SEDIMENTOLOGICAL INTERPRETATION
OF THE TONTO GROUP STRATIGRAPHY
(GRAND CANYON COLORADO RIVER)
G. Berthault
The article was published in the journal of the Russian Academy of Science, Lithology and Mineral Resources, vol.39. No. 5, 2004. Used by Permission.
Abstract – Sedimentological analysis and reconstruction of sedimentation conditions of the Tonto Group (Grand Canyon of Colorado River) reveals that deposits of different stratigraphic sub-divisions were formed simultaneously in different litho-dynamical zones of the Cambrian paleobasin. Thus, the stratigraphic divisions of the geological column founded on the principles of Steno do not correspond to the reality of sedimentary genesis.
INTRODUCTION
In my previous article (Berthault, 2002), I demonstrated on the basis of experiments in sedimentation of heterogeneous sand mixtures in flow conditions that the three principles of superposition, continuity and original horizontality of strata affirmed by N. Steno should be re-considered and supplemented.
Because Steno assumed from observation of stratified rocks that superposed strata of sedimentary rocks were successive layers of sediment, a stratigraphic scale was devised as a means of providing a relative chronology of the Earth’s crust. It was not constructed strata by strata because of the impossibility of following the same strata around the earth. It was built at a higher level of stratified layer called a “stage”. According to the classical definition: A stage is a unit defined from a “reference cut” (stratotype) characterised by a group of paleontological, lithological or structural criteria of universal value (Aubouin, 1967, p. 229). It corresponds to an “age”, e.g. the black marls of Oxford define the “Oxfordian” stage, which in relative chronology corresponds to “Oxfordian” age. In theory, the formations which have the same “stratotype” all around the earth have the same age. This results in stratigraphic correlations between them.
The reality is not so simple because changes in lithological facies are discovered when a layer is followed. This is why, to establish their correlations, geologists refer to marine index fossils over large geographical areas by applying the principle of paleontological identity based upon the affirmation that an ensemble of strata having the same paleontological identity has the same age (Aubouin, 1967).
Locality-types are situated principally in the Anglo-Parisian basin where the stratigraphic scale started to be constructed. It can be verified that the layers, correlated respectively with the stratotypes, are superposed in the same vertical, and thus a classification in time of the stratotypes can be made by application of the principle of superposition. For example, the Oxfordian precedes the Kimmeridgian.
Geologists have recognised the existence of marine transgressions and regressions in sedimentary basins. They are characterised by discordances between two superposed formations (change in orientation of stratification and an erosion surface). Inasmuch as “stages” and “series” are defined mostly by paleontological composition of the strata (Geological …, 1960), stratigraphic units do not take into account lithological features and discordances of the sequences. Since stage (age) is the primary element for construction of the stratigraphic units of higher rank – series (epoch), system (period), erathem (era), these stratigraphic subdivisions also do not take into consideration sedimentological processes.
This abridged summary of the stratigraphic scale, is a necessary prologue to substantiate or justify a new approach of interpreting the stratigraphic column by sedimentology.
STRATIGRAPHY OF SEDIMENTARY STRATA OF GRAND CANYON
To illustrate the difference between sedimentological (particularly our experiments) (Julien et al, 1993) and stratigraphical interpretation, I would refer to the sedimentary sequence of the Grand Canyon (Colorado River) and especially the Tonto Group (Fig. 1) as described in (Grand Canyon, 1989).
The Precambrian basement of the sequence consists of a complex group of highly metamorphic and intensively folded rock (Vishnu Group), especially chlorite-mica schist, with minor amounts of amphibolite, gneiss and calc-silicate rocks. Zoroaster pink feldspars granite occurs with intrusive contact with Vishnu rocks as vertical dikes and pegmatic veins up to a few tens of meters thick. Both Vishnu and Zoroaster rocks are assigned to the Lower Proterozoic. In some tectonic depressions these rocks are overlain by tilted Upper Proterozoic volcanic-sedimentary Grand Canyon Supergroup. The Tonto Group assigned to the Cambrian System directly overlies Vishnu and Zoroaster rocks (Grand Canyon, 1989).
Three Formations have been recognized in the Tonto Group. From the bottom to the top these Formations are: Tapeats Sandstone, Bright Angel Shale, and Muav Limestone.
Tapeats Sandstone is the lowest horizontal formation of enormous lateral extent in the Grand Canyon. It is medium- to coarse-grained, quartz-rich sandstone, with a thickness usually between 40 and 100 meters. The base of the formation is often dominated by pebbles and boulders. The central portion of the formation is dominated by coarse-grained sandstone having cross beds with westward and southwestward dips (indicating water current flowing westward). The top of the formation is dominated by plane beds of sand, with ripples, and by thinner, fine-grained sand and silt beds which form a gradational contact with the overlying Bright Angel Shale.
