorini," American Association of Petroleum Geologists Bulletin 37:2196-2206 (1953); J. C. Maxwell, "Turbidite, tectonic and gravity transport, northern Apennine Mountains, Italy," American Association
of Petroleum Geologists Bulletin 43:2701-2719 (1959).
9 P. Elter and L. Trevisan, "Olistostromes in the tectonic evolution of the northern Apennines." In K. A. De Jong and R. Scholten, eds., Gravity and Tectonics (New York: John Wiley and Sons, 1973), pp. 175-188.
10 L. U. de Sitter, "Gravitational gliding tectonics: an essay in comparative structural geology," American Journal of Science 252:321-344 (1954).
11 W. R. Normark, "Ranger submarine slide, northern Sebastian Vizcaino Bay, Baja California, Mexico," Geological Society of America Bulletin 85:781-784 (1974); B. C. Heezen and C. L. Drake, "Grand Banks slump," American Association of Petroleum Geologists Bulletin 48:221-233 (1964); T. C. Moore, Jr., and T. J. H. Van Andel, W. H. Blow, and G. R. Heath, "Large submarine slide off north eastern continental margin of Brazil," American
Association of Petroleum Geologists Bulletin 54:125-128 (1970).