{"id":273,"date":"2021-09-16T19:29:01","date_gmt":"2021-09-16T19:29:01","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/accphysicalgeography\/chapter\/6-4-sedimentary-structures-and-fossils-physical-geology-2nd-edition\/"},"modified":"2022-02-03T19:17:08","modified_gmt":"2022-02-03T19:17:08","slug":"6-4-sedimentary-structures-and-fossils-physical-geology-2nd-edition","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/chapter\/6-4-sedimentary-structures-and-fossils-physical-geology-2nd-edition\/","title":{"raw":"6.4 Sedimentary Structures and Fossils \u2014 Physical Geology \u2013 2nd Edition","rendered":"6.4 Sedimentary Structures and Fossils \u2014 Physical Geology \u2013 2nd Edition"},"content":{"raw":"<div>\r\n<div>\r\n<h1 class=\"entry-title\">6.4 Sedimentary Structures and Fossils<\/h1>\r\nThrough careful observation over the past few centuries, geologists have discovered that the accumulation of sediments and sedimentary rocks takes place according to some important geological principles, as follows:\r\n<ul>\r\n \t<li>The\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"283-1488\">principle of original horizontality<\/button><\/strong>\u00a0is that sediments accumulate in essentially horizontal layers. The implication is that tilted sedimentary layers observed to day must have been subjected to tectonic forces.<\/li>\r\n \t<li>The\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"283-1489\">principle of superposition<\/button><\/strong>\u00a0is that sedimentary layers are deposited in sequence, and that unless the entire sequence has been turned over by tectonic processes, the layers at the bottom are older than those at the top.<\/li>\r\n \t<li>The\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"283-1490\">principle of inclusions<\/button><\/strong>\u00a0is that any rock fragments in a sedimentary layer must be older than the layer. For example, the cobbles in a conglomerate must have been formed before the conglomerate was formed.<\/li>\r\n \t<li>The\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"283-1491\">principle of faunal succession<\/button><\/strong>\u00a0is that there is a well-defined order in which organisms have evolved through geological time, and therefore the identification of specific fossils in a rock can be used to determine its age.<\/li>\r\n<\/ul>\r\nIn addition to these principles, that apply to all sedimentary rocks (as well as volcanic rocks), a number of other important characteristics of sedimentary processes result in the development of distinctive sedimentary features in specific sedimentary environments. By understanding the origins of these features, we can make some very useful inferences about the processes that led to deposition the rocks that we are studying.\r\n\r\n<strong><button class=\"glossary-term\" aria-describedby=\"283-1492\">Bedding<\/button><\/strong>, for example, is the separation of sediments into layers that either differ from one another in textures, composition, color, or weathering characteristics, or are separated by <strong><button class=\"glossary-term\" aria-describedby=\"283-1493\">partings<\/button><\/strong>\u2014narrow gaps between adjacent beds (Figure 6.4.1). Bedding is an indication of changes in depositional processes that may be related to seasonal differences, changes in climate, changes in locations of rivers or deltas, or tectonic changes. Partings may represent periods of non-deposition that could range from a few decades to a few millennia. Bedding can form in almost any sedimentary depositional environment.\r\n\r\n<\/div>\r\n<div>\r\n\r\n<img class=\"aligncenter\" src=\"https:\/\/upload.wikimedia.org\/wikipedia\/commons\/5\/51\/Morrison_Formation_%28Upper_Jurassic%3B_Dinosaur_Ridge%2C_west_of_Denver%2C_Colorado%2C_USA%29_1_%288278744810%29.jpg\" \/>\r\n<div id=\"caption-attachment-282\" class=\"wp-caption-text\">Figure 6.4.1 The Morrison Formation (Upper Jurassic in age) at Dinosaur Ridge near Morrison, CO. Bedding is defined by differences in color and texture, and also by partings (gaps) between beds that may otherwise appear to be similar.<\/div>\r\n<\/div>\r\n<strong><span class=\"glossary-term\">Cross-bedding<\/span><\/strong> is bedding that contains angled layers within otherwise horizontal beds, and it forms when sediments are deposited by flowing water or wind. Some examples are shown in Figures 6.0.11, 6.1.7b, and 6.4.2. Cross-beds formed in streams tend to be on the scale of centimeters to tens of centimeters, while those in <strong><span class=\"glossary-term\">aeolian<\/span><\/strong> (wind deposited) sediments can be on the scale of meters to several meters.