BASIC CONCEPTS

USED

IN

HISTORICAL GEOLOGY

CHAPTER SUMMARY

1. 1.  
   2. The rock record looses information about the complete process-response models.

V. CONCEPT OF GEOLOGICAL MODELS

All of the key ideas underlying geological thought are connected with the interaction between time and change. In essence, in the vastness of geological time everything changes and nothing is permanent. This dynamic system, the universe we exist in, is constantly changing. The rate of change may be different for different things in the universe but the basic concept that the whole system is changing with time holds true, even for the sub-atomic particles.

If we take a simple elementary Physical Geology phenomena such as the weathering process, we see that weathering is merely the attempt by a rock formed at a specific temperature and pressure to adapt to atmospheric pressures and temperatures.

Photo-series: WEATHERING OF LAVA FLOWS OF DIFFERENT AGES.

Similarly, in Neontology [that branch of biology which studies living as opposed to Paleontology which is concerned with fossil remains] dynamic changes are observed in populations of organisms.

Photo-series: THE BRITISH PEPPER MOTH CHANGING DUE TO VARYING ENVIRONMENTAL SELECTION PRESSURE.

Photo-series: A LOUISIANA CYPRESS SWAMP BEFORE AND AFTER CARELESS DRILLING.

Many of these changes observed on earth are slow, such as a river eroding a landscape; a raindrop dissolving away a soluble rock particle; the development of an open oceanic basin. Some changes take only a few thousand years, such as the silting-up of a lake, or the switching of the Mississippi River Delta. Some changes are catastrophic, such as a volcanic eruption, an earthquake, or a hurricane. Nevertheless, all of the changes are manifestations of the fundamental idea that everything is unstable with time and actually provided the proof of the dynamic nature of the universe. Associated with this realization historical geologists developed certain BASIC CONCEPTS to help them understand the past history of the earth. These basic concepts are associated with the thought that all natural systems are metastable. As soon as a stable condition seems to set-in some change occurs which stresses the system, and everything has to start adapting to the new set of conditions once more. In natural systems we can regard all objects, at any particular moment, as being in the process of either adapting to the systems conditions, or apparently adapted to the systems conditions, or failing to adapt to the systems conditions. These ideas are embodied in the Law of Instability as follows.

Every system that is stable imposes upon all phenomena that occur within it a restricted amount of action. Some phenomena are optimum, others can occur because of special transient conditions, others can never occur.

Corollaries of the law of instability are:

A knowledge of this simple law allows a natural scientist to begin to understand the earth system.

Returning to the example of an earthquake we can perceive this as a situation where the system [build-up of stress] is changing and the phenomena or objects [rocks in the upper crust] must adapt to the changing conditions. They do so suddenly. Sometimes this is minor [a small crack], sometimes it is moderate [landslides and faulting], and sometimes it is major [the Alaskan Earthquake].

The way in which a natural scientist begins to understand such things is by observation of specific phenomena within specific systems. Geologists do this constantly as they build and use conceptual models as an aid to understanding the earth system. An interesting characteristic of the dynamic nature of natural systems is that the changes that take place with time are conditional changes. By this is meant that what happens now is totally or partially dependant, in some way, upon what happened previously. We can express this as a concept of evolutionary sequence. If conditional changes are fairly obvious they are termed trends or sometimes cycles (if they twist back on themselves).

Photo-series: TREND TOWARDS CHANGES IN SIZE IN CERTAIN ORGANISMS.

Photo-series: CYCLE REPRESENTED BY VARVITES.

Numerous trends and cycles are observed in nature. Sometimes the cause of a trend or cycles may be known. In the evolution of HOMO SAPIENS, fossilized mammals of certain types can be placed along a series of trends, from which we are able to suggest the probable sequence of types from Australopithecus africanensis, to forms such as Homo erectus and Homo sapiens. Similarly we are able to trace the evolution of fish into amphibians and then into reptiles. We may not always understand why such trends occur or stop (e.g. evolution of Dinosaurs but by looking at natural phenomena in the form of trends we are often able to understand the phenomena better. The main reason why it is it often difficult to understand trends and cycles is that the way in which selection pressure acts upon a phenomenon is rarely DETERMINISTIC. By this is meant that if something happens it does not following that a specific and definite response will always occur. If only one response can occur this is deterministic , however, generally a number of possible responses could occur but only one of them will [this is probabilistic]. Geological problems are always approached knowing that the system is not only dynamic but also probabilistic. This is the reason why geological events rarely truly repeat themselves. For example, why all river deltas do not look alike or even resemble the MISSISSIPPI DELTA.

