Sedimentary StructuresTypes of Sedimentary StructuresGEOL 342 Sedimentation and StratigraphyLecture 4: Sedimentary structures8 February 2005Assoc. Prof. A. Jay KaufmanSedimentary StructuresSedimentary structures are probably the most critical means of interpreting sedimentaryand post-depositional processes. Their recognition and application are key to definingdepositional environments, geological history, and surface processes. Types of Sedimentary StructuresMany different sedimentary structures correspond to different types of formation. Theyall reflect a series of processes: physical, chemical, and biological. We will discuss mostly thephysical here; in later lasses we’ll go into depth about some chemical and biological structures.Below is a list of the classic break down associated with classification of structures:Type Class Key processes Some examplesPhysicalConstructional Flow of currents &sedimentary settlingRipples, dunes, antidunes,hummocks; normal grading, clastimbrication; laminationErosional Flow of currents (inexcess of deposition)Scours, flutes, guttersExternal action Many Tool marks, rain drops, desiccationcracksModifying Post-depositionalmobilizationSoft-sediment deformation,injections, flame structures, dishes BiologicalConstructional Many Reefs, stromatolites, termitemoundsModifying Many Burrows, foot prints, rootsChemicalPrimary Evaporation Crystal casts & formsSecondary Dissolution &precipitationSoil horizonization, vugs, karstA. Plane beddingThree basic mechanisms can form plane bedding, including 1) sedimentation fromsuspension, 2) horizontal accretion from a moving bedload, and 3) encroachment into the leeof an obstacle. Fine scale bedding (< 1cm) is called lamination.Where would one most likely expect to find plane bed laminations? 1What factors might disrupt fine scale laminations in mudrocks?1) flocculation of clays – clumping before particles settle2) bioturbation – disturbance by organismsB. Bedforms generated by unidirections currentsAs soon as flow attains a force sufficient to erode particles, sediments are transported in aset of structures on the surface of beds called bedforms. There is a predictable sequence ofbedforms that depends on velocity, grains size and depth of flow. Temperature and viscosity (claycontent) can also alter bedforms. Current ripples are generally assymetrical.In sand finer than 0.7mm (coarse sand or silt) the first features to form are ripples.Generally their spacing is 10-20 mm or less, and their height is a few centimeters. As flowvelocity increases the ripples enlarge to form sand waves and then dunes, which have spacingfrom 0.5 to 10m and heights of tens of centimeters to a meter or more.Small irregularities in the bottom cause a slight turbulence as flow is diverted up andaround them such that the flow over an obstacle no longer hugs the bottom, but separates from itat the point of flow separation. The point of flow reattachment is where most erosion occursoften forming troughs with long axes parallel to current flow. The zone of reverse circulation isan area of turbulence and backflow on the lee side of a ripple.Sediment migrating up ripple avalanches down lee face to produces inclined forset bedsthat produce cross-bedding. These forms migrate downstream. The shape of the ripple dependson a balance between the bedload and deposition of suspended load such that if there is littlesuspended load the forests are steeper.2When current velocities are high turbulent flow turns to sheet flow (FR > 1) and planebeds are formed. Antidunes are formed by even higher velocities – bedform in phase withsurface waves and so they migrate by accretion on the upstream side. Often the migration of a ripple is interrupted and the ripple is eroded back and thenburied by a new advancing bedform. Such an interruption produces a tiny erosional surfacebetween cross-strata known as a reactivation surface.C. Bedforms generated by multidirections currentsWave ripples on beaches are formed in a similar manner to current ripples – a rotatingeddy precedes a wave as it moves onshore, precipitating the sand load into troughs and ripples.As the wave crest passes, the eddy rises with the crest and disperses into the backwash. However,because of the bi-directional flow, wave ripples are often symmetrical with sharp peaks and broadtroughs in contrast to current ripplesIn marine environments dominated by tides, structures like herringbone cross-bedding and interference ripples are common. The most distinctive feature of tidal regions are caused by the mixing of sand and mud sized fractions to form lenticular, wavy, and flaser bedding.3D. Bedding plane structuresAnother class of sedimentary structures form on the interface between beds, usually onthe exposed surface of a recently deposited bed before it is buried. These features are usefulbecause they indicate current direction and post-depositional deformation of the sediment. Theseinclude 1) sole marks formed by currentsflute cast – elongate teardrop shaped depression that tapers upstream tool mark – indention of the cohesive mud bottom by a “tool” 2) mudcracks, and 3) raindrop impressions. Geopetal structures indicate the top of beds, and these can be found as:-scoured tops of ripple crests-graded bedding-infilling of fossils-sole marks Soft sediment deformation structures are usually due to a density instability betweendifferent sediments layers. The most common are load structures, irregular bulbous featuresformed with a denser material has sunk into a less dense material.Sometimes balls of sand load downward into mud and are pinched off to formpseudonodules or “ball and pillow” structures. Tongue like protuberances of mud into overlyingsoft sediment is known as flame structures. Finally, deformation of soft sediment leads toconvolute bedding, suggesting intense structural