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Sedimentary Rock

The great depths of sedimentary rock on earth are something that we cannot ignore. Surely these huge deposits must tell us something about the age of the earth. And indeed they do, but not necessarily what most people expect. We may have heard urban myths such as that cross bedded sandstone in the middle of the Grand Canyon demonstrates that there was a period of desert conditions producing massive sand dunes in the middle of deposits from Noah’s flood. In that case there obviously was no worldwide flood! And some of us may have heard that shales and claystone indicate that these sediments were deposited from tranquil seas over long periods of time. Again, the obvious conclusions are contrary to young earth geology. It is evident that a consideration of what the sedimentary rocks demonstrate, is very important.

One excellent resource on this topic is chapter 3, “Interpreting Strata of Grand Canyon” in Grand Canyon: Monument to Catastrophe edited by Steven A. Austin. 1994.  This book is available on Amazon and is well worth purchasing. Its chapters on sedimentary rocks and radiometric dating are extremely good. Although the reference is old, it is well documented and it is hard to argue with empirical observations. So, let’s take a look at some of the key points in this chapter.

The first point is that rock layers (strata) have historical significance. Each layer represents processes which operated during a past interval of time, and, therefore are worthy of historical study. It is evident however that the rock strata do not come with labels attached indicating their past history. Having collected information, we need to interpret it. Evolutionists tend to draw conclusions in keeping with their worldview, while creationists draw other conclusions. The differences in conclusions tend to boil down to a question of rates. Significantly however, Dr. Austin points out that some recent studies are indicative of rates of sedimentation which are in keeping with the expectations of flood geology.

Lime Muds: many modern examples of these deposits are recognized to be accumulating very slowly and only in quiet water. The obvious conclusion is that deep deposits of such sediments must be very old. However modern examples of rapid deposition of lime muds are now known from high energy situations such as hurricanes in the Bahamas. How can this be? Apparently, the mud particles aggregate into pelletoids which exhibit the hydraulic characteristics of sand which then settles out quickly. Thus Dr. Austin remarks: “The evidence of rapid accumulation of lime mud in modern sedimentary environments helps to interpret many ancient limestone deposits. Probably most significant are the so-called “lithographic limestones” with extremely fine texture, but with extraordinary fossil preservation. {the most famous of which is Archaeopteryx] ….. Clearly, catastrophic processes are needed to make these fine-grained limestones.” [book p. 26]

Dr. Austin also describes a situation in Grand Canyon where 18 inch (45 cm) shelled nautiloid fossils have been found buried in very fine-grained limestones. Dr. Austin ponders: “How did so many of these free-swimming deep-sea animals become fossilized in one location? How did the fine-grained lime mud accumulate, as it buried these animals?” [p. 27] [Nautiloids were like octopus but living in a shell like the extant Nautilus.]

There are many cross beds in Grand Canyon, some from lime mud (acting like sand) and more from sandstone itself. These rock forms have popularly been imagined to result from desert conditions and wind. Observations from nature and laboratory studies using large flumes, however have demonstrated that the cross bedding comes from high velocity water currents moving in very deep water. The first currents drop some of their loads of sediment. Subsequent dunes chop off the top half of the previous dune and deposit a new one on top. This is how the cross bed pattern is developed. From laboratory studies and computer models, graphs of conditions which generate such sand waves have been developed. Some of the cross beds observed in Grand Canyon’s Supai and Kaibab sandstone could have come from sand waves 18 m (60 ft) high. Reading from the model, these sand waves came from water 54 m (180 ft) deep and moving at velocity of 90-155 centimetres per second (3-5 ft per second). There are some localized places in the world today where such high velocity water currents have been observed. As far as a large region is concerned, Dr. Austin suggests that “a tsunami provides the best modern analogy for understanding how large-scale Grand Canyon cross beds form.” [p. 35]

Another feature of sedimentary rocks is shrinkage cracks, which again suggest drying conditions. But as Dr. Austin points out, a large body of geologic literature now indicates that shrinkage cracks can form in a clay-rich environment under water. The compressed clay can lose volume and cracks appear. [p. 41]. Dr. Austin continues through the list of issues coming from sedimentary deposition such as rapid deposition, flat deposition planes, and unconformities.