Whether dealing with historical geology the rate of change, and particularly the position of events within a time framework is important. Geologists have two kinds of techniques for developing an understanding of this time framework. These are absolute time measurement techniques and relative time measurement techniques.

Absolute time is the time as measured by a clock. Each unit of time is of a fixed duration and all units are equal. The methods used in absolute time determination rely primarily upon the radioactivity of radiogenic minerals. Although the techniques are technical and sophisticated they are routine and are of concern to the historical geologist particularly from the viewpoint of their reliability and accuracy.

The basic technical idea behind radiogenic methods of age determination is that many atoms have unstable nuclei. They are unstable in the sense that over a period of time the nucleus will spontaneously disintegrate and change to a more stable system (technically to a lower energy state). These radioactive atoms in fact change into other, different atoms called daughter atoms.

The important points about these techniques are as follows.

The most commonly known technique is probably the URANIUM-LEAD method whereby URANIUM 238 changes via a series of steps to LEAD 206. The age of the rock is determined essentially by the following.

1. Knowing the decay rate of the elements.

2. Knowing how much uranium is present and how much lead is present in the sample.

3. Assuming that no additional material has been added or lost from the sample since the original Uranium was formed.

4. Estimating how much material was there to start with and how long the material has been decaying i.e. the age of the rock.

The major groups of radiogenic minerals used to date rocks older than 10 * 10⁶ years, and particularly the age of the earth and the moon, include the following.

1. RUBIDIUM 87-STRONTIUM 87 with a 1/2 life of 47,000 * 10⁶ years.

2. THORIUM 232-LEAD 208 with 1/2 life of 13,900 * 10⁶ years.

3. URANIUM 238-LEAD 206 with a 1/2 life of 4,500 * 10⁶ years.

4. URANIUM 235-LEAD 207 with a 1/2 life of 710 * 10⁶ years.

A second group of radioactive elements are used for dating younger rocks.

1. POTASSIUM 40-ARGON 40 or POTASSIUM 40-CALCIUM 40 with a half life of 1,300 *10⁶ years. The POTASSIUM-ARGON method is not considered to be as reliable as the previous methods because it is difficult to assume that material has not been lost when dealing with a light gas such as argon (remember one of the assumptions of the method is that no material is added or subtracted).

2. CARBON 14-NITROGEN 14 with a 1/2 life of 5,570 years. Thus the method is particularly useful for dating recent events in the earth's history such as the archaeological remains of HOME SAPIENS. It can be applied accurately for at least rocks as old as 40,000 years.

The method works because normal carbon found in life-forms is composed of two types called carbon 12 (98.89%) and carbon 13 (1.11%) neither being radiogenic. Radioactive carbon 14 is continuously being produced in the atmosphere by the reaction of cosmic rays (neutrons) on nitrogen 14, changing it to carbon 14. The carbon 14 is rapidly oxidized to form CO₂ in the atmosphere and this CO₂ is distributed by winds, rivers and oceans. It becomes incorporated into organisms by the process of photosynthesis and thus all living matter tends to contain some carbon 14. This carbon 14 normally moves continuously throughout the body until the organism dies. Upon death any carbon 14 within the organism will start to decay back to nitrogen 14 with a 1/2 life of 5,570 years. Thus by measuring the amount of carbon 14 and nitrogen 14 in organic remains it is possible to date the sample.

There are a number of non-radiogenic methods of absolute age determination.

Astronomical absolute clocks. The earth's period of rotation and revolution is used to tell absolute time as measured by a clock. If these characteristics can be recognized from rocks then it is possible to develop a non-radiogenic method of absolute age determination e.g. varves.

