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Write-up - Cal State LA - Instructional We Server

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Write-up - Cal State LA - Instructional We ServerLA,We,we,Cal,State,cal,state

Sedimentary Petrology Winter ’2012

    SEDIMENTARY STRUCTURES

    This lab is designed to give you a quick introduction to sedimentary structures. For this lab, we will examine some of the larger scale features (ripples, cross bedding, burrows, flute casts, etc.) of sedimentary rocks. Sedimentary structures, especially those formed during the deposition of sediment, are extremely important for determining depositional environments, the up direction in rocks, and paleocurrent direction. Read the section on sedimentary structures in your text.

Lab Assignment

1. Make sketches of the structures found in the rocks provided

    2. Indicate the current direction and top direction. If it is not possible to determine current direction or top direction, please say so.

    3. Get into the habit of putting scales on your sketches.

    4. Note the major categories of sedimentary structuresdepositional, erosional, biogenic, etc. Be able

    to categorize structures.

    For exams, in addition to the above, you will be responsible for all of the information provided for each sedimentary structure here and in the text.

EPOSITIONAL STRUCTURES D

    Bedded and Laminated Samples 303 and RNBW-1. The most fundamental feature of sedimentary rocks

    is stratification or bedding. Stratification usually results from changes in composition or grain size. Beds are tabular or lenticular (refer to text or geology dictionary for definitions) layers in sedimentary rocks that that are distinguishable from layers above and below by lithologic, textural or structural characteristics. In many cases individual beds/laminae represent a single depositional event. Most beds have been deposited from hours to days in the case of turbidites or flood events or from years or tens of years in the case of marine shelf sandstones or limestones. The upper and lower surfaces of beds are known as bedding planes or bounding planes. These planes may represent quite a bit of time when little deposition occurred. Sediment could have been moved but not buried during this time because of strong current activity or perhaps there was simply no sediment transported or erosion was taking place. In the case of sample 303, chances are that the stratification resulted from deposition over a significant period of time because the rock represents deposition in a deep marine environment. In the deep marine environment, a 1 centimeter thick layer may represent about 1000 years of deposition. I don't see any indication that the bed represents a turbidite. Individual beds can be laminated.

    Massive bedding is used to describe beds that appear to be homogenous and lacking in internal structures. Massive beds are also termed structureless beds. However, upon close examination many of the massive beds are not truly structureless, but contain trace fossils such as burrows. Thorough churning of the sediment may lead to complete homogenization and the development of a structureless bed. Massive beds may also result from deposition through density currents such as turbidity currents. One would expect to find a random fabric or non-alignment of grains in a massive bed resulting from bioturbation. If the massive bed resulted from turbidity current deposition one would expect to find grain alignment.

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    1. According to your text, when is a layer termed a laminae? A bed?

    2. Are the layers observed in the fine-grained sample beds or laminae or both?

    3. Beds with surfaces that parallel each other and that do not contain internal dipping laminae (cross-

    laminae are termed parallel beds. Are these parallel beds or laminae?

    4. For Sample RNBW-1, assume that the median grain size is 0.5 mm and that the depth of deposition

    was 0.3m, what type of flow velocities would be need to form the bedform observed. See Fig. 2.15

    p.24

    5. Is the bedform present in RNBW-1 part of the upper or lower flow regime?

    6. According to the Fig. 2.15, p.24, which factors control bedform development in sandy sediments?

    7. Is there any alignment to the grains? If so, what does this indicate about current direction?

Ripples in fine grained Limestone and Sample IPC SED Pet. Ripple marks are the most common

    sedimentary structure in modern sedimentary environments. They are found in both siliciclastic and carbonate sediments. Ripples are either asymmetric or symmetric. Ripples resulting from unidirectional flow are typically asymmetric. Asymmetrical ripples are called current ripples. Wave formed ripples or oscillation ripples are common in shallow marine, deltaic and lacustrine sediments. They have a symmetrical profile as opposed to the typical asymmetrical profile of current ripples. Wave formed ripples can also be asymmetric. The wavelength of a wave ripple depends on such things as grain size, water depth and wavelength of waves on the water.

