Hummocky cross-stratification is a type of sedimentary structure found in sandstones. It is a form of cross-bedding usually formed by the action of large storms, such as hurricanes. It takes the form of a series of "smile"-like shapes, crosscutting each other. It is only formed at a depth of water below fair-weather wave base and above storm-weather wave base. They are not related to "hummocks" except in shape.
Hummocky cross-stratification from the Book Cliffs, Grand Junction, Colorado.Hummocky cross-stratification from the Book Cliffs, Utah.Hummocky cross-stratification from the Arisag Formation (Silurian), Nova Scotia.
The name was introduced by Harms et al. in 1975. Before this time, these structures were recognized under many different names. When hummocky cross-stratification was founded, it was originally given the name “truncated wave-ripple laminae,” by Campbell (1966, 1971).[1] The main features were listed by Bourgeois (1980), Harms et al. (1982), and Walker (1983),[1] in order to identify the structure. Dott and Bourgeois launched an idealized hummocky stratification sequence. From bottom to top, these include: first-order scoured base (± sole marks); characteristic hummocky zone with several second-order truncation surfaces separating individual undulating lamina sets; a zone of flat laminae; a zone with well-oriented ripple cross-laminae and symmetrical ripple forms; all overlain by a more or less burrowed mudstone or siltstone.[2] Walker (1983) wanted to create a second sequence, but it was decided that this sequence offers the best basis for studying hummocky cross-stratification for the future.[1]
Compositionedit
This structure is commonly found in silt to fine sand. It is typically interbedded with bioturbated mudstone. It commonly contains concretions of abundant mica and plant detritus in the tops of many laminae. This helps indicate a shape sorting.[2] Although hummocky cross-stratification is usually found in shallow marine sedimentary rocks, it has also been found in some lacustrine sedimentary rocks.[1]
Common characteristicsedit
Isolated sandy swale within bioturbated mudrocks of the Pebbley Beach Formation (Permian), New South Wales.
In plan view (seen from above), it takes on the form of hummocks and swales that are circular to elliptical, with long wavelengths (1–5 m) but with low height (tens of centimeters). Laminations drape these hummocks; in cross-section view, these laminations have an upward curvature, and low angle, curved intersections. Hummocky cross-stratification can form in sediments up to about 3 cm in diameter, with near-bed water particle velocities between about 40–100 cm/s.
Formation of structureedit
This structure is formed under a combination of unidirectional and oscillatory flow that is generated by relatively large storm waves in the ocean. Deposition involves fallout from suspension and lateral tractive flow due to wave oscillation. As the large waves drape sand over an irregular scoured surface, this strong storm-wave action erodes the seabed into low hummocks and swales that lack any significant orientation.[1][2] It is usually formed by redeposition below normal fair weather wave base delivered offshore by flooding rivers and shoals by large waves.[2]
Depositional environmentsedit
During ancient times, hummocky cross-stratification was located in shallow marine environments, on the shore face and shelf by waves. It can also form on land during especially large storms when large amounts of water are pushed up onto the tidal flat. These landward deposits feature smaller bed forms due to the attenuation of storm waves as they move onto the land. While it is usually formed in marine settings by the action of storms (e.g.hurricane) it may also be deposited in fluvial strata; a fluvial origin is more likely if the unit solely comprises sand.[3]
^ abcdeSurlyk, F., and N. Noenygaard. "Hummocky Cross-stratification from the Lower Jurassic Hasle Formation of Bornholm, Denmark." Sedimentary Geology 46.3-4 (1986): 259-73. Print.
^ abcdDott, R. H., and Joanne Bourgeois. "Hummocky Stratification: Significance of Its Variable Bedding Sequences." Geological Society of America Bulletin 93.8 (1982): 663-80. Print.
^Woolfe, K.J. (1993). "Devonian depositional environments in the Darwin Mountains: Marine or non-marine?". Antarctic Science. 5 (2): 211–220. Bibcode:1993AntSc...5..211W. doi:10.1017/S0954102093000276. S2CID 129509428.