\r\n<div class=\"wp-caption aligncenter\" style=\"width: 650px\">\r\n\r\n<img class=\"wp-image-267\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Cross-bedded.png\" alt=\"\" width=\"650\" height=\"487\" \/>\r\n<div class=\"wp-caption-text\">Figure 6.4.2 Cross-bedded Jurassic Navajo Formation aeolian sandstone at Zion National Park, Utah. In most of the layers the cross-beds dip down toward the right, implying a consistent wind direction from right to left during deposition.<\/div>\r\n<\/div>\r\nCross-beds form as sediments are deposited on the leading edge of an advancing ripple or dune under steady state conditions (similar flow rate and same flow direction). Each layer is related to a different ripple that advances in the direction of flow, and is partially eroded by the following ripple (Figure 6.4.3). Cross-bedding is a very important sedimentary structure to be able to recognize because it can provide information on the process of deposition, the direction of current flows and, when analyzed in detail, on other features like the rate of flow and the amount of sediment available.\r\n<div class=\"wp-caption aligncenter\" style=\"width: 800px\">\r\n\r\n<img class=\"wp-image-268\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/cross-beds.png\" alt=\"\" width=\"800\" height=\"220\" \/>\r\n<div class=\"wp-caption-text\">Figure 6.4.3 Formation of cross-beds as a series of ripples or dunes migrates with the flow. Each ripple advances forward (right to left in this view) as more sediment is deposited on its leading face (small arrows). (On each ripple the last deposited layer is represented by small dots.)<\/div>\r\n<\/div>\r\n<strong><span class=\"glossary-term\">Graded bedding<\/span><\/strong> is characterized by a gradation in grain size from bottom to top within a single bed. \u201cNormal\u201d graded beds are coarse at the bottom and become finer toward the top.\u00a0 They are a product of deposition from a slowing current (Figure 6.4.4).\u00a0 Most graded beds form in a submarine-fan environment (see Figure 6.4.1), where sediment-rich flows descend periodically from a shallow marine shelf down a slope and onto the deeper sea floor.\u00a0Some graded beds are reversed (coarser at the top), and this normally results from deposition by a fast-moving debris flow (see Chapter 15).\r\n<div class=\"wp-caption aligncenter\" style=\"width: 750px\">\r\n\r\n<img class=\"wp-image-269\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/bouma-2.png\" alt=\"\" width=\"750\" height=\"470\" \/>\r\n<div class=\"wp-caption-text\">Figure 6.4.4 A graded turbidite bed in Cretaceous Spray Formation rocks on Gabriola Island, B.C. The lower several centimeters of sand and silt probably formed over the duration of less than an hour. The upper few centimeters of fine clay may have accumulated over several hundred years.<\/div>\r\n<\/div>\r\n<strong><span class=\"glossary-term\">Ripples<\/span><\/strong>, which are associated with the formation of cross-bedding, may be preserved on the surfaces of sedimentary beds. Ripples can also help to determine flow direction as they tend to have their steepest surface facing in the direction of the flow (see Figure 6.4.3).\r\n\r\nIn a stream environment, boulders, cobbles, and pebbles can become <strong><span class=\"glossary-term\">imbricated<\/span><\/strong>, meaning that they are generally tilted in the same direction. Clasts in streams tend to tilt with their upper ends pointing downstream because this is the most stable position with respect to the stream flow (Figure 6.4.5 and Figure 6.1.7c).\r\n<div class=\"wp-caption aligncenter\" style=\"width: 597px\">\r\n\r\n<img class=\"size-full wp-image-270\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/imbrication-of-clasts.png\" alt=\"\" width=\"597\" height=\"279\" \/>\r\n<div class=\"wp-caption-text\">Figure 6.4.5 An illustration of imbrication of clasts in a fluvial environment.<\/div>\r\n<\/div>\r\n<div><img class=\"aligncenter\" src=\"https:\/\/coloradogeologicalsurvey.org\/wp-content\/uploads\/G-20180722-163733-600x450.jpg\" alt=\"Ripple marks in the Dakota sandstone, Dinosaur Ridge, near Morrison, Colorado. Photo credit: CGS.