Criteria for Distinguishing Microbial Mats and Earths

Braided-river gravels and sands form important aquifers in the Quaternary fluvioglacial outwash deposits of many parts of the world. A detailed understanding of these deposits is vital for modeling groundwater flow and contaminant transport. A quantitative, 3-D depositional model that can aid characterization of gravelly fluvial aquifers is developed based on existing published information and extensive new data from the Sagavanirktok River in northern Alaska. Sagavanirktok River deposits were studied using trenches, cores, wireline logs, porosity and permeability measurements, and ground-penetrating radar profiles. The mode of origin of the deposits was interpreted using knowledge of: (1) channel geometry and mode of erosion and deposition derived from annual aerial photos, and (2) bed texture and bed topography during erosion–deposition events (floods). Recognition of different scales of bedform and associated stratification is essential to the accurate modeling of fluvial deposits. Within a channel belt, the deposits of compound braid bars, point bars, and major channel fills are represented by compound sets of large-scale inclined strata. These compound sets fine upward, fine upward then coarsen upward, or show little vertical variation in grain size, and commonly have open-framework gravel near their bases. Unit bars and minor channel fills (associated with cross-bar channels) are represented by simple sets of large-scale inclined strata. These simple sets generally fine upward, and open-framework gravel commonly occurs at the bases and downstream ends of these sets. Superimposed simple sets form compound sets. Dunes and bed-load sheets that migrate over bars and in channels are represented by sets of medium-scale trough cross strata and gravelly planar strata, respectively. Cross strata in a medium-scale set can alternate between open-framework and closed-framework gravel. Ripples and upper-stage plane beds are represented by sets of small-scale trough cross-stratified sand and planar-laminated sand, respectively. At the top of the channel belt, these sands contain drifted plant remains, roots, and burrows. The 3-D depositional model represents the geometry and spatial distribution of the different scales of strata that occur in all river deposits. Furthermore, the length: thickness ratios of different scales of strata are similar to the length: height ratios of the formative bed forms (e.g., bars, dunes) and scale with the channel geometry, suggesting that the model can be applied to different scales of river deposits. Distributions of porosity and permeability are related to sediment textures and can be included in the model by predicting the spatial distribution of sediment textures within different scales of strata. Of particular importance is the distribution of high-permeability open-framework gravel strata that may be continuous for tens to hundreds of meters. Permeabilities of open-framework gravels can be two or three orders of magnitude greater than permeabilities of surrounding sediments, and significantly influence fluid flow and contaminant transport within the aquifer. Stochastic predictions of the spatial distribution of different scales of strata and their associated porosities and permeabilities in an aquifer will benefit from site-specific data (e.g., geophysical profiles, borehole logs, wireline logs, and pumping tests) combined with this three-dimensional model of gravelly fluvial deposits.