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April 11, 2024
hummocky, cross, stratification, type, sedimentary, structure, found, sandstones, form, cross, bedding, usually, formed, action, large, storms, such, hurricanes, takes, form, series, smile, like, shapes, crosscutting, each, other, only, formed, depth, water, b. Hummocky cross stratification is a type of sedimentary structure found in sandstones It is a form of cross bedding usually formed by the action of large storms such as hurricanes It takes the form of a series of smile like shapes crosscutting each other It is only formed at a depth of water below fair weather wave base and above storm weather wave base They are not related to hummocks except in shape Hummocky cross stratification from the Book Cliffs Grand Junction Colorado Hummocky cross stratification from the Book Cliffs Utah Hummocky cross stratification from the Arisag Formation Silurian Nova Scotia Contents 1 History 2 Composition 3 Common characteristics 4 Formation of structure 5 Depositional environments 6 See also 7 ReferencesHistory editThe name was introduced by Harms et al in 1975 Before this time these structures were recognized under many different names When hummocky cross stratification was founded it was originally given the name truncated wave ripple laminae by Campbell 1966 1971 1 The main features were listed by Bourgeois 1980 Harms et al 1982 and Walker 1983 1 in order to identify the structure Dott and Bourgeois launched an idealized hummocky stratification sequence From bottom to top these include first order scoured base sole marks characteristic hummocky zone with several second order truncation surfaces separating individual undulating lamina sets a zone of flat laminae a zone with well oriented ripple cross laminae and symmetrical ripple forms all overlain by a more or less burrowed mudstone or siltstone 2 Walker 1983 wanted to create a second sequence but it was decided that this sequence offers the best basis for studying hummocky cross stratification for the future 1 Composition editThis structure is commonly found in silt to fine sand It is typically interbedded with bioturbated mudstone It commonly contains concretions of abundant mica and plant detritus in the tops of many laminae This helps indicate a shape sorting 2 Although hummocky cross stratification is usually found in shallow marine sedimentary rocks it has also been found in some lacustrine sedimentary rocks 1 Common characteristics edit nbsp Isolated sandy swale within bioturbated mudrocks of the Pebbley Beach Formation Permian New South Wales In plan view seen from above it takes on the form of hummocks and swales that are circular to elliptical with long wavelengths 1 5 m but with low height tens of centimeters Laminations drape these hummocks in cross section view these laminations have an upward curvature and low angle curved intersections Hummocky cross stratification can form in sediments up to about 3 cm in diameter with near bed water particle velocities between about 40 100 cm s Formation of structure editThis structure is formed under a combination of unidirectional and oscillatory flow that is generated by relatively large storm waves in the ocean Deposition involves fallout from suspension and lateral tractive flow due to wave oscillation As the large waves drape sand over an irregular scoured surface this strong storm wave action erodes the seabed into low hummocks and swales that lack any significant orientation 1 2 It is usually formed by redeposition below normal fair weather wave base delivered offshore by flooding rivers and shoals by large waves 2 Depositional environments editDuring ancient times hummocky cross stratification was located in shallow marine environments on the shore face and shelf by waves It can also form on land during especially large storms when large amounts of water are pushed up onto the tidal flat These landward deposits feature smaller bed forms due to the attenuation of storm waves as they move onto the land While it is usually formed in marine settings by the action of storms e g hurricane it may also be deposited in fluvial strata a fluvial origin is more likely if the unit solely comprises sand 3 See also editWave base Sedimentary structuresReferences edit a b c d e Surlyk F and N Noenygaard Hummocky Cross stratification from the Lower Jurassic Hasle Formation of Bornholm Denmark Sedimentary Geology 46 3 4 1986 259 73 Print a b c d Dott R H and Joanne Bourgeois Hummocky Stratification Significance of Its Variable Bedding Sequences Geological Society of America Bulletin 93 8 1982 663 80 Print Woolfe K J 1993 Devonian depositional environments in the Darwin Mountains Marine or non marine Antarctic Science 5 2 211 220 Bibcode 1993AntSc 5 211W doi 10 1017 S0954102093000276 S2CID 129509428 nbsp Wikimedia Commons has media related to Hummocky cross stratification Retrieved from https en wikipedia org w index php title Hummocky cross stratification amp oldid 1142157095, wikipedia, wiki, book, books, library,