\" \/><\/div>\r\n<div style=\"text-align: center\">Figure 6.4.6 Ripple marks preserved in sandstone, Dinosaur Ridge, Morrison, CO.<\/div>\r\n<strong><span class=\"glossary-term\">Mud cracks<\/span><\/strong> form when a shallow body of water (e.g., a tidal flat or pond or even a puddle), into which muddy sediments have been deposited, dries up and cracks (Figure 6.4.6). This happens because the clay in the upper mud layer tends to shrink on drying, and so it cracks because it occupies less space when it is dry.\r\n<div class=\"wp-caption aligncenter\" style=\"width: 700px\">\r\n\r\n<img class=\"wp-image-271\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/myvatn-mudcracks-1.jpg\" alt=\"\" width=\"700\" height=\"421\" \/>\r\n<div class=\"wp-caption-text\">Figure 6.4.7 Modern-day mudcracks in volcanic mud at a hot-spring area near Myvatn, Iceland.<\/div>\r\n<\/div>\r\n<div><img class=\"aligncenter\" src=\"https:\/\/coloradogeologicalsurvey.org\/wp-content\/uploads\/G-20120829-201057-600x450.jpg\" alt=\"Fossilized mudcracks in the Mesa Verde Formation near Paonia Reservoir, Gunnison County, Colorado. Photo credit: CGS.\" \/><\/div>\r\n<div style=\"text-align: center\">Figure 6.4.8 Fossilized mudcracks in limestone near Paonia Reservoir, Gunnison County, CO.<\/div>\r\nThe various structures described above are critical to understanding and interpreting the conditions that existed during the formation of sedimentary rocks. In addition to these, geologists also look very closely at sedimentary grains to determine their mineralogy or lithology (in order to make inferences about the type of source rock and the weathering processes), their degree of rounding, their sizes, and the extent to which they have been sorted by transportation and depositional processes.\u00a0 Some of the types of differences that we might want to look for are illustrated in Figure 6.4.7.\r\n<div class=\"wp-caption aligncenter\" style=\"width: 750px\">\r\n\r\n<img class=\"wp-image-272\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/grain-differences.png\" alt=\"\" width=\"750\" height=\"261\" \/>\r\n<div class=\"wp-caption-text\">Figure 6.4.9 Thin section photos of two sandstones with very different grain characteristics. The one on the left has angular grains with a wide range of different types (quartz, feldspar, biotite, rock fragments), and is poorly sorted (grains range from less than 0.05 mm to ~1 mm).\u00a0 The one on the right has relatively well-rounded grains of quartz only, and the size range is much less (approx. 0.25 to 1 mm).\u00a0 (Scale bars are 1 mm.)<\/div>\r\n<\/div>\r\nWe won\u2019t be covering fossils in any detail in this book, but they are extremely important for understanding sedimentary rocks. Of course, fossils can be used to date sedimentary rocks, but equally importantly, they tell us a great deal about the depositional environment of the sediments and the climate at the time. For example, they can help to differentiate marine versus terrestrial environments; estimate the depth of the water; detect the existence of currents; and estimate average temperature and precipitation. For example, the tests of tiny marine organisms (mostly foraminifera) have been recovered from deep-ocean sediment cores from all over the world, and their isotopic signatures have been measured. As we\u2019ll see in Chapter 19, this has provided us with information about the changes in average global temperatures over the past 65 million years.\r\n<div class=\"textbox textbox--exercises\">\r\n<div class=\"textbox__header\">\r\n\r\nSedimentary rocks can tell us a great deal about the environmental conditions that existed during the time of their formation. Make some inferences about the source rock, weathering environment, type and distance of sediment transportation, and deposition conditions that existed during the formation of the following rocks:\r\n<ol>\r\n \t<li><strong>Quartz sandstone:<\/strong> no feldspar, well-sorted and well-rounded quartz grains, cross-bedding<\/li>\r\n \t<li><strong>Feldspathic sandstone and mudstone:<\/strong> feldspar, volcanic fragments, angular grains, repetitive graded bedding from sandstone upwards to mudstone<\/li>\r\n \t<li><strong>Conglomerate:<\/strong> well-rounded pebbles and cobbles of granite and basalt; imbrication<\/li>\r\n \t<li><strong>Breccia:<\/strong> poorly sorted, angular limestone fragments; orange-red matrix<\/li>\r\n<\/ol>\r\nSee Appendix 3 for <a href=\"back-matter-005-appendix-3-answers-to-exercises.html#exercisea6.4\">Exercise 6.4 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\n<h3>Media Attributions<\/h3>\r\n<ul>\r\n \t<li>Figure 6.4.1: Wikimedia Commons<\/li>\r\n \t<li>Figures 6.4.2, 6.4.3, 6.4.4, 6.4.5, 6.4.7., 6.4.9.: \u00a9 Steven Earle. CC BY.