Strata of the Grand Canyon - Grand Stairacse
04/06/2002 11:17 PM
Using the well studied and comparatively accessible strata of the Colorado Plateau as a case study, we will evaluate the major predictions of the YEC 2-step creation/flood model of sedimentary rocks. In the first section, we will outline a description of the Precambrian and Paleozoic strata of the Colorado Plateau, especially that portion of it accessible in the Grand Canyon area.
Precambrian and Paleozoic Strata of the Grand Canyon
KF = Kaibab
Formation. TF = Toroweap Formation. CS
= Coconino Sandstone. HS = Hermit Shale.
Supai Group: ES = Esplanade Sandstone, Wes = Wescogame Formation, Man = Manakacha Formation, Wat = Watahomigi Formation. SCF = Surprise Canyon Formation. RF = Redwall Formation. TBF = Temple Butte Formation
Tonto Group: ML = Muav Limestone, BAS = Bright Angel Shale, TS = Tapeats Sandstone
VG = Vishnu Group. From Rockhounds.com, Overview of Grand Canyon Geology and Rock Formations
1. The Vishnu Complex
The oldest exposed rock group in the Colorado plateau, the Vishnu complex, is thought to have been formed by the accretion of volcanic arcs and/or microplates onto the western side of the North American craton (Babcock, p 11). The Vishnu consists of both volcanic and sedimentary rocks. Calc-silicate laminations are preserved in some weakly metamorphosed sections, and these have been interpreted as stromatolitic in origin. Babcock (p. 15-17) writes:
"Rocks of the Vishnu complex originated as sediments and volcanic rocks laid down on the sea floor . . . The sediments consisted mostly of quartz-rich sand, silt and clay, with lesser amounts of feldspar, mica, and iron oxide. Interspersed with the sediments were basaltic to adesitic laval flows and their feeder dikes, along with ash layers, which probably were associated with a chain of volcanic islands. It has been estimated that the total thickness of sediments and volcanics is greater than 40,000 ft (12,200m) (Maxson 1961). However, the most interesting rocks in the Vishnu complex are discontinuous layers or lenses of carbonate that make up less than one percent of the total section. . .
"After burial, the sediments and volcanics were subjected to at least two seperate episodes of regional metamorphism, The first event converted the sediments into slate or phyllites and the volcanics into greenstones and greenschists. because the metamorphism was low grade, many relict sedimentary and volcanic structures such as cross bedding or graded bedding can be found. . .
"In most places, evidence for this early metamorphism has been obliterated by a second, higher grade period of regional metamorphism. . . The metasediment formed during this metamorphism consist mostly of micaceous and quartzose schists or gneisses, while the metavolcanic rocks are amphibolites and horneblend schists"
Because the Vishnu complex has been subjected to at least two periods of metamorpic activity (dated at 1.72 and 1.68 billion years ago, respectively), precise dating of its original formation is problematic. However, neodymium isotope models presented by Bennett and DePaolo (1987) indicate a maximum age of about 1.8 - 2.0 billion years. More recently, Ilg et al. have reported an age of 1750 Ma and 1742 Ma for these metavolcanic schists (GSA Bulletin: Vol. 108, No. 9, pp. 11491166).
The Vishnu complex was intruded by large quantities of magma before the overlying Bass Limestone was deposited. These large bodies of magma cooled (relatively) slowly within the crust, and their granitic and granodioritic remnants are referred to as plutons. At least 20 plutons are known within the Vishnu. Some plutons are highly folded and foliated by metamorphic activity (Ruby Superunit), others less so (Phantom superunit), still others totally unfoliated (Surprise Canyon superunit). This suggests that plutonic bodies were being emplaced within the Vishnu before, during, and after metamorphic deformation occured there. Other intrusive igneous structures within the Vishnu include dikes and sills, which form the extinct magmatic plumbing which fed growing plutons. Interestingly, according to Babcock, some of these dikes "can be traced directly [from the plutonic bodies] into the magma chambers from which they originated" (p. 21). Again, most of these display little or no foliation, and seem to have been emplaced after the last stage of Vishnu metamorphism. Hawkins summarizes these relationships:
New field observations and U-Pb zircon and monazite ages are used to outline a geologic history of Paleoproterozoic rocks beneath the Colorado Plateau in the Grand Canyon of northern Arizona. The Upper Granite Gorge of the Grand Canyon exposes three lithotectonic units, including supracrustal rocks, mafic to intermediate-composition plutons, and peraluminous granite dikes, that are variably deformed and metamorphosed. New U-Pb ages indicate that layered supracrustal rocks were deposited or erupted over at least 8 m.y., between 1750 ± 2 and 1741 ± 1 Ma. Mafic to intermediate-composition plutons intruded the supracrustal rocks at 1741 ± 1 Ma (Zoroaster granite), 1737 ± 1 Ma (Grapevine Camp granite), 1730 ± 3 Ma (Trinity granodiorite), 1717 ± 1 Ma (Ruby granodiorite), and 1713 ± 2 Ma (Horn diorite). Both supracrustal and plutonic rocks are intruded by 16981662 Ma peraluminous granite and pegmatite dikes that locally form 50% of the rock volume. U-Pb ages of foliated plutonic rocks and crosscutting granite dikes bracket the timing of fabric development for three groups of fabrics. Group 1 fabrics formed between 1730 ± 3 and 1698 ± 1 Ma. Group 2 fabrics formed between 1713 ± 2 and 1685 ± 1 Ma. The timing of group 3 fabric formation is poorly constrained, although the 1662 Ma Phantom pluton contains shear bands that offset pegmatitic dikes that crosscut the pluton. Metamorphic grade varies across the transect from lower amphibolite facies to lower granulite facies. We suggest that single-crystal U-Pb ages (17061697) of metamorphic monazite, from a 1741 Ma supracrustal rock, directly date the timing of lower granulite-facies metamorphism in the eastern Upper Gorge. U-Pb ages (16851680 Ma) of granite dikes that postdate the development of leucosomal pegmatite pods in migmatitic supracrustal rocks in the eastern Upper Gorge are consistent with this suggestion.
