PARADOX IN STRATIGRAPHY
PARADOX IN STRATIGRAPHY:
WHERE ARE THE ANCIENT ROCKY LANDSCAPES AND SEASHORES?
When researching ancient rocky landscapes, we face a serious paradox. Ancient rocky seashores and true rocky landscapes are missing from the Paleozoic and Mesozoic eras. The ancient erosions surfaces are at odds with what we see today. They are mostly flat and lacking the normal erosion and alteration as we see in the present Earth. To tease your curiosity, we invited our readers to have a first look at these odd surfaces recorded along what we call the unconformities. An unconformity marks the limit between a younger sedimentary rock laid above an older eroded rock.
Geologists with notions of geomorphology (the study of landforms) are baffled by the flatness of these rocky surfaces. These ancient unconformities should be adorned with
typical features of differential erosion whatever the climate conditions:
Ribs shapes reliefs
Etching effect on folded rocks
Ribs shape accentuated by boring marine organisms in shallow water.
Each stratum of layered rocks has its own resistance to alteration, waves abrasion, frost, and bioerosion. This effect, named differential erosion, affects all types of rock in any climatic conditions on earth today. Moreover, erosion of low reliefs would never evenly level rock strata. Even powerful waves abrasion is unable to sweep away the effect of differential erosion. Storm waves and torrents abrasion might blunt small reliefs, but differential erosion will always amplify its effect.
The richness and the beauty of our landscapes today are the results of differential erosion on a wide variety of strata. Nature is like an artist that needs time to sculpt his masterpiece made of forms and colors. Rock surfaces exposed to an “aging” process give the best result.
The Paleozoic and Mesozoic unconformities are rather flat and dull with no traces of aging, depleted of differential erosion features. Instead, nature was generating super razing/bulldozing events, unlike anything we see today. In the end, we would have to doubt seriously about the uniformitarian doctrine.
In Ice or Water? 1905, Sir Henry Howorth raised the question by the following statement:
"The absence of the erosive agency of water, as manifested in cutting valleys and gorges in the under strata of the earth, is fatal to the theory that each formation has successively emerged from the sea and become the surface of the habitable world . . . What we want to see is a plain instance of valleys excavated and mountains formed in the ancient strata of the earth as we find them existing in the present day . . . "
Surprisingly, when geologists are exposed to this controversy, they admit the reality of a paradox. Unfortunately, this debate was ignored from the arena of earth science
At this point our readers will learn some principles of earth science.
Principles in geomorphology and stratigraphy
The following slideshow presents how a continent should be eroded accordingly to the processes observed today. At any moments of this long process, our nature-artist would always produce differential erosion masterpiece on any rock surfaces.
To illustrate this concept, we use 6 artistic licenses:
The vertical scale is exaggerated.
The continent is a cake built of ancient horizontal rock strata.
The sea level is fixe with no fluctuation.
No continent rebound from the unloading erosion.
No sinking of the continental margin under the overload of sediment.
Orogeny or epeirogeny are omitted for this example.
Through time, erosion warns down the relief into detritus which are transported and laid down as sediment deposit at the margin. It is well established that the continent would be flatted down if no uplifting tectonics force comes into play.
Some geomorphologists might insist that a total flattening would transform a continent into what we call a peneplain. As we will see this is just a desperate hypothesis that does not fit with the nature of our continental crust. Meanwhile, the peneplanation process was prized by some stratigraphers to explain the odd aspect of the ancient unconformities. In fact, this concept is just an escaping door to ignore the paradox concerned by this webpage.
For some, peneplain occurs when a continent is warned out to his base level (≈ sea level) then erosion stops. At any moment in time up to the end, the continent surface will bear the stairsteps aspect produced by differential erosion.
Today, flat lands are fluvial plains which are formed by sediment deposition, not by bedrock erosion. The only eroded flat bedrock on earth is the Hudson Bay. Even there, the ice age abrasion left reliefs adorned with differential erosion. So, in fact, we find no modern examples of the “so-called” peneplains.
The next slideshow presents a cross-section at one stage of this process. Erosion of the continent was omitted. The new sediment deposits cover a rocky seashore which was sculpted with differential erosion features. This sort of landscape burial occurs often during a sea rise, named a transgression. This surface will become a future unconformity.
In the above example, we insist on a sort of stairsteps seashore. This is not just due to the stratified aspect of the rocks. Because of the hydrodynamics of waves, even a very gradual rise of the sea level will always produce a series of ramps or terraces. This is well known from coastal geomorphologists and geographers. In the following example, you see a theoretical continent made of a homogeneous rock, let say a massive granite. With the freedom of an artistic license:
Continental fluvial erosion was omitted
Sediment deposits were omitted except the beach deposits.
The last slide shows the relief without sea water.
Notice the fractal aspect of the shoreline carved with caps and bays.
