Stratigraphy ( Lithostratigraphy )

I. Lithostratigraphy

     Actually determining that a sandstone seen at two outcrops a mile apart is part of the same formation. In the desert, where you have great exposure, you can often be pretty sure of a good correlation. Around Pittsburgh, one sandstone looks much like the next, and the exposures are too limited to see much of the rocks that occur above or below (stratigraphic context can be extremely helpful). Also, in a humid climate, a fresh exposure is often a very different color from an old weathered one. So, don't underestimate the difficulties of correlating rocks based on their physical appearances. This is one reason that biostratigraphy becomes so important

B.   Rocks do not equal time. On the scale that we see outcrops, either in road cuts or across whole mountain sides, many sedimentary formations show up as nice layers with more or less parallel tops and bottoms. Almost every outcrop we see, even if it crosses a whole mountain side in the desert, looks like a beautiful layer cake. The layers were clearly originally horizontal, with the oldest on the bottom and the youngest on the top, and they appear to be quite laterally continuous. If you accept this impression as fact, it perverts everything you have learned in class thus far. Handout: Trangression/Regression general shelf box drawings. There are 3 points: First, at the scale of the drawing, it is clear that the sandstone beds in fact dip toward the sea. HOWEVER, this dip is obvious ONLY because of the extreme vertical exaggeration of the drawings. Recall that the shelf dips on average only 0.1° seaward. As far as your eyes are concerned, this is in fact horizontal. But, when gazing at a given outcrop, remember that the layers are really generally dipping toward the center of the basin. Second, a given formation can span a considerable time range. While this is obvious when it comes to considering a single vertical section (old on bottom, younger on top), it is harder to keep this in mind when you consider the age of the base and top of a formation as you walk over a wide area. On these figures, the passage of time is marked by time lines, which define ancient active depositional surfaces. Lines or surfaces like these are termed "isochronous. Looking at the regression/transgression figures, you can see that the bases and tops of formations start and end at different times. If you were measuring sections around the basin, you would find sequences of layered beach sands deposited immediately above an underlying formation. They might all look very similar, because they each represent the same depositional environment,  if you look at the figures, you can see that they will be of different ages that depend on the nature of the transgression or regression. The only way that you can be sure that a set of sand layers was deposited at the time (i.e., along strike parallel to the shoreline) is by using biostratigraphy to constrain the relative ages of the strata. This biostratigraphic time frame will allow you to put the basal sand units, or whatever rock bodies you want to consider, into their proper depositional frame work. Notice how the overall thickness of the formations varies depending on whether they are parts of transgressive and regressive sequences. Third, once time lines are established, you know that if you follow them into deeper water, you will go from sand (beach) to silt to clay (shelf). If you stay at a given water depth and move parallel to the shore line, you can stay within a given sandstone formation at the same time horizon. Handout: Devonian Catskill sequences of New York. Both figures are based on a compilation of vertical sections measured at a number of localities across southern New York. If you are thinking 'layer cake', your interpretive cross section will look like the upper drawing. Note how the Catskill Group appears to have just somehow rained down from Heaven to lay nicely on the underlying formations. It is tough to reconcile such a cross-section with an understanding of how sedimentary environments work. The lower drawing illustrates how, with a better prior understanding of depositional systems and how sedimentary facies change through space and time, the formations are now seen as a part of a prograding sequence. Biostratigraphy is necessary to draw the time lines that help estimate the ancient depositional surfaces. In this example, the base of the sequence has several limestone beds that indicate outer shelf sediments. Shelf shales come next, apparently pushed in from the east. As the available accommodation space fills, the sediments coarsen up to beach facies. Finally a sequence of redbeds indicating continental deposition sweep west across the area.

C.    When faced will real outcrops in the wild, it is a challenge to force yourself to not see the world as a layer cake. Outcrops just look like layer cakes because their original depositional dips are so shallow!

D.    Sedimentary rocks are not a continuous record of time! They are full of time gaps when no sediment was deposited and preserved or when preserved sediment was later eroded.

1.    Ancient record: measure thickness of 5, 10, or 100 Myr sequence of sedimentary rocks, divide by the total age range they represent, and sedimentation rates are meters/ka. Modern environments: sediments accumulate 100 to 1000 times faster (100 to several 1000 meters/ka). Clearly, a great deal of sediment accumulating today will not make it into the sedimentary rock record. There is a lot of erosion! From calculations like those above, geologists conclude that only <0.1% to 1% of geologic time is typically represented by sedimentary rock. This is true even in the deep sea. One wonders if this 0.1 to 1% of sediment is a representative sample of sediments originally deposited in the ancient sedimentary environments, or if it all represents extraordinary events like huge rare storms.

2.   Example: Beach sediments. Over the course of 6 months, sediments are during fair weather, then small storms strip some off, more fair weather deposition follows, big storm strips off a lot of beach, then a long period of fair weather deposition follows, etc., etc. Maybe only tiny snippets of longest periods of fair weather deposition will survive to next season. How to get beach into ancient record? An exceptionally big storm (once every 1000 or 10,000 years?) sweeps beach sediments into deeper water, where they can get buried? Perhaps a big (HUGE) earthquake drops the beach surface down enough meters to preserve much of it below wave base?

3.    Unconformity: a much longer break in the sedimentary record (generally one to multi-Myr time scale). Often marked by erosion (as opposed to nondeposition), and generally caused by significant changes in the sedimentary regime (as opposed to normal processes occurring at at given environment). These gaps in time can be estimated using stratigraphic techniques. The shifts in sedimentary environments may be caused by a major sea-level changes or by regional uplift and erosion.