Vegetation-induced sedimentary structures (VISS) are primary sedimentary structures formed by the interaction of detrital sediment with in situ plants. VISS provide physical evidence of vegetation's fundamental role in mediating sediment accumulation and erosion in clastic depositional environments.[1] VISS can be broken into seven types, five being hydrodynamic and two being decay-related. The simple hydrodynamic VISS are categorized by centroclinal cross strata, scratch semicircles and upturned beds. The complex hydrodynamic VISS are categorized by coalesced scour fills and scour-and-mound beds. The decay-related VISS are categorized by mudstone-filled hollows and downturned beds.[1]
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♫MUSIC♫ MILES O'BRIEN: It's another summer day on the lake. [BOAT SOUNDS] But this is no pleasure cruise. With support from the National Science Foundation, lake ecologist Jasmine Saros and her team from the University of Maine are plying the lake waters of Southwestern Greenland, gathering samples of diatoms to study how climate change is affecting this Arctic ecosystem. JASMINE SAROS: A diatom is a type of algae and it is different from other types of algae because it has what you could call a glass cell wall. [INSTRUMENTS] MILES O'BRIEN: They are gathering water and mud samples from lakes around the edge of the Russell glacier. Saros is particularly interested in diatom population dynamics – how different species thrive or falter under changing conditions. JASMINE SAROS: They can be early indicators of environmental change, so those silica cell walls basically can be deposited in sediments and we can look over thousands of years and see which diatoms were there in the past. [BOAT MOTOR] MILES O'BRIEN: Greenland is an ideal place to take these samples because the Arctic is warming at a relatively fast pace compared to other parts of the globe. Even so, the water is still frigidly cold here, and wet socks are a daily hazard. But, Saros' research shows diatom populations respond to more than just rising temperatures. JASMINE SAROS: When temperature increases you can see a change in how productive plants are on the landscape, and that can have implications for lakes as well because plants secrete organic material that drains into lakes and that affects how much light you'll see on lakes. So, there is several different ways climate change can affect lake ecosystems. MILES O'BRIEN: It is a long, cold, hard miserable day in the field. But when the field work is done, the work is not. Because of the nature of this science, there's a lot of work to do in the lab. Each night they filter out the diatoms and culture them under a mix of different temperature, nutrient and light conditions. But, Saros stresses the work isn't just about diatoms. She's focused on the broader picture. JASMINE SAROS: When we look at changes in diatoms in these lakes it's not because we're so focused on the fact that you know, oh no, diatoms are changing, what's going to happen? It's the bigger implications of that. MILES O'BRIEN: Using diatoms as a tool to better understand how environmental change affects lake ecosystems – for Jasmine Saros, it's well worth the long, cold days on the water. For Science Nation, I'm Miles O'Brien.
Hydrodynamic structures
Upturned beds (vegetation shadow)
Upturned beds are mounds of elongated sediment that are deposited on the lee (downflow) side of an obstruction, containing form-concordant stratification.[2][3] These may also be described as tongue shaped.[4] These beds are formed where plants were once standing and caused a decrease in bed shear, allowing sediment to deposit near their base.[2] This commonly occurs with meandering flow that deposits sediment on the lee side of a plant. Sediment is also accumulated on the front and sides of an obstruction when deposition is high. The presence of upturned beds in cross-section is consistent with modern vegetation shadows.[1]
Scratch circles
Sets of concentric grooves that form in sand or mud by the action of sharp objects anchored at the center of a curvature set. Most likely formed from some sort of free moving plant under water.[2] Plants are bent by the current, causing them to scratch concentric grooves into the adjacent substrate. The grooves are most likely formed in a muddy substrate, which preserved them during deposition of the overlying sediment. These scratch semicircles can record currents in wind or water during ancient deposition.[1][2]
Centroclinal cross strata
Bodies of fine to very fine grained sand that fill symmetrical, cone shaped, depressions centered on tree trunks by swirling flood water. The fill is generally organized into form-concordant, concentric laminae that dip towards the tree.[5] Centroclinal cross strata form from the infilling of scours created around trees by swirling flood water.[5] Studies and piers show us just how the scouring process works. In front of these piers, decelerating flow and friction with the bed material causes a downward pressure gradient that leads to erosive downflow.[2] Downflow is accompanied by horseshoe shaped sediment deposits that generate a U-shaped scour around the front and sides of the obstruction.
Scour-and-mound beds
Diffuse sandy lenses associated with standing vegetation at numerous horizons, within the poorly drained floodplain assemblage containing heterolithic bedding. These typically occur above rooted horizons.[1] Scour-and-mound beds that are between sand and heterolithic bedding would suggests that they are formed in forested areas with standing water. Where sand charged flows rapidly decelerated and deposited large amounts of sediment with only minimal scour.[6]
Coalesced scour fills
Large, internally complex sandstone bodies in well-drained floodplain strata, which superficially resemble small channel bodies. These discrete, locally thickened accumulations are laterally equivalent to thin sheet (crevasse splay) sandstones and are strongly incised into red mudstones.[1] Coalesced scour fills are strongly erosional structures formed where interconnected scours between trees are infilled with sandy sediment during waning flow. Strong incision into underlying strata and downflow tapering suggests that the precursor scours formed in response to vigorous floods across the well-drained floodplain. These essentially are unconstrained by channels and vegetation. The current flow carried only enough sand to fill the scour and blanket the floodplain with a thin sand layer.[1]
Downturned beds and mudstone-filled hollows
Sedimentary structures that appear to "protrude" into underlying strata.[1] Most likely from the decay of entombed plants. These may have a "pothole-like" form. They reflect a prominent component of soft-sediment deformation in overlying and adjacent strata, but may also represent hydrodynamic activity around a plant that was not preserved.[1]
References
- ^ a b c d e f g h i Rygel, M.C., Gibling, B.C. and Calder, J.H. (2004) Vegetation-induced sedimentary structures from fossil forests in the Pennsylvanian, Joggins Formation, Nova Scotia; Sedimentology, v. 51, p. 531-552.
- ^ a b c d e Allen, J.R.L. (1982) Sedimentary Structures: Their Character and Physical Basis, 2. Elsevier, Amsterdam, 663 pp.
- ^ Karcz, I. (1968) Fluviatile obstacle marks from the wadis of the Negev (southern Israel). J. Sed. Petrol., 38, 1000–1012.
- ^ Clemmensen, L.B. (1986) Storm-generated eolian sand shadows and their sedimentary structures, Vejers strand, Denmark, Journal of Sedimentary Research, v. 56, p. 520–527.
- ^ a b Underwood, J.R. and Lambert, W. (1974) Centroclinal cross strata, a distinctive sedimentary structure. J. Sed. Petrol., 44, 1111–1113.
- ^ Gastaldo, R.A. (1986) Implications on the paleoecology of autochthonous lycopsids in clastic sedimentary environments of the Early Pennsylvanian of Alabama. Palaeogeogr. Palaeoclimatol. Palaeoecol., 53, 191–212.
This page was last edited on 7 June 2019, at 17:35