Biological absolute clocks. Biological clocks can be used only in very special cases for age determination e.g. tree ring analysis and growth line analysis. For very young rocks, coordination of phenomena within the rocks with the historical record can provide an absolute time scale e.g. introduction of certain plants into specific regions, forest clearing, and reduction of farming activities in an areas because of disease.

Relative time is time measured by the relative position of an event. The method divides geological time relative to a past, present, or future event and is the most commonly used technique for telling time used by the historical geologist.

The Law of Superposition or Steno's Law uses the relative position of layers of rock to determine relative age.

To get some idea of the law of superposition let us imagine a large tank into which we pour successively different colored sand, so that layers of sand build-up with the passage of time. We can relate the position of any one layer of sand relative to any another by its colour. Even if we cut the tank in two or take a section from one part of the tank and compare it with a section from another part of the tank, we can still determine the relative ages of the sand layers. Moreover, provided all the colors are different we can take a single small sample and date it relative to other sands. This is the essence of the Law of Superposition which stated simply says that in layered beds of rock the rocks lower in the sequence are the oldest. Steno's Law applies most of the time to layered sequences because there is usually some attribute of the layer [bed] that is characteristic of each layer [colour, grain size, mineral content]. There are cases where earth movements make Steno's Law inapplicable such as when beds are vertical or overturned. A major complicating factor is that layers of sedimentary rocks are normally time transgressive [diachronous] when traced over any significant distance. Thus the same rock sequence may occur in two different localities but the actual ages of the apparently similar rocks are different from one location to the next.

The Law of Evolutionary Development or Smith's Law uses biological trends and associations to determine relative age. It relies on the fact that all organisms seem to evolve i.e. there is a gradual change or modification in the appearance of an organism when one traces any individual trend of ancestor-descendant. e.g. coiling in an oyster shell. Successive layers of rock will therefore contain successive parts of an ancestor-descendant trend. Imagine a large pond or lake in which a population of oysters are growing. It would be expected that variation within the population of oysters will occur, just as we get variation in any interbreeding population. If the population continues to exist through time and successive populations are produced i.e. the young grow older, give birth to young, the older ones die, the younger ones grow older, and give birth to young, etc., then provided the population exists for sufficient length of time e.g. 5 million years modification in the population make-up will occur. If we know the structure of the population through time i.e. how it changes then it is clear that we can take an individual fossilized oyster and date it relative to the sequence of oysters. Also, if we have a number of oysters from the same part of the sequence then we can probably get a better estimate of exactly where in the sequence of oysters our sample came from. The major complicating factor is that all organisms adapt to their depositional environment therefore it is possible that two similar environments of different ages have similar sets of fossilized remains. Paleontologists must be able to separate the effects of the time [temporal effects] from the effects of location [areal effects] before providing an accurate age to a rock. Benthonic [or benthic] organisms are those that live on, or in, the sea floor and they tend to die in the environment in which they lived. They are said to be autochthonous meaning they are found in the rocks representing the depositional environment in which they lived. Planktonic [or planktic] and Nektonic organisms, which are the floaters and swimmers, sink to the bottom after death and thus are not found in the environment within which they lived. They are said to be allochthonous.

Good age determinations are based upon rock sequences that allow sampling from similar depositional environments in different regions or use fossils that are known to be independent of environment so that any environmental effect is minimized. The planktonic and nektonic organisms usually have characteristics which make them useful for age determination [these are termed good key or zonal fossils]. The desired characteristics include moderate to fast evolution, wide geographic spread, and fairly high abundance.

In addition, fossilized planktonic micro-organisms [microfossils] are usually both small and abundant and thus they can be found in a small sample such as would be derived from the cuttings resulting from drilling an oil well. Having lots of fossils means we have a good chance of finding key fossils. In the marine environment there are some very important microfossil groups which we will deal with later. However, they principally include fossils with calcareous shells such as foraminifera and coccoliths; with siliceous shells such as radiolarians and diatoms; and, with organic shells such as dinoflagellates. In the continental environment there are few such fossils, although continental aqueous environments may contain abundant algae. However, one important group of microfossils that are abundant in the continental environment are parts of plants. In particular the spores and pollen [miospores and megaspores] are regularly found as fossils. A very important aspect of spores and pollen is that they are taken by rivers into the sea and therefore they can be used to determine the age of both continental and marine sediments.