    1.What type of ripple (wave or current) is present in the limestone? What is the current direction?

    2. For IPC Sed Pet, answer the questions listed in #1.

    3. Without looking at samples, make a sketch of a current ripple and a wave-formed ripple.

    4. What is the ripple index for the ripples in the limestone? What is the range of the ripple index

    values of current ripples? Ripple index should only be used as an approximate guide to ripples.

    5. For the sake of discussion (refer to figure 2.15, P. 24 in your text), let’s assume that the original

    grain size for IPC Sed Pet was 0.4 mm and that the water depth was 0.3 m, which bedform will

    form first? Second? Third? Fourth?

    5. Are the ripples in the limestone straight crested or are they wavy and bifurcating?

    6. According to your book, are the ripples in the limestone wave or current formed?

    7. Assume for IPC that the grain size was 1.5mm, would we expect to see ripples in the sample? Why?

    What would be the succession of bed forms developed?

    8. How would the bedforms differ if the grain size for IPC were 0.1mm?

4553 Herringbone cross bedding(large sample on counter). Ripples and dunes produce cross bedding.

    The migrations of ripples and dunes, when conditions of net sedimentation occur, produce inclined bedding termed cross stratification. This cross-stratification is also termed foresets. The inclined bedding marks the former position of the lee side of ripple or dune. Two general types of cross bedding occur: planar and trough. (see p. 28 of text). According to your book planar cross beds or foresets dip up to 30 degrees or more. They have an angular or tangential basal contact with the horizontal. Tangential basal contacts can be used as top/bottom indicators. The tangential contacts mark the

    bottom of the foreset. Planar cross beds usually form tabular sets. Trough cross beds are produced by bedforms with curved crests and typically form scoop-shaped beds. Herringbone cross beds result from two adjacent sets of cross beds showing currents flowing in opposite directions. In other words, the foresets of the different sets dip in opposite directions. Herringbone cross beds can be found in tidal

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    environments where flood tides (incoming tides) produce one set of cross beds, and the ebb tide (outgoing tide) produces the other set. Make sure that you see the herringbone cross beds

    1.Sketch a couple of ripples (you will not find any ripples in this sample) and label wavelength, lee

    side, stoss side, crest, trough, height, cross strata, eddy, and point of reattachment.

    2.In this sample, is the cross stratification planar or trough?

    3.If planar cross beds occur in the sample are they tabular. Label a couple of the sets of cross

    stratification on your sketch.

    4. How can you determine top/bottom on this sample?

5020E cross stratification. Observe the well-preserved wavy cross stratification. Where rapid

    sedimentation occurs the ripples will grow upwards as well as forward. The ripple will climb up the stoss side of the one downstream. These are called climbing ripples or ripple drift. With rapid deposition laminations will form on the stoss side and be continuous with lee side laminations of the ripple. Do you see this in the sample?

    1. Are these ripples part of the upper or lower flow regimes?

    2. Which bedforms characterize the upper and lower flow regimes?

4693 Cross bedding in coarse grained pebbly sandstone

Sample 3982 A typical turbidite (refer to pages 81 to 86 of text and figure 2.78(h) of typical Bouma

    sequence shown). The coarser thicker and sandier interval represents the turbidite. A turbidite is the deposit left by a turbidity current. A classic and complete turbidite consists of the units listed for the above sample. Together the units represent deposition from a waning turbidity current. The coarser grains found in the A unit are deposited first followed by the remaining units. The laminated B represents deposition from a high flow regime and the development of upper plane beds. Cross laminated C represents deposition under low flow regime. While the upper laminated D unit represents further decreasing flow velocities. Unit E represents the settling out of suspension of the finer particles. Typically you will not encounter the complete Bouma sequence. In a low density turbidity current, the resulting turbidite may consist of the A through E units. However, low density turbidity currents, depending on grain size and distance of transport may only show the upper units, such as the C through E units (listed as Tc, Td, & Te). Downcurrent transport will result in the loss of the lower units. Very dilute turbidites may only show the Td & Te units. High density turbidity currents will show the R1-S3 units.

