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Sediments to Sequences EPSC 425: Sediments to Sequences Galen Halverson Winter Semester, 2014 Contents 1 Introduction to Stratigraphy 1 1.1 Sedimentary Geology . 1 1.2 What is Stratigraphy? . 1 1.3 Strata as Temporal Archives . 2 1.4 Lithostratigraphy . 5 1.5 Depositional Environments . 8 1.6 Lithofacies and Associations . 14 1.7 Correlation . 16 2 Geophysical Data 18 2.1 Well Logs . 18 2.2 Seismic Exploration . 24 3 Sequence Stratigraphy 27 3.1 Introduction . 27 3.2 Fundamental concepts . 29 3.3 Shoreline trajectories and sediment stacking patterns . 30 3.4 Base-level changes and the development of systems tracts . 32 3.5 Sequence boundaries . 36 3.6 Stratal terminations . 38 3.7 Types of Sequences . 38 3.8 Sequence Stratigraphy of Carbonate Platforms . 42 4 Learning to Tell Geological Time 44 4.1 Introduction . 44 4.2 The Geological Time Scale . 45 4.3 Telling Time in the Stratigraphic Record . 47 5 Radiometric Techniques 51 5.1 Introduction . 51 5.2 U-Th-Pb System . 52 5.3 K-Ar System . 57 5.4 Re-Os System . 59 5.5 Radiocarbon Dating . 59 6 Magnetic Stratigraphy 62 i ii 7 Biochronology 65 7.1 Biostratigraphy . 65 7.2 Biochronology . 65 8 Sedimentary Cycles 67 8.1 Introduction . 67 8.2 Stratigraphic cycles . 67 8.3 Climate-independent cyclic sedimentary patterns . 68 8.4 Sedimentary parameters linked to climate change . 68 8.5 Astronomical Forcing . 71 9 Chemical Stratigraphy 76 9.1 Introduction . 76 9.2 Underlying Principles . 76 9.3 Development and Application of the Method . 78 9.4 Stable Isotope Systems . 79 9.5 Radiogenic isotope systems . 89 Chapter 1 Introduction to Stratigraphy 1.1 Sedimentary Geology Sedimentary Geology encompasses any field of science dealing with sediments or sedimen- tary rocks { that is rocks formed by Earth surface processes that begin with weathering on land, followed by erosion, and finally the physical settling of grains or chemical precip- itation of minerals from air, water, or ice. Sediments (unlithified) and sedimentary rocks (lithified) are archives of information about environmental, tectonic, and biological condi- tions that prevailed at the time they were laid down; even their very existence is revealing to the interested geologist. Sediments are largely derived from older sedimentary, igneous and metamorphic rocks, and themselves may be metamorphosed or melted to form other rock types. Hence, they represent one arc in the continuous and long cycle of the shaping and reshaping of Earth's landscape, as envisioned by James Hutton. 1.2 What is Stratigraphy? Stratigraphy is the study of geological strata. It may not be immediately obvious to a non- stratigrapher why it is important and it may come across as rather boring to many. But stratigraphy is a process for applying order to the world of stratified rocks. Stratigraphy involves documenting sedimentary successions, interpreting them, correlating sedimentary units across basins, and where possible, dating them, either directly or indirectly. Stratig- raphy is the temporal and environmental framework within which much of Earth's history has been studied, debated, and deduced. Stratigraphy is also indispensable in petroleum geology, for all petroleum source rocks are sedimentary, as is virtually every reservoir. Similarly, stratigraphy is important both in exploration of stratabound ores and in hy- drogeology, because most major aquifers are in porous sediments, and their lithology and spatial distribution control the extent of the reservoir and fluid flow within it. Stratigraphy encapsulates both sedimentary and volcanic rocks. However, it is funda- mentally intertwined more closely with sedimentary geology than igneous petrology, and volcanic rocks are studied only insofar as they may comprise a component of a stratigraphic succession. That said, volcanic rocks can be extremely useful in stratigraphy as markers for correlation and for the potential for radiometric dating. Geology has only been a scientific discipline since the early 19th century. However, curi- 1 Chapter 1: Introduction to Stratigraphy 2 ous intellectuals contemplated the deeper meaning of sedimentary rocks long before that. Aristotle, Leonardo Da Vinci, and Nicolas Steno all famously endeavoured to understand sedimentary rocks, with varying degrees of success. Steno (1638{1686), a Danish anatomist, was certainly the most successful geologists of these early dilettantes; for his efforts, he is often regarded as the proto-father of geology. This early curiosity about stratigraphy was certainly not a coincidence: sediments and sedimentary rocks cover much of Earth's sur- face. They also contain fossils, which are captivating to many. Steno formulated three basic principles (Steno's Laws) about sedimentary rocks that con- tinue to govern stratigraphic approaches today: • Law of Superposition • Law of Original Horizontality and Lateral Continuity • Law of Cross-cutting Relationships We can add to this list another basic principle: the law of inclusion, which states that if one rock is incorporated into rock (say a pebble, or even a fossil in most cases), it the former must be older than the latter. These laws are all fairly obvious to any geologist, but they are powerful nonetheless, because they enable us to order geological events chronologically. Time is of course indispensable into trying to reconstruct Earth history, and most often, we cannot rely upon radiometric dating techniques to provide ages for rocks. 