Bright Angel Shale—Here, greenish-gray, silty-to-sandy shale is 100 to 120 meters thick. Prominent beds of sandy dolomite and silty limestone are very persistent within the shale, throughout the Canyon. Green sandstones, containing glauconite and dark-brown ironstone are also common. Bright Angel Shale represents deeper water and slower currents than does the Tapeats Sandstone. The top of the Bright Angel Shale intertongues with Muav Limestone.
Muav Limestone—Yellowish-brown, impure, silty and sandy limestone is from 100 to 300 meters thick. Small, irregular inclusions of clay within Muav occur above the Bright Angel Shale. Limestone thickness and purity increases toward the west.
Because the Tapeats, Bright Angel, and Muav are not separated by unconformities but grade into each other, they have been collectively called the Tonto Group. The deposits are overlain by the sequence of sandstones, siltstones, shales and carbonate rocks of Devonian, Carboniferous and Permian Systems.
SEDIMENTOLOGICAL FEATURES OF THE TONTO GROUP
Fig. 2. A model for the formation of sedimentary deposits during Cambrian transgression in Nevada, Arizona, and New Mexico.
Zones: (1) upper part of continental slope; (2) the adjacent shallow-water area; (3) submarine sand waves; (4) plane beds of sand with ripples; (5) silicate clay- and silt-size particles with graded beds; (6) lime mud of deepest zone.
The Tonto Group resulted from a large erosive transgression. The erosion appears greatest in the western and central Grand Canyon, where Tapeats Sandstone most often rests directly on schist and granite (Vishnu and Zoroaster). The depth of erosion is least on the eastern Grand Canyon where Tapeats rests directly on tilted strata of the Grand Canyon Supergroup. A diagram of the Tonto Group formation is shown on Fig. 2.
To understand how these formations (Tapeats, Bright Angel and Muav) superposed each other and juxtaposed as shown in the diagram, one must start with the powerful current which eroded the granites and schistes of the Vishnu Group and Zoroaster granites. An erosion produced clasts of all sizes, particles of quartz and clay, pebbles and boulders. Lebedev (1959) indicated for each size of clast, the velocity of incipient motion relative to the depth of water. For pebbles it was 2 to 3 m/s and boulders more than 6 m/s. The velocity of the frontal mass of water was initially above 6 m/s sufficient for transporting the boulders as far as zone 2 (see Fig.2) and thus it was more than 1.5 m/s.
As the transgression advanced, the depth of the water increased causing the current to reduce. The ensuing reduced current, nevertheless retained an erosive capacity sufficient to cause clasts smaller than boulders, such as pebbles, to be transported as far as zone 2 where velocity of the current was about 1.5 m/s.
In shallow zone 1 the erosive current diminished in velocity and transported clasts of terrigeneous (gravel, sand, silt and clay) and carbonate material (particularly lime). A regressive current started, which carried westwards the largest particles in a bed load, and the smallest in a suspended load. The first deposited from zone 2 to 6, and the second in zones 5 and 6 only.
Zone 3 is composed of sand waves forming thinly cross-bedded sands, which compose the middle of the Tapeats. Here the water velocity was about 1.0 meter per second. Westward and southwestward dips of cross-beds indicate predominant direction of the current.
Zone 4 represents the deepest and lowest-velocity waters depositing the uppermost Tapeats.
Zone 5 is located in still deeper and slower-moving waters. The silicate clay- and silt-size particles were accumulating as graded silt and clay beds of Bright Angel shales. Here, the water velocity was about 0.5 meter per second.
Zone 6 is farthest to the west, in the deepest and slowest-moving water, where there was a deficiency of silicate clay and silt-sized particles. Lime mud was accumulating as rhythmically laminated and bedded flat strata, where the water current velocity was less than 0.5 meter per second.
At the level of the bottom of the paleobasin, particles become finer from east to west. The thickness of the deposit shows a similar pattern with particles becoming finer from the bottom of the deposit to its surface. This can be explained as follows. The deposit of a particle occurred when the current velocity transporting it became less than the critical velocity which caused the particle to fall and deposit. This critical velocity is near to “incipient motion”, which usually refers to the threshold conditions between erosion and deposit of a single particle (Julien, 1995). The incipient motion increased with the size of the particle. Consequently, the diminution in the size of particles from east to west, and from the base to the surface of the deposit, resulted from a decrease of the current velocity during the time of the deposit. This diminution of velocity could result from the water withdrawing from zones 1 and 2 to the deeper water of zone 3 where the current was slowing.
Consider a particle A which deposited at time t0 on the Zoroaster and Vishnu base in Zone 6. At the following time t1, a finer particle B deposited on the level of the bottom of the paleobasin to the west of A in the direction of the current. At the same time t1, another particle C, finer than A, deposited on top of A. An ensemble of particles, amongst which B and C, deposited simultaneously at t1 (Fig. 3).