    1. Make a sketch showing the complete Bouma sequence, label the units, and indicate upper or lower

    flow regime.

    2. Carefully examine the sample and label the turbidite units observed in this sample.

    3.Which way is up?

    4.What are the alternating light and dark laminae, found in the lower half of the sample, composed of?

    5.Do you any ripples, cross lamination or burrows?

    .What are the possible depositional environments? 6

3074A 2478 Graded Bedding. Graded bedding refers to changes in grain size upwards. Normal

    grading is a gradual decrease in average grain size upwards or a gradual decrease in the size of the coarsest particles upwards. Graded bedding is generated by waning density currents and is typically

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    associated with turbidites, which are the deposits of turbidity currents. Decrease in the strength of storm currents can also generate graded bedding. Also, decreasing flow velocities from waters associated with a breach in a river levee can also generate graded bedding in a river floodplain. Reverse grading or inverse grading is a gradual increase in grain size upwards. Reverse grading occurs in beach laminae, cross-beds as a result of grain flows, and in mass flow deposits such as a debris flow. If a bed is normally graded then look for other sedimentary structures associated with turbidity current deposition such as the graded A unit, laminated B unit, the cross laminated C unit, the parallel laminated D unit and the structureless mud unit (E unit). Note the grain alignment in this sample

EDDING PLANE MARKINGSBedding plane markings occur on the underside of beds or soles of beds. B

    Thus they are commonly referred to as sole markings.

    Ehlig Flute casts. Flute casts are a type of sole mark. They are not tool marks but they are scour marks. Turbulent current eddies moving over an irregular surface cut into or scour a semi-consolidated substrate forming depressions (scour marks) that are subsequently infilled by sediment. Irregularities in the surface result in flow acceleration and erosion. Flute casts occur individually or typically in groups. Usually, on an individual sole the flutes tend to be nearly the same size. A rounded and sometimes tightly curved nose at one end characterizes flutes. The deepest part of the flute occurs near the nose. Opposite the nose the flute flares out and dies out. Flutes range from about 5-50 cm in length, 1-20 cm in width, and up to 10 cm in depth. The bulbous or nose end of the flute points up current.

Groove casts Sample 1512. Groove casts are a type of tool mark. Tool marks result from objects

    carried by the current interacting with the bed, rather than by the current itself. For example a stick carried by the current can cut into the underlying bed leaving a groove. Subsequent deposition results in the infilling of the groove, which in turn results in the formation of a relatively straight ridge-like feature at the base of the deposited unit. This feature is called a groove mark. Groove marks occur isolated or in groups. Groove marks give generalized current direction. Objects bouncing on the substrate can produce skip marks which are discontinuous and repeated marks on the substrate. Tool marks are a type of sole mark. Sole marks are structures found as casts on the bases of coarser grained beds interbedded with mudstones.

MARKS GENERATED BY DEFORMATION.

2640 Convolute laminations and bedding consist of irregular folds and contortions of planar laminations

    or cross laminations. These convolutions result from the instability of the layer upon which the laminae were deposited. Typically, the underlying substrate contains water and is not stable when the denser sand is deposited on top of it. The substrate gives way and deformation of the laminae occurs. Associated with convolute laminations are load casts, which are bulbous features found at the bottom of beds. Load casts result when a sand layer sinks into an underlying less dense mud layer. Dewatering structures, such as dish a pillar structures form as a result of the movement of water upwards through the sediment following the deposition or from some stress such as earthquakes, which liquefy a sediment.

1785 and 1778 Soft sediment deformation. Note the deformed cross laminae.

