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GLY 471 Stratigraphy and Sedimentology

Guide to Field Description of Sedimentary Rocks

Field stratigraphy requires astute observation and interpretation of sedimentary rocks at hand-sample to outcrop scales. In order to interpret the depositional environment of sedimentary rocks it is necessary to describe them in detail in three-dimensions. Only rarely are sedimentary rocks exposed extensively in three-dimensions. In most cases three-dimensional description must be pieced together from spatially correlatable one- or two-dimensional exposures. Descriptions will begin at the base of each section and proceed upwards. Descriptions of rocks require measurement of strata thickness and notation of physical characteristics. For this project, in addition to stratigraphic thickness, each student will collect information pertaining to color, bedding, texture, fossils, and unit geometry. Data will be recorded in the field notebook following the required format (see attached). The notebook must be kept up-to-date by each student and will be checked and part of your assessment. Description normally follows a logical sequence as follows: 1. Brief reconnaissance of the whole outcrop to gain a preliminary impression of the main

sedimentary features such as sedimentary rock types present (as defined by color, composition, and texture), geometry and orientation of beds and bedsets, sedimentary structures, and associated paleocurrents, trace and body fossils, vertical and lateral variation in these sedimentary properties. An effort should be made to define preliminary lithofacies and lithofacies associations (sequences). You will need a notebook, hand lens, clinocompass, measuring tape or stick (Jacob staff) , grainsize comparator, acid bottle, etc.

2. Detailed description and stratigraphic logs. The best way of recording sedimentological

information in the field is to construct a graphic log and write detailed observations next to it. The type and style of log produced at the outcrop will depend largely on the scale and nature of the project; however, in the data collection phases of a project, it is better to err on the side of collecting too much information rather than too little. There are reasonably standard ways of compiling the descriptive information graphically. Examples are given on attached sheets. Notice that it is possible to display a large amount of information on graphic logs, including mean grain size, composition, bed geometry, sedimentary structures, paleocurrents, fossils, colors, etc.. It is critical to think about the best scale to use. When measuring sedimentological logs, start at the base of the outcrop and, where possible, record the following information for each bed or bed set:

Lithology. Composition of grains and cement/matrix. Mean grain size, sorting, shape, fabric.

Degree of cementation or weathering. Color on fresh and weathered surfaces. Spatial variations of lithology in a bed or bed set. Name the sedimentary rock based on this information.

Bedding Geometry. Thickness, including lateral variation. Are beds sheet-like, lenticular,

etc.? Nature of upper and lower boundaries (erosional, sharp, gradational, etc...)

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Sedimentary Structures. Describe internal and external structures, including their dimensions,

orientations, spatial variation, etc. If difficult to see, collect a sample for laboratory analysis.

Direction Indicators. Record direction azimuth separately for each structure. If regional dip

exceeds approximately 15°, a correction is required to obtain the true paleocurrent azimuth.

Fossils. Identify body and trace fossils, and collect samples if possible. Record variation in

size and shape, abundance and spatial distribution, orientation, sediment-fossil and fossil-fossil associations, nature of preservation.

Recognition of Mappable Units One of the goals of field stratigraphy is to recognize distinct stratigraphic units based upon their physical characteristics and are useful in mapping and correlation. The fundamental unit is a formation that is defined as a distinct mappable rock unit. Keep the scale of the project in mind. Do not become obsessed with a two-foot unit that does not differ markedly from the strata below and above. Step back from the outcrop and get a feeling for any natural “breaks” in the sequence. For example, as you look at a 100 foot tall cliff exposure try and determine changes in such physical characteristics as bedding style, color, or erosional profile; these initial observations may expedite the measuring processes. “distinction” is the word of the day. Don’t be thrown off by subtle variations in rock characteristics—try to identify unifying features. A formation may have much variation within it. For example, a 100 foot sequence containing alternating 20 foot intervals of thin beds and thick beds may be observed. Each of these 20 foot intervals may be described individually and assigned an informal unit designator (e.g., A1, A2, etc…). this is fine simply as a matter of convenience. However, it may be determined that grain size, fossils, and other physical characteristics of each 20-foot-thick vary only slightly. Therefore, these alternations should be combined into a single mapable formation. The basic questions to ask is this: If I were in a pasture and came upon three feet of this 20 foot section of thin bedded strata, could I say exactly where I was in the 100 foot sequence? If the answer is “NO”, then do some lumping; if it’s “YES” consider other alternatives like a member or marker bed before giving the interval formational status. As the exercise progresses prospective formations, or mappable units, may reveal systematic vertical changes. These changes may be due to grain size variations (e.g., coarser upward), bedding changes, or a combination of certain features such as fossils or bedding, for example. Under such circumstances the designation of “members” may be appropriate. Although details ARE important, try not to be overcome by the minutia when measuring sections. Be prepared to move to the left and right along strike (perpendicular to the line of section) in order to allow complete description. As sedimentary rocks tend to occur in repetitive sequences, you will find that, after the initial stages, it is not necessary to determine all of the above sedimentary properties for every bed or bedset. It is obviously an advantage to recognize distinctive facies at an early stage. Look for the significant changes.

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