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Nonconformity
A "nonconformity" is a type of discontinuity surface where sedimentary rocks lie above crystalline rocks. The dashed line in the picture marks a nonconformity between the Mesoproterozoic Pikes Peak Granite (bottom/right) and the Pennsylvanian conglomerates of the Fountain Formation (top/left), near Denver, CO. The contact in this outcrop (near Red Rocks Amphitheater) is marked by a plaque.
Principle of inclusion
One of the basic principles of stratigraphy is the "principle of inclusion." The principle states that if fragments of a rock unit are found included in a second rock unit, the second unit is younger than the first. In this picture, fragments of the Mesoproterozoic Pikes Peak granite (like the clast on which the pencil is resting) are included in a conglomerate layer of the Pennsylvanian Fountain Formation, CO. We infer that the granite must have existed as a solid rock body that was broken and eroded before and during the deposition of the conglomerate layer. Therefore, this principle helps establishing a relative sequence of events. The principle was already understood by Nicolas Steno, the father of stratigraphy. In his "Prodromus," published in 1669, he writes: "these same bodies had already become hard at the time when the matter of the earth and rock containing them was still fluid. And not only did the earth and rock not produce the bodies contained in them, but they did not even exist as such when those bodies were produced in them” (pp. 15-16).
Clastic dykes
Clastic dykes are sedimentary features consisting of seams of sediment truncating the surrounding host rock or deposit. These dykes can form through different processes. For example, fluidized sand can be injected through fractures, cutting through underlying and overlying deposits. Alternatively, pre-existing open cracks can be passively infilled by accumulation of a different kind of sediment. This picture shows three sub-vertical sandstone dykes, with parallel orientation, cutting through sub-horizontal mudstone and sandstone layers of the Pennsylvanian Fountain Formation, at the Balanced Rock site of Garden of the Gods Park, CO. Although different interpretations have been presented to explain the dykes in this outcrop (e.g., infill after thermal contraction of unconsolidated deposits in a cold environment), fieldwork by GRI scientists suggests that they should be interpreted as injection features, possibly related to seismic activation of small normal faults. Pencil for scale (resting vertically on right dyke).
Recumbent anticline
Anticlinal folding in Pennsylvanian strata of the Belden Fm. (Sweetwater Creek, near Dotsero, CO). Monoclinal to recumbent folds (similar to this one) can be caused by displacement connected with the termination of a thrust fault buried at depth. Some have interpreted this particular fold in this way, linking it with thrust activity during the Laramide orogeny. Note that the lithology of the Belden Fm., which consists of bedded shales, limestone, and evaporites, favors ductile deformation. Layers that tend to resist plastic deformation under stress are termed "competent," and a contrast between competent (e.g., sandstone, chert) and less competent (e.g., shale, gypsum) beds can produce spectacular folding.
Hyperpycnite
You may have heard of turbidites. This type of deposit forms from pulse-like flows, where sediment travels rapidly down a slope because of a gravity contrast with the surrounding fluid. Turbidites typically show an upward decrease in gain size and a vertical succession of structures (massive/planar laminated/ripple cross-laminated) indicative of decreasing energy of the flow. There are, however, some deposits that differ from turbidites by showing a symmetric internal organization, with increasing and then decreasing grainsize, and with structures pointing to increasing and then decreasing energy of the flow. Hyperpycnites are such kind of deposit, forming when a river, laden with sediment during its flooding stage, flows into a basin. The flow typically waxes, reaches peak strength, and then wanes. Therefore, the resulting hyperpycnite reflects the waxing and waning cycle in its internal structure. This picture shows a nice hyperpycnite from the Pennsylvanian Minturn Formation, near McCoy (CO, USA). The bed displays reverse grading in its bottom part and a gradual upward transition from ripple laminated, to planar laminated, to massive (waxing phase). The upper part of the bed shows normal grading and hints of ripple-cross lamination (waning phase).
Sole marks
Sole marks are sedimentary structures found at the base of beds, best seen in surface view. They are typically preserved as casts of incisions/depressions created at the top of the underlying bed and filled with sediment of the overlying bed, often with a grainsize contrast between the beds (sand over mud). Sole marks are common in deposits of sediment gravity flows (like turbidites), where turbulence or bedload transport in an energetic flow can erode or leave marks on the underlying substrate in conjunction with active sedimentation. This picture, showing the base of a bed from the Pennsylvanian Minturn Formation, near McCoy (CO, USA), illustrates several kinds of sole marks: a) Flute casts, elongated scours with a wider round concavity at the upstream end, tapering to a pointed termination down current. Flute casts are formed by turbulent eddies in the flow and are excellent paleocurrent indicators; b) Prod marks, small irregular indentations, caused by particles suspended in the flow that occasionally hit the substrate; c) Groove casts, linear features, parallel to the current direction, formed by particles (like pebbles or sticks) dragged with the bedload and eroding the substrate. Prod marks and groove casts are sometimes called "tool marks," because they form as objects in the flow interact with the substrate. See if you are able to identify the three types of marks in the picture. In which direction do you think the flow was moving, towards the top or the bottom of the image? Lens cap for scale has a diameter of 6 cm.
Bedforms
Bedforms are morphological patterns that form at the interface of flowing fluid and a bed of moving sediment. Typical examples include dunes and ripples. Lateral migration of bedforms can generate sedimentary structures, like ripple cross-lamination or cross-stratification. Geology students often mix up the concept of bedform and sedimentary structure: ripples are bedforms (not sedimentary structures) and ripple cross-lamination is a sedimentary structure (not a bedform). Sometimes, like in this picture of hyperpycnites from the Pennsylvanian Minturn Formation near McCoy (CO, USA), both sedimentary structure and associated bedform are preserved in a bed. The asymmetric profile of unidirectional current ripples (flow towards the left) and foreset laminae dipping to the left are clearly visible at the top of two consecutive sandstone beds, overlain by mudstone beds. Draping of bedforms by finer sediment indicates a decrease in hydrodynamic energy within a rapidly discharged, sediment-laden flow. Pencil for scale.
Clinostratification
The principle of original horizontality is one of the basic principles of stratigraphy. It states that sediments are deposited in essentially horizontal layers, and it was enunciated already by Steno in his Prodromus (1669, p. 30): "...adeoque strata omnia praeter infimum duobus planis horizontis parallelis contineri" (...all strata, therefore, except the lowest, were bounded by two planes parallel to the horizon). There are, however, exceptions to this principle. Strata can be deposited at an angle when sediment is being transported over a sloping substrate. This occurs, for example on the stoss face of sand dunes, on fluvial bars, or, at an even larger scale, on the front of submerged deltas. This type of stratification is known as "clinostratification" and the inclined layers or laminae are called "foresets." This picture shows nice clinoforms (foresets dipping to the left) overlain by horizontal beds, in the Pennsylvanian Minturn Formation (CO, USA). Backpack for scale, about 50 cm in size.
Normal grading
Normal grading is a type of arrangement of particles in a sedimentary bed, where coarser grains lie at the bottom and finer grains at the top, resulting in a general fining-upward size distribution. Several physical processes can produce normal grading, but most commonly it is indicative of a decrease in the energy of the flow transporting the particles. The bed in the lower half of this picture shows normal grading from microconglomerate angular pebbles to granules to coarse sand at the top. Pennsylvanian Minturn Formation (CO, USA), scale in cm increments.