Fig. 3. Scheme of deposition of heterogranular deposits in current conditions.
The sedimentary genesis of the Tonto Group is explained by simultaneous prograding of the strata both laterally and vertically and movement of the sedimentation area from east to west. This explanation does not corresponds to the stratigraphic concept of successive horizontal layers. It may be asked whether this simultaneity of deposit in Tapeats, might extend to Bright Angel and Muav. In other words could a particle of clay (Bright Angel) deposit in zone 5 at the same time as a particle of sand in zone 4? Without going into the calculations, an answer can be obtained by examining the data.
In the first place, the velocity of the particles of silt and clay transported by the current was greater than that of the particles of sand transported at a lower level.
Secondly the velocity of the particles decreased from the maximum permissible non-erosive velocity (Fortier, Scobey, 1926), to the incipient motion (Shields, 1936) modified by Yalin and Karahan (1979), corresponding to the threshold conditions between erosion and sedimentation. Consequently, considering two particles, one of silt, the other of sand, travelling over the same distance, but with a certain delay of time of departure between them, the silt particle would deposit by flocculation in Bright Angel at the same time as the sand particle in Tapeats.
It should be added that clay was the first to fall on the subjacent Tapeats in zone 6 where the current was the weakest and where, therefore, its velocity dropped first below the incipient motion of clay. It then fell in zone 5, for the same reason that the velocity of current diminished to the point where it was below the incipient motion.
Bright Angel, therefore, progressively covers Tapeats from west to east in zones 5 and 6. At the same time, in zones of higher hydrodynamic (zones 2, 3 and 4) deposit of Tapeats sandstone continued. Thus, deposition of the Tonto Group occurred not successively as expressed by the principle of superposition. The same reasoning applies to Bright Angel and Muav in zone 6.
This analysis is in conformity with the results of our laboratory experiments in the University of Colorado (Julien et al., 1993).
STRATIGRAPHIC INTERPRETATION
Let us now see how stratigraphy interprets the Tonto Group. According to the principle of continuity each layer is of the same age at every point. Sedimentation is held to be vertical and the velocity of sedimentation uniform and very low (from 8 to 17 mm per thousand years) to justify the total time of deposit, i.e. 30 million years, corresponding to the epoch or series of theLower to Middle Cambrian" Paleozoic… 2003).
In the stratigraphic scale, the Cambrian is composed of three series (Lower, Middle and Upper) total duration over 50 million years. Whereas, the Tonto Group which corresponds to a transgression, includes three facies which deposit simultaneously, so they finally appear superposed and juxtaposed. Actual deposition was much more rapid than the calculated stratigraphic velocity of deposition. The stratigraphic scale at the level of the Cambrian System of Grand Canyon of Colorado, therefore, does not take into account the reality of sedimentary genesis either in time or extent.
Discordances exist between certain sequences superposed on top of the Tonto Group. Our flume experiments showed that an increase in current velocity caused partial erosion of the deposit, and a diminution of the velocity following the increase caused a deposit of sediment on the surface of the erosion, without discontinuity of sedimentation. This is the scour and fill effect (Julien et al., 1993). Similar erosion surfaces are observed in the sedimentary cover of the Russian platform (Ignatiev, 1971). Also, a variation in velocity can change the orientation of the strata. It is necessary, therefore, to take into account that an apparent interruption of sedimentation can be the result of variation of hydraulic conditions of sedimentation without chronological hiatus.
The above reasoning for the Tonto Group applies equally to all the series and systems and, consequently, to all parts of the stratigraphic scale. The scale has not taken into account the reality of the genesis of sedimentary rock concerning either time or extent. In time because the transressive/regressive sequences start and finish by powerful erosive currents, which rapidly transport and deposit enormous masses of sediment. The time, therefore, of sedimentary formation is much less than the geological time correlated with the scale.
The actual time is evaluated by paleohydraulic analysis. In some cases actual time of sedimentation is only 0.0001 % (Romanovsky, 1988) or 0.01–0.001 % (Meien, 1989) of the stratigraphic time for formation of the sediments. At the same time, a significant part of the time responsible for formation of the sequence belongs to latent gaps of sedimentation (diastremes) (Romanovsky, 1988).
In extent because the sequences are composed, like those of the Tonto Group, of formations superposed and juxtaposed which form partly simultaneously and not successively as shown in the scale. Moreover, if two superposed sequences are considered showing discordance between them, it is not a hiatus except for the Grand Unconformity separating the Tonto Group from the subjacent base. Otherwise such discordance could characterise a variation of flow velocity during sedimentation.
As regards the Grand Canyon the stratigraphic position of the superposed formations should be interpreted by taking into account transgressive and regressive currents as the principal agent of their formation. In consequence, the scale must be considered in the light of observation and experimentation, i.e. an analysis of the paleohydraulic conditions which determined the deposition of sediment, and the creation of sedimentary rocks by diagenesis.
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