<\/li>\r\n \t<li>6.4.6, 6.4.8.: Colorado Geological Survey<\/li>\r\n<\/ul>\r\n<\/div>\r\n<!-- pb_fixme -->","rendered":"<div>\n<div>\n<h1 class=\"entry-title\">6.4 Sedimentary Structures and Fossils<\/h1>\n<p>Through careful observation over the past few centuries, geologists have discovered that the accumulation of sediments and sedimentary rocks takes place according to some important geological principles, as follows:<\/p>\n<ul>\n<li>The\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"283-1488\">principle of original horizontality<\/button><\/strong>\u00a0is that sediments accumulate in essentially horizontal layers. The implication is that tilted sedimentary layers observed to day must have been subjected to tectonic forces.<\/li>\n<li>The\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"283-1489\">principle of superposition<\/button><\/strong>\u00a0is that sedimentary layers are deposited in sequence, and that unless the entire sequence has been turned over by tectonic processes, the layers at the bottom are older than those at the top.<\/li>\n<li>The\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"283-1490\">principle of inclusions<\/button><\/strong>\u00a0is that any rock fragments in a sedimentary layer must be older than the layer. For example, the cobbles in a conglomerate must have been formed before the conglomerate was formed.<\/li>\n<li>The\u00a0<strong><button class=\"glossary-term\" aria-describedby=\"283-1491\">principle of faunal succession<\/button><\/strong>\u00a0is that there is a well-defined order in which organisms have evolved through geological time, and therefore the identification of specific fossils in a rock can be used to determine its age.<\/li>\n<\/ul>\n<p>In addition to these principles, that apply to all sedimentary rocks (as well as volcanic rocks), a number of other important characteristics of sedimentary processes result in the development of distinctive sedimentary features in specific sedimentary environments. By understanding the origins of these features, we can make some very useful inferences about the processes that led to deposition the rocks that we are studying.<\/p>\n<p><strong><button class=\"glossary-term\" aria-describedby=\"283-1492\">Bedding<\/button><\/strong>, for example, is the separation of sediments into layers that either differ from one another in textures, composition, color, or weathering characteristics, or are separated by <strong><button class=\"glossary-term\" aria-describedby=\"283-1493\">partings<\/button><\/strong>\u2014narrow gaps between adjacent beds (Figure 6.4.1). Bedding is an indication of changes in depositional processes that may be related to seasonal differences, changes in climate, changes in locations of rivers or deltas, or tectonic changes. Partings may represent periods of non-deposition that could range from a few decades to a few millennia. Bedding can form in almost any sedimentary depositional environment.<\/p>\n<\/div>\n<div>\n<p><img decoding=\"async\" class=\"aligncenter\" src=\"https:\/\/upload.wikimedia.org\/wikipedia\/commons\/5\/51\/Morrison_Formation_%28Upper_Jurassic%3B_Dinosaur_Ridge%2C_west_of_Denver%2C_Colorado%2C_USA%29_1_%288278744810%29.jpg\" alt=\"image\" \/><\/p>\n<div id=\"caption-attachment-282\" class=\"wp-caption-text\">Figure 6.4.1 The Morrison Formation (Upper Jurassic in age) at Dinosaur Ridge near Morrison, CO. Bedding is defined by differences in color and texture, and also by partings (gaps) between beds that may otherwise appear to be similar.<\/div>\n<\/div>\n<p><strong><span class=\"glossary-term\">Cross-bedding<\/span><\/strong> is bedding that contains angled layers within otherwise horizontal beds, and it forms when sediments are deposited by flowing water or wind. Some examples are shown in Figures 6.0.11, 6.1.7b, and 6.4.2. Cross-beds formed in streams tend to be on the scale of centimeters to tens of centimeters, while those in <strong><span class=\"glossary-term\">aeolian<\/span><\/strong> (wind deposited) sediments can be on the scale of meters to several meters.