Monday, January 28, 2013

Stratigraphy and Time

Stratigraphy is the study of sedimentary rocks in space and time. It is the basis of interpreting what happened in the past. We use facies to interpret depositional environments from the rocks. Changes in facies both vertically and horizontally allow us to interpret changes in ancient landscapes and processes.

Example: Beach Facies. Beach environments grade laterally into each other. The offshore areas grade into the swash zone of the foreshore. The foreshore grades into the berm (the highest point of the beach) and backshore (if present). Eolian (wind) dunes, marshes or erosional cliffs can be present landward of the beach. Rock facies similarly grade into each other because they were deposited in different depositional environments. If the depositional environments stay in exactly the same place through time, a stratigraphic column in each place would consist of a uniform facies, but each stratigraphic column would have a different style of rock (facies). However, depositional environments tend to migrate back and forth as sea level rises or falls, basins fill in with sediment, etc. Thus, facies in stratigraphic columns tend to change upward. They also vary laterally. See figure 19.8 on pg. 308 of Nichols or this figure: from this page discussing stratigraphic correlations:

Changes in sea level and depositional environment lead to variations in stratigraphic columns both laterally and vertically. If you compare different stratigraphic columns, there are several ways you might "correlate" them. If you correlate different rock types, e.g. lithostratigraphy, you are marking regions with similar characteristics, but the sediments in each unit were not necessarily deposited at the same time. In contrast, if you correlate rocks that were deposited at the same time, e.g. chronostratigraphy, each unit often consists of more than one facies. This is obvious when you look at the distribution of depositional environments now. Different areas are accumulating different types of sediment at the same time.

Lithostratigraphic correlations are easy because you can directly observe rock type. These correlations are very useful for studies of reservoir properties, where one might want to identify a porous sand that acts as a water or hydrocarbon reservoir. However, these correlations do not help you interpret ancient depositional environments because they do not represent an ancient landscape. Chronostratigraphic correlations tell you the most about depositional environments and their distribution through time, but they can be VERY difficult because you have to have a time marker that tells you what deposits were synchronous. Sometimes volcanic ash beds or other depositional events allow you to directly observe which rocks were deposited at the same time, but these events are rare. Often, chronostratigraphic correlations require detailed facies analyses and an understanding of how depositional environments change through time.

Walther’s Law is key for understanding the differences between lithostratigraphy and chronostratigraphy. Walther’s Law states that environments that are adjacent to each other are represented as vertical successions of facies in the rock record if there is no break in sedimentation (no unconformity). If sea level is rising relative to the shore line, the different depositional environments are migrating inland. This leads to different facies accumulating progressively inland as well. The most landward deposits are river deposits and alluvial plain deposits, followed by marsh and then marine deposits. Vertically, you see the facies representing those depositional environments in the same order. At any given time, rocks are being deposited in all of the different environments.

Chronostratigraphy - Chronostratigraphy enhances the interpretation of the stratigraphic record in terms of Earth history. Even when one has a detailed map of the distribution of depositional environments, it is difficult to say exactly how to correlate section in terms of time. In real rocks, there are a number of tools that you can use to get correlations of various accuracy. These include: fossils (biostratigraphy); magnetic properties (magnetostratigraphy); absolute ages of interbedded volcanic ash beds and basalt flows; some chemical properties such as elemental isotope ratios in carbonates; geological instantaneous depositional events such as huge storms, meteorite impacts, etc.; and unconformities due to sea level falls and the geometry of sedimentary deposits (sequence stratigraphy). We will get back to all of these in more detail throughout the quarter, particularly near the end.

Distribution of Rock and Time - One might think that sections can be correlated based on assuming that the same amount of sediment gets deposited in all places in the same amount of time. This is a BAD assumption, although many researchers are forced to use it. It is important to understand that the preserved rock does not represent all of time. What I mean is that time is not evenly represented by rock thickness. For example, with turbidites, the sandstones may have been deposited in a couple hours to a day at most, whereas the shales (Bouma E) represent 100’s to 1000’s of years of fine grains settling out. Thus, most of the "time" is represented in the much thinner shales. In addition, there is erosion at the base of some of the turbidites. Thus, there is a significant amount of time that is only represented by an erosional surface which produces a gap in the rock record. Generally, sedimentation is thought of as a continuous processes. This is NOT true. Sedimentation is episodic and there are unconformities in the stratigraphic record spanning all time ranges from minutes to millions of years. Gaps of minutes might occur in a river if there is a burst of strong flow that is erosive rather than depositional. Gaps of hours occur at low tides when the intertidal zone is exposed. Gaps of years to thousands of years can occur in land environments where there is no source of sediment or the topography is too high to collect sediment. Gaps of millions of years also occur in terrestrial environments, especially if there is erosion. The longer time gaps usually represent regional changes in deposition and can be very useful for correlating rocks chronostratigraphically. Also, different depositional environments accumulate sediment at different rates: thickness does not equal time!

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