Sedimentation and sedimentary rocks. Sediments - sand, silt, mud, dust, and other less-familiar materials - are deposited in strata that can be broad and sheet-like or narrow and ribbon-shaped. These shapes occur on scales of meters to 100s of kilometers, and are the direct result of sedimentation such as flowing water, wind, or explosion from a volcano, to name a few more or less common processes. Virtually every geographical location we can think of - a river, a desert, a lake, an estuary, a mountain, the bottom of the ocean, the pampas - has sedimentary processes peculiar to it that will produce distinctive sediments and, with time and burial, distinctive sedimentary rocks.

Relative age dating. It is a fact that younger sediments are deposited upon older sediments (exemplified in Figure 2.2), and yet this apparently self-evident insight is the fundamental basis of all correlations of sedimentary strata in time. Ascertaining the relative ages of two strata is termed relative dating and, while not providing the age in years before present, provides the age of one stratum relative to another stratum. Here, then, is part of the solution to dating dinosaur bone. Suppose that a stratum containing a dinosaur bone is sandwiched between two layers of volcanic ash. Ideally, an absolute age date could be obtained from each of the ash layers. We would know that the bone was younger than the lower layer but older than the upper layer. Depending upon how much time separates the two layers, the bone between them can be dated with greater or lesser accuracy (Figure

But how can one tell that two geographically separated deposits were deposited at the same time if absolute ages are unknown? In this, fortunately, stratigraphers are aided by one last, extremely important tool: biostratigraphy.

Figure 2.2. Superposition of strata in Petrified Forest National Park, Arizona, USA. Thick stacks of red mudstones were deposited by rivers 213 million years ago, to produce the succession of layers visible in this photograph.

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