<\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 650px\">\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-267\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Cross-bedded.png\" alt=\"\" width=\"650\" height=\"487\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Cross-bedded.png 697w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Cross-bedded-300x225.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Cross-bedded-65x49.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Cross-bedded-225x169.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Cross-bedded-350x262.png 350w\" sizes=\"auto, (max-width: 650px) 100vw, 650px\" \/><\/p>\n<div class=\"wp-caption-text\">Figure 6.4.2 Cross-bedded Jurassic Navajo Formation aeolian sandstone at Zion National Park, Utah. In most of the layers the cross-beds dip down toward the right, implying a consistent wind direction from right to left during deposition.<\/div>\n<\/div>\n<p>Cross-beds form as sediments are deposited on the leading edge of an advancing ripple or dune under steady state conditions (similar flow rate and same flow direction). Each layer is related to a different ripple that advances in the direction of flow, and is partially eroded by the following ripple (Figure 6.4.3). Cross-bedding is a very important sedimentary structure to be able to recognize because it can provide information on the process of deposition, the direction of current flows and, when analyzed in detail, on other features like the rate of flow and the amount of sediment available.<\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 800px\">\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-268\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/cross-beds.png\" alt=\"\" width=\"800\" height=\"220\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/cross-beds.png 1024w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/cross-beds-300x82.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/cross-beds-768x211.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/cross-beds-65x18.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/cross-beds-225x62.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/cross-beds-350x96.png 350w\" sizes=\"auto, (max-width: 800px) 100vw, 800px\" \/><\/p>\n<div class=\"wp-caption-text\">Figure 6.4.3 Formation of cross-beds as a series of ripples or dunes migrates with the flow. Each ripple advances forward (right to left in this view) as more sediment is deposited on its leading face (small arrows). (On each ripple the last deposited layer is represented by small dots.)<\/div>\n<\/div>\n<p><strong><span class=\"glossary-term\">Graded bedding<\/span><\/strong> is characterized by a gradation in grain size from bottom to top within a single bed. \u201cNormal\u201d graded beds are coarse at the bottom and become finer toward the top.\u00a0 They are a product of deposition from a slowing current (Figure 6.4.4).\u00a0 Most graded beds form in a submarine-fan environment (see Figure 6.4.1), where sediment-rich flows descend periodically from a shallow marine shelf down a slope and onto the deeper sea floor.\u00a0Some graded beds are reversed (coarser at the top), and this normally results from deposition by a fast-moving debris flow (see Chapter 15).<\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 750px\">\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-269\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/bouma-2.png\" alt=\"\" width=\"750\" height=\"470\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/bouma-2.png 872w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/bouma-2-300x188.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/bouma-2-768x481.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/bouma-2-65x41.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/bouma-2-225x141.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/bouma-2-350x219.png 350w\" sizes=\"auto, (max-width: 750px) 100vw, 750px\" \/><\/p>\n<div class=\"wp-caption-text\">Figure 6.4.4 A graded turbidite bed in Cretaceous Spray Formation rocks on Gabriola Island, B.C. The lower several centimeters of sand and silt probably formed over the duration of less than an hour. The upper few centimeters of fine clay may have accumulated over several hundred years.