EROSION STRUCTURES

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    PP 5-5 Scours. Erosion structure include channels (structures that show a U-shape or V-shape in cross section and cut across earlier formed bedding and lamination). Channels are formed mostly by currents but may also develop as a result of mass movement. They are common in fluvial and tidal environments. Scours are also erosion structures. They are typically small troughs filled with finer or coarser sediments that cut underlying layers. They are thought to form by currents and are filled with material

    as the current decreases. They are found in fluvial to deep marine environments.

    1. Find what may be very small scale scours in this sample formed of sediment deposited in a deep

    marine environment. Indicate top/bottom direction.

    2. What type of current do you think formed the scours in the deep marine environment?

IOGENIC SBTRUCTURES

Trace fossils Sample MR 1-1A Wheeler Gorge & 302. Markings on and in beds attributed to the

    activities of fossil organisms have long been recognized. These organically produced markings are called trace fossils. The markings result from burrowing, boring, feeding, resting and locomotion activities of organisms. Organisms produce a wide variety of trace fossils. Biogenic structures can be subdivided into bioturbation structures (burrows, tracks, trails, root penetrations structures) derived through organic activity that tends to penetrate, mix, or disturb sediment 2) bioerosion structures (borings, scrapings, bitings) and excrement (coprolites such as fecal pellets or fecal castings). In general, vertical burrows (e.g. clam burrows and shrimp burrows) are more typical of shallow water environments and horizontal burrows (e.g. worm burrows) are more typical of deeper water environment. Caution is required however, because factors such as water depth, oxygen, and food supply control the distribution of organisms. Trace fossils are excellent indicators of relative oxygen levels in the water column at the time the sediment was deposited. In general, larger bodied trace fossils are associated with well-oxygenated environments. Contrastingly, smaller trace fossils are associated with lower oxygen levels. However, both large and smaller bodied trace fossil may occur in the same environment. As mentioned, thorough bioturbation may result in the complete destruction of bedding, leading to a homogenized deposit/bed.

    1. Wet the cut surface of Wheeler Gorge to observe the numerous small chondrites burrows. The

    burrows were made by a worm-like organism that fed on organic material mined from the sediment.

    3385(sample on counter) What do you suppose the linear features on the surface of this rock are? Do you think the sediment was deposited in a deeper water environment? Why?

MISCELLANEOUS STRUCTURES

Mudcracks. Sample on table RNBW-1. Mudcracks are downward pointing, v-shaped structures that

    show a crude polygonal structure. The polygons are centimeters to meters in diameter and the cracks are up to a few centimeters wide and a few 10s of centimeters deep. The mudcracks indicate periodic subaerial exposure and desiccation. Subsequent sedimentation over the mudcracks fills the cracks.

    1. On your sketch indicate top/bottom direction

    Parting Lineations. Sample 304 & 305. Parting lineations or current lineations occur on bedding surfaces of parallel laminated sandstones. They consist of subparallel ridges and grooves. Relief is on

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    the order of a single grain size and the spacing of the lineations is a few millimeters. Paring lineation appears to be most common in deposits composed of medium sand or coarse silt. This parting lineations probably results from the current alignment of the grains. The grains in the sandstone are aligned parallel to the lineations. The lineations are oriented parallel to current direction and are useful as paleocurrent indicators, but the true current direction cannot be discerned.

    1.Sketch both sides of 304. Do the current directions indicated by the parting lineations on both side of the sample agree with each other. Why?

STRUCTURES OF SECONDARY ORIGIN

    ConcretionsSee samples on table.. One of the more common secondary structures in sedimentary rocks are concretions. Concretions are typical formed of calcite, but they are also composed of chert, dolomite, pyrite, hematite or gypsum. They form through the precipitation of minerals around a nucleus, such as a shell fragment. They range from spherical to disc-shaped to pipe-shaped, and range in size from less than a centimeter to as large as three meters. They are not useful as top/bottom or paleocurrent indicators.

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