Hawkins et al. U-Pb geochronologic constraints on the Paleoproterozoic crustal evolution of the Upper Granite Gorge, Grand Canyon, Arizona. GSA Bulletin: Vol. 108, No. 9, pp. 11671181.
2. The Grand Canyon Supergroup
The Grand Canyon Supergroup is about 12,000 ft thick (Baar, p. 8), and of Precambrian age. It was deposited atop the Vishnu, after which time the Vishnu was faulted and the fault bound sections were titled. The result is that the GCS was also faulted and broken into large tilted blocks. The basin-and-range surface of the these tilted blocks was eroded into a flat plain before overlying sediments were deposited, as indicated by their angularly unconformable boundary with the overlying Cambrian Tapeats Sandstone. Sears summarizes the reconstructed history of the Grand Canyon Supergroup, noting that it was deposited "during Middle and Late Proterozoic time upon the deeply eroded Lower Proterozoic Vishnu crystalline complex. It was folded, faulted, and beveled by erosion before deposition of the Middle Cambrian Tapeats Sandstone" (p. 72). In places, the GCS is seperated from the overlying Cambrian strata by a weathered zone or regolith up to 50ft thick (Sharp, 1940), indicating that the fault blocks were exposed to the atmosphere for a long time before the Cambrian transgression.
The Cardenas Lava at the top of the Unkar group is suitable for radiometric dating. McKee and Noble (1974) have published dates of 1.15 billion years for the sills and dikes associated (?) with the Cardenas flows, which agrees with ages of 1.1- 1.2 billion years published by Ford et al, and with paleomagnetic data presented by Elston and Scott (1976). More recent 40Ar/39Ar age determinations indicate that the Unkar Group was deposited between ca. 1.1 and 1.2 billion years ago (Timmons et al., Geological Society of America Bulletin: Vol. 113, No. 2, pp. 163181, 2000). A U-Pb zircon age of 742 ± 6 Ma has been reported for an ash layer at the top of the Chuar Group (Karlstrom et al., Geology: Vol. 28, No. 7, pp. 619622).
Life was abundant when the GCS was deposited, but it was all microscopic. Bacterial and eukaryotic fossils, including Melanocyrillium, Chuaria, and algal filaments, as well as bacterially constructed stromatolites, are found in several strata of the GCS, and are most abundant in the Chuar group. In fact, the Chuar group is rich in hydrocarbons and associated kerogen (P.K. Link et al., p. 527). Similar fossils are found in deposits of similar age around in the world. Melanocyrillium, for example, has been found in approximately 20 Late Proterozoic formations (ibid., p. 565), including correlative mid to late Proterozoic formations for instance in the Pahrump Group near the top of the Beck Springs Dolomite, and in shales of the Uinta Mountain Group ( p. 474). Elston notes that microbiota from the Chuar Group and the Red Pine Shale of the Uinta Mountain Group are identical to in form to middle and Late proterozoic (upper Riphean-lower Vendian) deposits in Sweden, Norway, Greenland, Svalbard, the Russian platform, and the southern Urals (p. 528).