<\/div>\n<\/div>\n<p><strong><span class=\"glossary-term\">Ripples<\/span><\/strong>, which are associated with the formation of cross-bedding, may be preserved on the surfaces of sedimentary beds. Ripples can also help to determine flow direction as they tend to have their steepest surface facing in the direction of the flow (see Figure 6.4.3).<\/p>\n<p>In a stream environment, boulders, cobbles, and pebbles can become <strong><span class=\"glossary-term\">imbricated<\/span><\/strong>, meaning that they are generally tilted in the same direction. Clasts in streams tend to tilt with their upper ends pointing downstream because this is the most stable position with respect to the stream flow (Figure 6.4.5 and Figure 6.1.7c).<\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 597px\">\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-270\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/imbrication-of-clasts.png\" alt=\"\" width=\"597\" height=\"279\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/imbrication-of-clasts.png 597w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/imbrication-of-clasts-300x140.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/imbrication-of-clasts-65x30.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/imbrication-of-clasts-225x105.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/imbrication-of-clasts-350x164.png 350w\" sizes=\"auto, (max-width: 597px) 100vw, 597px\" \/><\/p>\n<div class=\"wp-caption-text\">Figure 6.4.5 An illustration of imbrication of clasts in a fluvial environment.<\/div>\n<\/div>\n<div><img decoding=\"async\" class=\"aligncenter\" src=\"https:\/\/coloradogeologicalsurvey.org\/wp-content\/uploads\/G-20180722-163733-600x450.jpg\" alt=\"Ripple marks in the Dakota sandstone, Dinosaur Ridge, near Morrison, Colorado. Photo credit: CGS.\" \/><\/div>\n<div style=\"text-align: center\">Figure 6.4.6 Ripple marks preserved in sandstone, Dinosaur Ridge, Morrison, CO.<\/div>\n<p><strong><span class=\"glossary-term\">Mud cracks<\/span><\/strong> form when a shallow body of water (e.g., a tidal flat or pond or even a puddle), into which muddy sediments have been deposited, dries up and cracks (Figure 6.4.6). This happens because the clay in the upper mud layer tends to shrink on drying, and so it cracks because it occupies less space when it is dry.<\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 700px\">\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-271\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/myvatn-mudcracks-1.jpg\" alt=\"\" width=\"700\" height=\"421\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/myvatn-mudcracks-1.jpg 1024w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/myvatn-mudcracks-1-300x180.jpg 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/myvatn-mudcracks-1-768x462.jpg 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/myvatn-mudcracks-1-65x39.jpg 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/myvatn-mudcracks-1-225x135.jpg 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/myvatn-mudcracks-1-350x211.jpg 350w\" sizes=\"auto, (max-width: 700px) 100vw, 700px\" \/><\/p>\n<div class=\"wp-caption-text\">Figure 6.4.7 Modern-day mudcracks in volcanic mud at a hot-spring area near Myvatn, Iceland.<\/div>\n<\/div>\n<div><img decoding=\"async\" class=\"aligncenter\" src=\"https:\/\/coloradogeologicalsurvey.org\/wp-content\/uploads\/G-20120829-201057-600x450.jpg\" alt=\"Fossilized mudcracks in the Mesa Verde Formation near Paonia Reservoir, Gunnison County, Colorado. Photo credit: CGS.\" \/><\/div>\n<div style=\"text-align: center\">Figure 6.4.8 Fossilized mudcracks in limestone near Paonia Reservoir, Gunnison County, CO.<\/div>\n<p>The various structures described above are critical to understanding and interpreting the conditions that existed during the formation of sedimentary rocks. In addition to these, geologists also look very closely at sedimentary grains to determine their mineralogy or lithology (in order to make inferences about the type of source rock and the weathering processes), their degree of rounding, their sizes, and the extent to which they have been sorted by transportation and depositional processes.\u00a0 Some of the types of differences that we might want to look for are illustrated in Figure 6.4.7.