1.3 Strata as Temporal Archives The stratigraphic record is the closest thing we have to an almanac of past geological events on the surface of the earth. Based on Steno's Laws, we can assume that strata that have not been horribly tectonized or otherwise perturbed should provide some sort of temporal record, with the lowest rocks being the oldest and the youngest rocks of partic- ular succession being the youngest. As such, they are an Archive of Earth history. This is why Earth historians turn to the sedimentary record to reconstruct and interpret past geological events, as varied as the origin of animals to meteorite impact events. However, the stratigraphic record is deeply flawed. For one, sedimentary basins do not cover the globe and many sedimentary basins that have existed over the history of the Earth have been partially or wholly destroyed. Another problem is that the record is not a truly continuous record of past events{that is, it is no tape recorder. Imagine a shallow marine sedimentary environment. Many of the sediments deposited and ultimately preserved are most likely deposited in a series of geologically instantaneous or short-lived events, such as storms, earthquakes, and other unusual conditions. During the intervening time, little or no deposition may take place, or what sediments are deposited may be wiped out by a subsequent event. Hence, the shallow marine sedimentary record only captures snapshots of conditions in any one basin. This irregularity and discontinuity in sedimentation makes it difficult to transport stratigraphic height or depth into time. Because most of the sedimentary rocks found on the continents are from the continental shelf environment, stratigraphers face a severe uphill challenge to reconstruct past geologi- cal events from the extant rock record. They can improve their odds by studying an entire Chapter 1: Introduction to Stratigraphy 3 sedimentary basin{or at least, more than a single drill core a stratigraphic section. By in- tegrating data from across a basin, on can fill in more details of past geological events and at the same time, better interpret the depositional history of a basin. Hence, such basin- wide studies are important both in Earth history and in exploration for petroleum or other strata-bound mineral resources, whose distribution is controlled by the temporal-spatial evolution of the basin fill and its subsequent burial and diagenesis. As will be discussed throughout the remainder of this course, many different tools are available to stratigraphers to reconstruct basin-wide stratigraphic geometries and basin evolution. Biostratigraphy is a tried and true method of establishing time equivalence, hence correlation of sedimen- tary strata. However, a variety of other sedimentologicaly, geochemical, and geophysical techniques are also applied. Indeed, the state of stratigraphy is such that many geologists become specialists in just one or a small handful of these techniques. The issues of incompleteness and disturbance to the sedimentary record are greatly reduced in the study of deep sea sediment cores. A consortium of scientists has been drilling the seafloor for over four decades now. It started as the Deep Sea Drilling Project (DSDP) in 1968 by the United States, was internationalized as the Ocean Drilling Program (ODP) in 1985, and became the Integrated Ocean Drilling Program (IODP) in 2004. The objective of the program is to sample sub-seafloor environments, which are of course overwhelmingly sedimentary (although they do drill into both oceanic and continental crust as well). This program has produced countless drill cores from all of the ocean basins and is far and away the greatest source of information on Earth history over the past 200 million years. 1.3.1 Stratigraphic subdivisions Stratigraphers naturally want to subdivide the sedimentary record into discrete units for the purpose of correlation, evaluating spatial variability of time equivalent units and fossils, and many other purposes. If all sedimentary rocks could be easily dated, this would be a rather trivial affair. However, it turns out that it is often extremely difficult to date rocks directly, and because the stratigraphic record is so often disturbed by erosion, tectonism, and other indignities, discerning ages of rocks is often very difficult. Even applying relative ages often involves correlation. Consequently, the subdivision of the stratigraphic record is not straightforward. In the early days of geology, in the 1800's, two different approaches emerged to apply order to the stratigraphic record: lithostratigraphy and biostratigraphy. Lithostratigraphy, de- scribed in more detail below, entailed the physical description of the rocks and correlation between separated sections based on lithological divisions and contrasts.
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