2a. The Unkar Group
The Unkar Group consists of the Bass Limestone (up to 330ft), the Hakatai Shale (up to 985ft), the Shinumo Quartzite (1328ft), the Dox Formation (3100ft), and the Cardenas Lava (~900ft). In each formation, the evidence suggests a shallow marine to subarial depositional environment. Hendricks and Stevenson write:
"During the nearly 250 million-year time span postulated for Unkar deposition, the region was at (or very near) sea level. Only one unconformity is documented within the Unkar -- between the Hakatai Shale and the Shinumo Quartzite. Minor fluctutations of sea level or sediment surface elevation is recorded by features suggesting both subarial and marine deposition throughout the sequence . . . The only suggestion of relatively deep-water deposition is noted by textural features in dolomites and mudstones in the middle of the Bass Limestone in the western Grand Canyon" (p. 35).
The Unkar Group as a whole appears to record 4 cycles of west-to-east transgressions and regressions of the sea, with subarial deposits alternating with tidal flat and shallow marine sediment deposits. This is implied not only by sedimentary properties and bedform stuctures, but also by the corresponding cyclic changes in the colors of the deposits. Elston notes that:
"clastic and carbonate deposits of the Unkar Group that are considered to be marine are characteristically dark to light purple and greyish red. In contrast, red beds that are considered to have accumulated subarially from a variety of stratigraphic characteristics are red-brown to brick red and red orange. . . The red brown hematite is associated with dessication features, whereas purple hematite is associated with the stromatolitic (tidal flat) horizons. Diversely colored mud chips in dessication crack fillings indicate that the different hematite pigmentations formed during accumulation and immediate post-depositional lithification of the sediments, and that the various colors were not the result of diagenesis occuring long after deposition. The color of hematite pigment thus has aided in the discrimination of four depositional, marine to tidal flat and subarial, cycle in the Unkar Group" (in P.K. Link et al., p. 525).
The Bass limestone preseves stromatolitic beds, symmetrical ripple patterns, and even dessication cracks. The stromatolitic beds and ripple marks suggest a shallow marine environment, and the dessication cracks show that the Bass Limestone was exposed to the air for substantial periods of time. Hendricks and Stevenson again:
"The lithology and sedimentary structures observed in the Bass Limestone suggest deposition in an easterly transgressing sea. During the maximum incursion of the sea, carbonates and deep-water mudstones accumulated in the western Grand Canyon. In the east, stromatolites were forming, and shallow water mudstones were being desposited. Following this period of transgression, the sea slowly regressed. Evidence for this includes ripplie-marks, mudcracks, and deposits of oxidized shales in the upper part of the Bass [actually the lower Hakatai. ed.] -- all suggesting periods of subarial exposure" (p. 37).
The Hakatai Shale, which lies atop the Bass Limestone, also shows evidence of shallow marine deposition (cross bedding, ripple marks), grading upwards to subarial deposits which include mudcracks. Also present are stromatolites, pictures of which can be seen on this page: Stromatolite Fossils in the Hakatai Shale. Other periods of dessication are preserved in the middle member of the Shinumo Quartzite (cross beds, mudcracks, and clay galls), and the upper member of the Dox Formation (stromatolites, salt casts, and mudcracks in the Commanche and Ocha Point members). Above the Dox Formation lies the Cardenas Basalt (up to 900ft), most of which appears to have been extruded subarially.
Modern Pollen in the Proterozoic Hakatai Shale: Disproof of Plant Evolution?
In a 1966 CRS article, creationist C. L. Burdick claimed to have found modern pollen in the Hakatai Shale (Microflora of the Grand Canyon. Creation Research Society 1966 Annual 3(1):38-50). This finding was lauded by creationists as definitive "disproof" of plant evolution, and even today is presented as such on numerous creation "science" web sites.
Unfortunately for Burdick and others who have promoted this claim, the supposed precambrian pollen is apparently nothing more than surface contamination. In 1980, another creationist, Arthur Chadwick of Loma Linda University, published an article in the journal Origins summarizing the results of his attempts to confirm Burdick's claims. Precambrian Pollen in the Grand Canyon - A Reexamination. Origins 8(1):7-12 (1981). He concluded:
But what of Burdick's supposed precambrian pollen? Where did it come from? As Chadwick points out, the samples were taken from a surficially exposed portion of the Hakatai, and this immediately suggests contamination of modern pollen in Burdick's samples. This suggestion was amply confirmed by Chadwick, who notes:
In many cases, there are criteria for distinguishing between original pollen and pollen contaminates. These have not been adequately adressed. For instance, ancient pollen should be darkly colored, not clear or yellowish like fresh pollen. Another problem is that the Hakatai was "baked" by intrusive igneous sills sometime after deposition. Thus the pollen, if it was originally present, should be baked also, probably baked into unrecognizability. Chadwick again:
Which is the more likely scenario: a) the surface exposure of the Hakatai Shale in the Grand Canyon contains original pollen grains which just happen to match the pollen spectrum of the Grand Canyon area, while the overlying 10,000 ft of sediments contain no evidence at all of any metazoan life of any kind and the first indisputed pollen grains occur much higher still, or b) the supposed precambrian pollen grains are simply surficial contaminants which entered the exposed shale very recently, after the Hakatai bed was exposed to the air by erosion? Chadwick notes that "More difficulties are created than are solved by Burdick's report since it would require the explanation of the accumulation of all the Upper Precambrian sediments (10,000 ft.), their lithification and subsequent erosion before the first additional fossil forms were buried. Add to this picture the many thousands of macerations of lower Paleozoic and Precambrian rocks which have been carried out in scores of palynology laboratories around the world which have not supported Burdick's claims. There is a general absence of evidence for flowering plants below the middle Cretaceous. It is a responsibility and challenge to creationists to develop a model of earth history which explains this absence."