<\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 750px\">\n<p><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-272\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/grain-differences.png\" alt=\"\" width=\"750\" height=\"261\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/grain-differences.png 1024w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/grain-differences-300x104.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/grain-differences-768x267.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/grain-differences-65x23.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/grain-differences-225x78.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/grain-differences-350x122.png 350w\" sizes=\"auto, (max-width: 750px) 100vw, 750px\" \/><\/p>\n<div class=\"wp-caption-text\">Figure 6.4.9 Thin section photos of two sandstones with very different grain characteristics. The one on the left has angular grains with a wide range of different types (quartz, feldspar, biotite, rock fragments), and is poorly sorted (grains range from less than 0.05 mm to ~1 mm).\u00a0 The one on the right has relatively well-rounded grains of quartz only, and the size range is much less (approx. 0.25 to 1 mm).\u00a0 (Scale bars are 1 mm.)<\/div>\n<\/div>\n<p>We won\u2019t be covering fossils in any detail in this book, but they are extremely important for understanding sedimentary rocks. Of course, fossils can be used to date sedimentary rocks, but equally importantly, they tell us a great deal about the depositional environment of the sediments and the climate at the time. For example, they can help to differentiate marine versus terrestrial environments; estimate the depth of the water; detect the existence of currents; and estimate average temperature and precipitation. For example, the tests of tiny marine organisms (mostly foraminifera) have been recovered from deep-ocean sediment cores from all over the world, and their isotopic signatures have been measured. As we\u2019ll see in Chapter 19, this has provided us with information about the changes in average global temperatures over the past 65 million years.<\/p>\n<div class=\"textbox textbox--exercises\">\n<div class=\"textbox__header\">\n<p>Sedimentary rocks can tell us a great deal about the environmental conditions that existed during the time of their formation. Make some inferences about the source rock, weathering environment, type and distance of sediment transportation, and deposition conditions that existed during the formation of the following rocks:<\/p>\n<ol>\n<li><strong>Quartz sandstone:<\/strong> no feldspar, well-sorted and well-rounded quartz grains, cross-bedding<\/li>\n<li><strong>Feldspathic sandstone and mudstone:<\/strong> feldspar, volcanic fragments, angular grains, repetitive graded bedding from sandstone upwards to mudstone<\/li>\n<li><strong>Conglomerate:<\/strong> well-rounded pebbles and cobbles of granite and basalt; imbrication<\/li>\n<li><strong>Breccia:<\/strong> poorly sorted, angular limestone fragments; orange-red matrix<\/li>\n<\/ol>\n<p>See Appendix 3 for <a href=\"back-matter-005-appendix-3-answers-to-exercises.html#exercisea6.4\">Exercise 6.4 answers<\/a>.<\/p>\n<\/div>\n<\/div>\n<h3>Media Attributions<\/h3>\n<ul>\n<li>Figure 6.4.1: Wikimedia Commons<\/li>\n<li>Figures 6.4.2, 6.4.3, 6.4.4, 6.4.5, 6.4.7., 6.4.9.: \u00a9 Steven Earle. CC BY.<\/li>\n<li>6.4.6, 6.4.8.: Colorado Geological Survey<\/li>\n<\/ul>\n<\/div>\n<p><!-- pb_fixme --><\/p>\n","protected":false},"author":32,"menu_order":50,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-273","chapter","type-chapter","status-publish","hentry"],"part":17,"_links":{"self":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/273","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/users\/32"}],"version-history":[{"count":4,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/273\/revisions"}],"predecessor-version":[{"id":1163,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/273\/revisions\/1163"}],"part":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/parts\/17"}],"metadata":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/273\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/media?parent=273"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapter-type?post=273"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/contributor?post=273"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/license?post=273"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}