Following the Ice Age melt, a worldwide sea transgression had drowned innumerable “stairsteps” seashores. Buried under new sediments, these submarine rocky seashores become modern unconformities. All geologists should be astounded by the absence of stairsteps topography along Paleozoic and Mesozoic unconformities.
Rock erosion does not occur under the sea. The chemical alteration has almost no effect on deeply drowning rock surface. The hydrothermal alteration would be efficient around the volcanic areas like the ocean ridge or the guyots. Bioerosion is efficient mainly above 200 m of depth where light sustain an ecosystem. To resume, landscapes are shaped when a regressing sea (a regression) exposes the rocky surface. The usual sequence is:
During a regression, atmospheric agents produce a rocky landscape.
During a transgression, the sea drowns and preserve the rocky landscape.
New marine sediments deposits seal the rocky landscape as a future unconformity.
The erosion of a new regression will expose the unconformity in the present time.
Keep in mind that the sea might transgress over 3 types of surface:
A hard-rocky surface which will produce a true unconformity.
A poorly lithified sedimentary rock like the one of badlands topography which will produce paraconformity or blended unconformity.
Many, not all, Paleozoic and Mesozoic unconformities are clearly of the first type.
The following example illustrates a sea regression exposing 2 types of rock followed by a transgression. The underlayers are a well-lithified limestone. The layers above are poorly lithified sediment typical of many badlands. Differential erosion is less defined. The topography is round and scourged by ravines and gullies.
Around the world, badlands are mostly carved in sedimentary rocks deposited at the end of the Mesozoic era (Cretaceous) and during the Cenozoic era. Remember that this is not an exclusive rule for that many well lithified sedimentary rocks are of the same ages. The degree of lithification depends on the chemistry of the sediments and their burial pressure.
The following slideshow helps to visualize how a transgressing shoreline would carve and dissolve a badland like Bryce Canyon. It is likely that the resulting unconformity would be a sort of paraconformity or a blended unconformity.
Since the work of Laurence Sloss on sequential stratigraphy, we know that our continents were deeply eroded many times during the Phanerozoic Eon (Paleozoic, Mesozoic, and Cenozoic eras).
Geologists are aware that the stratal expression and time of earth history had far more gaps than the fossils and rock records:
90% in unconformities: gaps of the vanished records.
10% in precious fossils and sedimentary records.
The erosion is so deep that the Paleozoic and Mesozoic seas were transgressing mostly over hard-rocky surfaces. Because of this, we expect that most of these unconformities should be sculpted with stairsteps series of terraces capped with shingle beaches. Rocky shores should be more predominant than terrestrial rocky landscapes.
The next slideshow presents the reality of our present nature AND NOT AN EXPRESSION OF THE ENIGMATIC PAST.
Uniformitarianism faces a serious appraisal:
Rocky seashores are inexistent.
Shingle beaches are inexistent.
True rocky landscapes are none.
We must admit that our present nature is mocking our uniformitarian paradigms.
Many earth scientists are unaware that the actual uniformitarian tendency is a misleading path. Whoever enters this debate, he better be a well-known champion with decades of publications with a long emeritus professorate.
Derek Ager had such a solid authority when he published The nature of the stratigraphical record in 1973 (first edition). All stratigraphy geologists remember how his book was a challenge to uniformitarianism. Following his 3rd editions, antagonists were still unable to counterattack his view and, Ager decided to publish his new book with a more provocative title: The New Catastrophism — The importance of the rare event in geological history, 1993.
His publications had surely contributed to the recognition of great mass extinctions by the new generation of paleontologists. Curiously and understandably, Ager never treated the topics of the missing paleo landscapes, a discipline for geomorphologists. Meanwhile, he reminds his reader of an unsolved mystery related to ancient unconformity when he cited the work of Markes Johnson: Why Are Ancient Rocky Shores so Uncommon? 1988.
We might ask why geomorphologists are not so involved in studying the rocky surfaces along the ancient unconformities? The answer is simple: These surfaces are simply not rocky landscapes. This study is out of the competence of geomorphologists. There is a need for a new discipline; “cataclysmic erosiology”. Unfortunately for erosiologists, and fortunately for the inhabitants of our planets, there are not too many modern catastrophic erosional events to study.
The Geodoxa’s pages on the Ice Age outbursts are very relevant to the lack of interests in the topic. This is pure erosiology. While hydrodynamic engineers and physicists are very supportive of the subglacial water theory, many Ice Age geoscientists keep a bias attitude.
Coming next... Under construction
The shortcoming of karstic caves
No rocky landscapes with dinosaurs
The odd aspect of the Siccar Point unconformity
Carboniferous coal deposits:
So many transgressions and regressions
So many eroded channels in soft banks
No differentially eroded rock surface!
Many unconscious ad hoc