Another very important issue in this chapter is the Great Unconformity underlying the Cambrian rocks. Many evolutionists have claimed that this surface came from gentle erosion over long periods of time. Creationists declare that it happened under water as a result of catastrophically moving water currents. Many doubt that such under-water erosion could occur. However, geologist J. Harlan Bretz demonstrated, after 40 years of hostile opposition from the geological community, that deep gullies were eroded in bedrock in Washington state as a result of rushing torrents 400 feet deep involving 500 cubic miles of water. Thus declares Dr. Austin, “The best theory seems to be that significant erosion occurred on the Great Unconformity while it was under water, not while it was elevated out of the sea. Creationist ideas seem to be in line with the facts.” [p. 47]

Dr. Andrew Snelling reinforces Dr. Austin’s interpretation of the origin of the Great Unconformity with extensive additional research from the Tapeats Sandstone in Grand Canyon. He declares that the whole event was under water (subaqueous) and very fast. Thus he declares concerning all the evidence he considered: “when combined, the mineralogical content, textural features, sedimentary structures, the continental scale deposition, paleocurrent directions identical to the same continental pattern, and even the tracks and traces of transitory invertebrates that had to be buried and fossilized rapidly, are all consistent with the catastrophic erosion of the Great Unconformity near the onset of the global Genesis Flood cataclysm about 4350 years ago and the tsunami-driven rapid short-distance transport and deposition of the Tapeats Sandstone at the base of the fining upwards Sauk megasequence in the first few days or weeks of that year-long event.” [ Andrew Snelling. 2021. The Petrology of the Tapeats Sandstone, Tonto Group, Grand Canyon, Arizona. Answers Research Journal 14: 159-254. See p. 245. One has to read the whole article to appreciate the detailed research involved.]

Some of Dr. Snelling’s evidences for fast underwater events connected to the erosion of the Great Unconformity and deposition of Tapeats sediments above it include: “the thin, non-bioturbated, strongly cross-laminated beds, and eroded ‘channels’ filled with cross-laminated sand, both including likely hummocky cross-stratification, are consistent with rapid sequential deposition by high-energy storm surges.”[p. 246]  Also the existence of detrital mica in the deposits testifies to these sediments having been deposited under water. It is apparently well known in geological circles that “mica flakes are rare in modern eolian [wind] deposits and commonly present in subaqueously [underwater] deposited sands.” [p. 235]

The important criterion one must apply when interpreting the age of sedimentary rocks is how fast they were deposited. An important article on sedimentation rates was published by three scientists from University of Alberta. Concerning the rocks (Scollard Formation) in which the last of the dinosaurs were entombed in Alberta, they declared: “Considering the rate of sedimentation of the Scollard Formation to be representative of the Edmonton Group (about 65 m Myr [65 metres per million years]), one average metre represents about 15,000 yr.” [J. F. Lerbekmo, M.E. Evans and H. Baadsgaard. 1979. Magnetostratigraphy, biostratigraphy and geochronology of Cretaceous-Tertiary boundary sediments, Red Deer Valley.  Nature 279 May 3 pp. 26-30. See p. 29]. Their conclusions that it took about 15,000 years to accumulate 1 metre of sediment, however do not make sense. In these rocks we find the very large skeletons of T. rex and Triceratops suddenly and permanently buried. At their quoted sedimentation rate of 15,000 years per metre, it might take a minimum of 6 x 15,000 or 90,000 years to bury such a skeleton. The bones would not sit around that long waiting to be buried.

Another set of observations demonstrated how rapid and devastating underwater avalanches can be.  On November 18, 1929, a severe earthquake occurred at sea on the Grand Banks south of Newfoundland. This was in a crossroad region of submarine communications cables. Six cables within 95 km broke right away. Another one broke 13 minutes later, followed by five other breaks, one after the other over a period of 13 hours. Each break was downslope to the previous one and farther away. The last break was in the deep ocean basin 480 km away. Each break occurred in at least two widely separated places. Twenty years later some scientists studied what had happened. This turbidity current dumped about 100 cubic kilometres of sediment on the sea floor. Approximately 100,000 square kilometres of sea floor, extending 800 km south from the earthquake site, were buried under a depth of about one metre of sediment. [Bruce C. Heezen and Maurice Ewing. 1952. Turbidity Currents and Submarine Slumps and the 1929 Grand Banks Earthquake. Amer. J. of Science 250 December pp. 849-878] Since then a number of other similar situations have been observed. The speed of deposition of these underwater catastrophes is readily apparent. Applied to the flood, we can see plenty of potential for devastation.

Rock Strata, Fossils and the Flood (DVD, 47 min). Sedimentary rocks from all over the world are discussed under the theme of flood geology.

Evolution’s Achilles’ Heels (DVD, 96 min). Dr. Marcus Ross discusses flood geology and other scientists discuss other topics such as radiometric dating and origin of life.

Geology Tackles Great Anomaly (Great Unconformity)

The Coconino Sandstone: Desert or Flood? (YouTube, 7 min)

Related Terms

  • Cambrian Explosion
  • Burgess Shale