There are many factors that influence both the occurrence of sediments and the evolution and adaptation of organisms and these many factors complicate the issues. Nevertheless, using the sediments and the fossils in a rock the historical geologist usually can develop a fairly accurate time-scale for telling the relative age of sediments e.g. for the deep sea sediments and young sediments of the Atlantic seaboard of the Cenozoic Era units of 25,000 years are possible using techniques developed in the last 15 years. Accurate methods of this type forms the basis of much of the work done by major oil companies [who developed the technique for their own use].

Rocks generally do not occur as continuous layers over very large distances. More often they are cut by rivers, or by cracks and faults in the earth, or are disturbed by earth movements, so that part of a layer occurs on one region and the rest of it in another region. The process of relating the rocks in one area with the rocks in another area using age determination techniques is called stratigraphic correlation. It is important to note that time in rocks is only as accurate as the technique used to measure it! In many cases geological age determinations are subject to large error because the techniques used for age determination are not very accurate. Generally precision is lost if one tries to correlate rocks over large distances.

When examining the process of stratigraphic correlation it is useful to think of a rock as made-up of a biologic [or fossil] component, and a lithologic [or mineral] components called the biotype and the lithotype. Methods of stratigraphic correlation that use the biotype are referred to as biostratigraphic methods and when we refer to the methods we use the term biostratigraphy. Those methods that use the lithotype are called the lithostratigraphic methods and we refer to this area of study as lithostratigraphy. Biostratigraphy and lithostratigraphy are two different approaches to the same problem: how do we understand the historical geology of an area.

In practice, neither of these methods allow really precise methods of absolute time correlation. This is because most beds of rock are diachronous and most populations of organisms are strongly influenced by environmental adaptations. This results in stratigraphic matching as opposed to stratigraphic correlation. The most sophisticated techniques of stratigraphic matching are the geophysical and petrophysical methods. These use some physically measurable property of rocks to match rock successions e.g. seismicity, resistivity, magnetism and radioactivity.

True time correlation techniques are termed chronostratigraphic methods, and the subject in general is called chronostratigraphy. In this light HISTORICAL GEOLOGY can be defined as the understanding of EARTH CHRONOSTRATIGRAPHY.

The units of time division used by the geologist, whether derived by the absolute or relative method are used to form the geological time scale. Informally there is a division of the whole of the time since the beginning of earth into Archeozoic, Proterozoic, Phanerozoic. However, the main time scale is a set of hierarchical units the principal ones being ERA, which is the largest unit, PERIOD, which is a subdivision of an era, EPOCH, which is a subdivision of a period, and AGE, which is the subdivision of an epoch. These are all terms applied to the time units (somewhat akin to Monday, Tuesday etc.). Sometimes we talk about the rocks that were deposited during a particular time and then instead of using the term Period we use SYSTEM and instead of Epoch we use the word SERIES, which are divided into STAGES.

The various systems, series, and stages are defined and recognized by type sections. These are all located in Europe (simply because this is where geology as a formal science began). Type sections are sequences of rock that represent the sediments deposited through a specific time unit. One of the problems in historical geology is that the type succession for one period may be in one country and the type succession for another period is in another country. In order to get a complete time scale it is necessary to correlate form one country to another. There is no known part of earth where a continuous succession of rocks has been deposited throughout geological time, although there are certain areas where complete successions occur for rather long time intervals. Orogeny and Isostasy, associated with earth movements, and other earth changes cause sequences of rock to be missing [missing sections]. The task of creating a precise and accurate geological column is the principle problem that faces historical geology. If both the age and depositional environment of a rock is known then it is easy to make of make showing the distribution of the environments based on all rocks of the same age. This produces a paleo-environmental or paleo-geographic map.

