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Nontectonic structures are critical in determining the original orientation of strata. Primary structures like mud cracks, ripples, and sole marks provide insights into the facing direction of rock sequences. Unlike tectonic structures, these features form in primary depositional environments and may resemble tectonic forms due to soft-sediment folds or other processes. By analyzing sedimentary structures, sedimentary facies, and unconformities, geologists can learn about the geological history, including paleoenvironmental changes. This knowledge aids in better understanding sedimentary environments and the implications for stratigraphy and mineral exploration.
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Nontectonic Structures • Nontectonic structures help to determine the original orientation of strata • Primary structures such as mud cracks, ripples, sole marks, and vesicles help to determine original orientation • These features help us determine the facing direction of a sequence of rocks.
Nontectonic Structures • Structures that formed in primary depositional environments may mimic tectonic structures • Examples ductile flow in water-saturated silt, glacial ice, or evaporites • Primary structures are usually older than tectonic structures and cross-cutting relationships can help differentiate between them
Tectonic vs Nontectonic Structures Soft-sediment folds Slumping Tectonic folds
Primary Sedimentary Structures • Bedding • Bedding Planes • Graded Beds • Cross Bedding
Primary Sedimentary Structures • Mudcracks • Polygonal blocks formed by desiccation and shrinkage of saturated sediments • Taper down from the surface until they terminate • The surface layer may also separate and curl upward
Primary Sedimentary Structures • Ripple Marks • Current (translational) ripple marks form under a prevailing direction of fluid flow • Asymmetrical with the steep side toward downstream • Not useful to determine facing because they have the same shape whether they are upright or overturned • Oscillatory ripple marks form with back-and-forth fluid flow • Symmetrical with pointed peaks showing original facing direction
Primary Sedimentary Structures • Rain Imprints • Can determine facing direction • Tracks and Trails • May be used to determine facing direction • Sole Marks, Scour Marks, and Flute Casts • Are later filled with sediment and can be used to determine facing direction
Primary Sedimentary Structures • Load Casts • Form from dewatering of the underlying sediment from the weight of the newly deposited sediment • Can be used to determine the facing direction • The broadly convex bases of the load casts show the original bottom of the structure
Fossils • Fossils • Provide relative ages • May be used to determine facing direction based on fossil sequences or growing conditions in life assemblages • May also be used as strain indicators
Sedimentary Environments • Sedimentary Facies • Sedimentary rock units vary laterally and vertically as paleoenvironments change. • Facies are separated by composition, texture, sorting, physical and biogenic sedimentary structures. • Walther’s principle - Only those facies that once existed side-by-side can be observed vertically juxtaposed in outcrop.
Unconformities • Unconformity – A break in the sedimentary record where part of a stratigraphic succession is missing. • Produced by erosion or nondeposition • Disconformities • Angular unconformities • Nonconformities
Primary Igneous Structures • Igneous plutons and lava flows may form in shapes that resemble sedimentary features or may tectonically alter previously emplaced country rock
Primary Igneous Structures • Compositional banding • Occurs in igneous rocks and may result from crystal settling, differentiation, fractional crystallization, and multiple parallel intrusions or flows. • Study of these may reveal differentiation or fractional crystallization sequences
Primary Igneous Structures • Vesicles • Cavities left by gas bubbles that form in magma due to pressure release • Vesicles generally accumulate at the top of the magma chamber providing the facing direction
Primary Igneous Structures • Pillow Structures • Form from lava flows into or below water • Vesicular glassy curved tops and V-shaped nonvesicular bases indicate facing direction
Primary Igneous Structures • Contact Metamorphic Zones from lava flows • The metamorphic aureole forms beneath the lava flow and can be used to determine facing direction
Gravity-Related Structures • Landslide and Submarine Flows • May resemble tectonic structures because of the flow patterns of these sediments • Olistostromes are matrix supported bedded sediments • Diamictites are matrix supported sediments with no bedding • Turbidites are unsorted sedimentary deposits produced from rapid flow of sediment
Gravity-Related Structures • Salt Structures • Occur in evaporite sequences of anhydrite, gypsum, or halite • These rocks flow more readily than any other common rock type and can form folded structures under surface temperature and pressure • These structures that move upward and gravitationally intrude overlying sediments are called diapirs.
Impact Structures • Structures with circular or elliptical outlines can be formed from meteorite impacts as apposed to tectonic forces • Shatter cones are produced from brittle deformation and can be good evidence of impacts
Nontectonic Structures • Nontectonic structures can appear to be tectonic features • Nontectonic structures can also be used to determine facing direction, which can aid in the interpretation of tectonic features
