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Primary and Nontectonic Structures

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Primary and Nontectonic Structures Primary and Nontectonic Structures Differential erosion of bedding surfaces in the Wasatch Formation, Bryce Canyon, Utah. http://www.nps.gov/brca/photo_gallery.html Sedimentary Structures Terminology of Stratification Bedding: Primary layering in a sedimentary rock, formed during deposition, manifested by changes in texture, color, and/or composition; may be emphasized by the presence of parting. Compaction: Squeezing unlithified sedimentary in response to pressure exerted by the weight of overlying layers. Overturned beds: Beds that have been rotated past vertical in an Earth-surface frame of reference; as a consequence, facing is down. Parting: The tendency of sedimentary layers to split or fracture along planes parallel to bedding; parting may be due to weak bonds between beds of difference composition, or may be due to a preference for bed- parallel orientation of clay. Strata: A sequence composed of layers of sedimentary rock. Stratigraphic facing or younging direction: The direction to younger strata, or, in other words, the direction to the depositional top of beds. Bedding Parallel Parting Fissility: Parting forms when beds are unroofed, and uplifted to shallower depths in the crust. Consequently, the load pushing down on the strata decreases and the strata expand slightly. During this expansion, fractures from along weak bedding plane and define the parting. This fracturing reflects the weaker bonds between contrasting lithologies of adjacent beds, or the occurrence of a preferred orientation of sedimentary grains (e.g., mica). The Use of Bedding in Structural Analysis Law of Original Horizontality (Bedding is labeled as S0) Depositional environment: the setting in which the sediment was originally deposited. Stratigraphic facing or younging direction: the direction in which in a sequence are progressively younger. Current direction: the direction in which fluid was flowing during depositions. Facing Indicators Facing indicators allow you to determine whether a bed is right-side-up (facing up) or overturned (facing down) with respect to the Earth’s surface. Graded bedding in a turbidite sequences (Bouma sequence) Bouma sequence Graded Bedding Cross Beds 交錯層 Surface Markings Local environment phenomena, such as rain, desiccation (drying), current traction, and the movement of organisms, affect the surface of a bed of a sediment. If the sediment is unlithified, these phenomena leave an imprint. Animal tracks Clast imbrication Flute casts Mudcracks Raindrop impression Ripple Marks Traction lineation Worm burrows Mudcracks Asymmetric ripple marks Load Cast (Ball-and- pillow structure): 荷重鑄型 Clastic Dikes Disrupted bedding Conformable and Unconformable Contacts Three basic type of contacts: 1. Depositional contacts 2. Fault contacts 3. Intrusive contacts The principal types of unconformities Disconformity(平行不整合或假整合): At a discomformity, beds of rock sequence above and below the unconformity are parallel to one another, but there is a measurable age difference between the two sequences. The disconformity surface represents a period of nondeposition and/or erosion. Angular Unconformity (交角不整合) Strata below the unconformity have a different attitude than strata above the unconformity. Beds below the unconformity are truncated at the unconformity, while beds above the unconformity roughly parallel the unconformity surface. Angular unconformity Caledonides at Siccar Point, Scotland Nonconformity (非整合) Nonconformity is used for unconformities which strata were deposited on a basement of older crystalline rocks. The crystalline rock may be either plutonic or metamorphic. Buttress unconformity(拱壁不整合): onlap unconformity (超覆不整合) Occurs where beds of the younger sequence where deposited in a region of significant predepositional topography. Note that a buttress unconformity differs from an angular unconformity in that the younger layer are truncated at the unconformity surface. Some Features to Identify Unconformities: Scour channels in sediments Unconformable contact between mid-Proterozoic Grenville gneiss (dark gray)and Cambrain sandstone and Pleistocene soils Pitted Pebble Stylolites (壓溶縫合線) In limestones and sandstones that contains some clay, the clay enhances the pressure solution process, Specifically, pressure solution occurs faster where the initial clay concentration is higher. Distinct seams of clay residue develop in the rock. .
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