In practice, when a field geologist produces a map of an area generally it is simply a map of the distribution of different rock types, such as clays, sands, limestones and coals. Only later is the rock dated (given an age). For convenience, when making such maps, and until the age of rocks are known, a simple rock-bed terminology is used. The basic unit is a layer or bed of rock that represents a single lithotype. Successions of similar beds forming a distinct a layer over a large area are termed formations. (e.g. a thick layer consisting of alternating clays and sands both of which are red in color; or, a single thick layer of sandstone with thin laminations of clay). Formations can be recognized over large areas and are useful for mapping both at the surface and in the subsurface. In some cases formations can be related to one another by some characteristic e.g. they all contain marine fossils; or continental plant remains. In such cases formations are united into groups. The idea of dividing rocks into groups, formations, and beds is one of convenience until the age is known and does not allow chronostratigraphic historical geology. We simply get a picture of the rock geometry. In fact most geological problems are solved using this simple type of rock geometry approach. Fortunately, for most economic studies it works moderately well.

The concept of actualism means that the processes that alter the earth at the present time also altered the earth in the geological past e.g. rivers carry a load according to their velocity and availability of material; or, mud cracks are formed the same way today as they were a billion years ago; or ripple marks can be used as environmental interpretation tools in a similar way today as they could in any other age.

A typical example of actualism would be the growth of a massive coral reef. At the present day these are restricted to certain conditions. They flourish were the mean surface water is about 23-25^o C with a lower limit of tolerance of 18^o C. This effectively restricts coral reefs to 30^o N and S of the equator unless there are high temperature waters around e.g. Bermuda lies in the Gulf Stream. In addition, coral reefs require clear water, saline water, and sunlight to thrive: the sunlight means they do not grow below 100 feet. Finally under these conditions corals grow at about 1 foot in 20 years.

If we find great fossilized coral reefs in a rock sequence e.g. some of the rock sequences of Texas we can interpret the environment using actualism in the following way:

a. The water was saline, clear and had a surface temperature of about 24^o C.

b. The water depth from the beginning of growth until the end was not more that 100 feet.

c. By measuring the thickness of the coral we can estimate how long it took to form.

Actualism implies that it is not simply that the processes are the same but also that the responses are the same e.g. dehydration causes mud cracks; or, the combination of H₂ and 0 at atmospheric temperature and pressure gives water and the properties of water are basically constant both now and in the past. This is termed the process-response model and is applied regularly to the interpretation of geological sequences.

The main problem with the process-response model is that in the real world of today a process takes place and results in a response, however, in the rock record where the response is preserved i.e. the depositional environment there has generally been a loss of information. An additional, complicating factor is that a response observed in a rock can be due to more than one process. Rocks do not contain all of the information that was in the original environment. Because of this information loss it is not always possible to determine the detailed environment of deposition but only to offer a number of alternatives. These factors led geologists to adopt the word facies instead of environment when discussing the depositional environment. To a geologist the rock facies approximates the environment. Biofacies are facies characterized by the distribution of fossil remains and lithofacies are facies characterized by the distribution of mineral grains and sedimentary rock types. The concept of total facies pertains to the facies which most accurately reflect the original environment and are defined by combining the lithofacies and biofacies data.

The reason the geologist is interested in the environment is because he wants to recreate the conditions of environment and geography that existed in the past. That existed when the rocks were deposited. Facies can be studied at different levels of geography or environment: global, realm, domain, province, region, or local.

Clearly the rock record is not very good for precisely defining time over large distances because the basis of age determination is subject to a variety of statistical errors. The fundamental way in which historical geologists solve this problem is applying Walthers Law. This takes into account diachroneity and adaptations. It is based on an understanding of stratigraphic sequences and how they are formed, especially with regard to sea-level changes.

Walthers Law

The vertical succession of sedimentary facies mirrors the horizontal succession of sedimentary facies coexisting with each other on a synchronous depositional surface.

Geologists use the depositional environments recognized in the Modern time period as a model for past environmental conditions; and, they use shallow core sections through Recent sediments to understand how successive environments really succeed one another. Much of the modern view of the three dimensional model for past environments is derived from a study of borehole sections [wells] and geophysical seismic line obtained from a study of sedimentary sequences from the Cenozoic rocks from various parts of the world.

A major development in interpreting the subsurface using models of stratigraphic sequences is the concept of sequence stratigraphy.