Evidence for an Active Hydrologic Cycle at Mars through Sedimentary Structure and Texture of its Rock Sample

Active hydrologic cycle is the most important agent that influences the transport, erosion and deposition of sedimentary rock. Many of the characteristics features of sedimentary rocks are therefore a result of their deposition under huge influence of active hydrologic cycle. Therefore, by, identifying the sedimentary texture and structure of a rock that associated with hydraulic processes can give compelling evidence about the presence of water in a particular environment, in this case, Mars. Through sedimentary texture, fabric of sedimentary rock can indicate the presence of hydrologic flow. The fabric of the sedimentary rock, which is a function of grain orientation and packing are able to capture the depositional processes, particularly the flow direction and velocities of the flow itself. A study by Parkash and Middleton (1970), show that sand-size grain deposited by fluid flow tends to become aligned parallel to the current direction. Also, sediment that was deposited by turbidity current and glacial process also exhibit parallel orientation to the direction of the fluid flow (Hiscott and Middleton, 1980). Therefore, if this fabric is found at Mars, there is a high probability that there is a presence of water during the formation of sedimentary rock. Sedimentary structure in rock also is a great indicator of presence of water in the paleonvironment. Since the great majority of sedimentary rocks are laid down under water, depositional structures such as bedding, cross-stratification and lineation are mostly a result of hydraulic process. Although there are some sedimentary structure that are formed both from hydraulic flow and wind flow process, there are some sedimentary structure that are distinct only to hydraulic process such as graded bedding, massive bedding, flaser and lenticular bedding, hummocky cross stratification, parting lineation and erosional structures (Boggs, 2006). Normal graded bedding is characterized wherever the sedimentary particles within a single bed become finer upwards (See Figure 1). This usually occur under the influence of turbidity current that settled down sediment accordingly to grain size at different energy level. Larger grain size will be deposited first and followed by smaller grain size as energy level of the flow decreases. Massive bedding, which lack internal structure, also is proposed as one of the structure that was formed by turbidity current (Kneller and Branney, 1995). Massive bedding lacks internal structure because of homogenous sedimentary particles that flow in turbidity current is deposited very quickly and didn’t get sorted. The action of turbidity current directly indicates the presence of hydraulic flow in the depositional environment. Flaser and lenticular bedding is a stratification structure displaying alternating layers of mud and sand (see Figure 2). It was formed mainly where condition of fluid flow and wave action that cause sand and mud is deposited alternately. For example, in tidal flats and sub tidal environments, and marine delta-front environment, where sediments supply of sand and mud are varied in response to change in velocity. Hummocky cross stratification is a form of cross-bedding that characterized by rolling sets of cross-laminae that are both concave up and convex up that are cross-cutting each other (See Figure 3). Dumas and Arnott (2006) suggests that this structure is formed by strong flows of oscillatory stream that are formed by the action of relatively large storms, such as hurricanes in a body of water. It is commonly found in shallow marine environment and also some lacustrine setting. Parting lineation, a depositional structure that commonly associated with sandstone, consist of a series of low relief closely-spaced ridges (See Figure 4). The sand grains usually are oriented parallel to the lineation axes; indicate the current flow of hydrologic flow, especially at fluvial system. This sedimentary structure will give a good indicator for presence of water at Mars because the only condition that will produce this structure is through the action of active hydrologic flow. At a larger scale, erosional structures such as incised channels and scour fill structure are a good indicator of active hydrologic cycle because these structure are governed mostly by high magnitude of turbulent flow that are able to erode preexisting rock. Incised channels that are usually identified by a U-shape or V-shape in cross section are formed by turbulent flow movement that is very common in tidal and fluvial environment. Scour fill structure are similar to incised channel except it is smaller. It was formed as a result of scour by current that are usually associated with fluvial or glacial origin. Both of this erosional structure need active hydrologic cycle in order to occur, so occurrence of this structure in Mars topography, will directly indicate the presence of water in that particular environment during the erosional event. In principal, the stratigraphic texture and structure of sedimentary rock on Mars must be analogous to Earth at least in a broad way due to the constancy of physical law that govern the behavior of fluids. So, by comparing sedimentary structure of rock form Earth with Mars, this can help to determine whether there is a presence of water in Mars in the ancient past. List of Figures Figure 1: Graded bedding, shown diagrammatically in cross section (Washington Department of Natural Resources Geology, 2006) Figure 2: Flaser and lenticular bedding in cross section (Kansas Geological Survey, 2008) Figure 3: Hummocky cross stratification, in cross section (Dumas and Arnott, 2006) Figure 4: Parting lineation and flow direction (UW-Milwaukee, 2003) Works Cited Dumas, S., & Arnott, R. (2006). Origin of hummocky and swaley cross-stratification− The controlling influence of unidirectional current strength and aggradation rate. Geology, 34, 1073-1076. Hiscott, R., & Middleton, G. (1980). Fabric of coarse deep-water sandstones, Tourelle Formation, Quebec, Canada. Journal of Sedimentary Petrology , 50, 703-722. Kneller, B., & Branney, M. (1995). Sustained high-density turbidity currents and the deposition of thick massive sands. Sedimentology, 42, 607-616. Parkash, B., & Middleton, G. (1970). Downcurrent texturl changes in Ordovician turbidite graywackes. Sedimentology, 14, 259-293. Sam Boggs, J. (2006). Principles of Sedimentology and Stratigraphy. New jersey: Pearson Education.