Surfaces sequenece boundary and their types
- 1. Surfaces Subaerial Unconformity (Sequence Boundary) Subaerial unconformities can expressed in several different ways, depending on geomorphology and the type of rock or sediment on which they are developed. In siliciclastic systems, downcutting rivers will create incised valleys, and areas between rivers will be subject to weathering and the formation of paleosols. In areas underlain by carbonate rocks, sequence boundaries are expressed by various forms of paleokarst. Erosion and incised valleys Erosion at a sequence boundary can manifest in several ways. At an individual outcrop, there may be marked truncation of meters to tens of meters of underlying strata. Such high-relief surfaces are often associated with incised valleys
- 2. • Channelled incision surface developed on offshore mudstone facies. Overlying facies are siltier and contain evidence of tidal influence. Large dolomite-cemented bodies are also associated with the channelled surface. The channel itself can be traced for at least one mile. Cretaceous Mancos Shale, Woodside, Utah.
- 3. Erosion surfaces are commonly overlain by a lag, often composed of coarse siliciclastic gravel or other material not obviously derived from underlying strata. Chert-pebble conglomerate (in shade), overlying shallow-water carbonates. The base of the conglomerate is a subaerial unconformity. Jurassic Sundance Formation, Tensleep, Wyoming.
- 4. • Paleosols • In areas undergoing subaerial weathering, paleosols form, principally on upland areas between valleys (called interfluves) and on the sides of valleys and terraces within valleys. Typically, the soil surface is perched well above the water table, causing well-drained soils to develop, fostering bleaching and anomalous deep red to brown colors.
- 5. Paleokarst In carbonate rocks, subaerial weathering consists primarily of dissolution, causing a variety of paleokarst features to form, including caves and collapse breccias. Limestone breccia at paleokarst surface. Knox Group, Dandridge, Tennessee..
- 6. • Surface of forced regression • A wave-dominated coast has a concave-up up depositional profile that reflects an equilibrium between sediment supplied to the coast and the characteristics of the waves that strike the coast. During a relative fall in sea level, this equilibrium profile is shifted seaward and downwards, such that the equilibrium profile is above the sediment surface in the upper shoreface, but below the sediment surface in the lower shoreface and seawards. As a result, sediment accumulates in the upper shoreface when sea level falls, but erosion occurs on the lower shoreface through offshore until the sediment surface reaches the level of the equilibrium surface. Once it does, sedimentation can resume in those areas.
- 7. A surface of forced regression, with cross-bedded grainstone shoal facies abruptly overlying deep-subtidal storm-bedded facies. Jacob staff is marked in 10 cm increments. Upper Ordovician Hermitage Formation, Woodbury, Tennessee.
- 8. Flooding surface Flooding surfaces are sharp contacts, with relatively deeper-water facies abruptly overlying shallow-water facies. In many cases, the overlying facies will be finer-grained than the underlying facies, but this is not always true. Flooding surface separating underlying red bayhead-delta facies from overlying white shoreface sandstone. This is one example of a flooding surface where the overlying facies is sandier than the underlying facies. Upper Ordovician Harding Formation, Cañon City, Colorado.
- 9. Combinations of surfaces Some surfaces reflect multiple, distinct sets of sedimentary processes and can therefore be difficult to interpret, at least initially. For example, it is common for subaerial unconformities to be flooded by the ocean during the transgressive systems tract, resulting in a single surface that records subaerial weathering and stratigraphic condensation. Combined sequence boundaries and transgressive surfaces are extremely common in cratonic basins and the updip, slowly subsiding portions of passive margins and distal foreland basins.
- 10. A prominent transgressive surface that is also a subaerial unconformity (sequence boundary) sequence boundary. This surface separates underlying light-colored shallow-subtidal carbonate from overlying darker deep-subtidal carbonate and mudstone. Note the pyritization, visible as a rusty stain, at this surface. This surface has meters of erosional truncation regionally, as well as meteoric cements, and a light carbon isotope spike indicative of a soil. Upper Ordovician Carters Limestone and Hermitage Formation, airport at Nashville, Tennessee. This outcrop has subsequently been removed and is no longer visible.