Addendum: Carl Froede has an article available online entitled Precambrian Plant Fossils and the Hakatai Shale Controversy, CRSQ 36:3:106-113, Dec. 1999. Froede does note that Chadwick "found no evidence of any plant fossils, and proposed that contamination was the cause of the earlier findings of pollen and spores," and that these findings are "very controversial." Froede notes that Steve Austin and Kurt Wise also reject the existence of plant fossils in Precambrian deposits.
However, after reading the article, twice, I was amazed to find that Froede does not even mention Chadwick's demonstration that the putative 'ancient' pollen assemblage just happens to match the present day pollen spectrum in the Grand Canyon region, even though that very paper is cited within the text! Nor does he mention the other palynological criteria, mentioned above, which indicate that these pollen grains were incorporated into the shale after they were diagenetically altered. Instead, Froede pretends that the "controversy surrounding Burdicks work centered around his finding both modern and ancient fossilized forms of plant spores and pollen in the Hakatai Shale." Amazing.
2b. The Nankoweap Formation
The Nankoweap Formation rests unconformably atop the Cardenas Lava. Apparently, there was a period of weathering and erosion of the Cardenas before deposition of the Nankoweap, since the lower member of the Nankoweap preserves accumulated piles of Cardenas detritus. Link et al., notes that this detrital zone, which is up to 10 meters thick, "records accumulation of iron-rich deposits derived from the subaerial weathering and alteration of basalt" (p. 526). Ford writes:
" . . . it seems that there was a considerable period of erosion after the eruption of the Cardenas Lava. As a result of this erosion, the lava was beveled by a North-east facing scarp that was perhaps as much as 650ft high. The lava itself was deeply weathered and affected by ferruginous alteration before the lower part of the scarp was buried by the lower member of the Nankoweap" (p 52).
This lower member is about 10m thick. The upper member of the Nankoweap is about 100m thick and consists of a cross bedded, redbed lithology. This member is characterized by shallow water to subarial depositional features (ripple marks, salt pseudomorphs, red hematite pigmentation, mudcracks).
2c. The Chuar Group
The Chuar Group consists of three formations and seven members, which include dolomites, limestones, and shales of varying thicknesses. Most formations within the Chuar are fine grained, laminated to thin bedded mudstone and dark gray shales. The total correlated thickness is approximately 5000ft. Some members preserve stromatolites, ripple marks, raindrop prints, mudcracks, and gypsum crystal casts, once again suggesting a predominantly shallow depositional environment subject to periodic dessication. Numerous microfossils of bacteria and acritarchs have been identified in thin sections from Chuar strata. Stromatolites bioherms are known from some members. For instance, the Awatubi Member of the Kwagunt Formation preserves a massive biohermal stromatolite layer 12ft thick (Ford, p. 59). These are assigned to the genus Baicalia. Also present are the stromatolite form-taxa Boxonia, Inzeria, and Stratifera. The Chuar Group also contains numerous occurences of the enigmatic Chuaria, which ranges in size from 70µ to about 5mm. Chuaria is considered to be a large acritarch by most authors.
3. The Sixtymile Formation
The Sixtymile formation is only about 200ft thick, and is composed of material derived from the Chuar Group. The lowest member (about 90ft thick) of the Sixtymile formation "is a landslide deposit that slid into the eastern side of the contemporarily growing Chuar syncline . . . Parts of the member are coarse breccias full of many different types of clasts- including chert and dolomite derived from the underlying Chuar Group. Among these are large derived blocks, one being a dolomitic limestone block 26 x 130 ft that matches the dolomite horizon in the Walcott Member of the Kwagunt -- some 230ft stratigraphically lower" (p. 62). According to Ford, these blocks and breccias "are derived from the Chuar Group in such as way that they demonstrate, first, that the Chuar syncline was developing during sedimentation and, second, that the Butte fault was active at the time and that the block to the east of it was being progressively uplifted" (p. 63).
Section II: Paleozoic Formations