Basin Histories and Properties

(adsbygoogle = window.adsbygoogle || []).push({}); Basin histories and properties are the geological characteristics and evolutionary processes that shape sedimentary basins. Understanding the history and properties of a basin is important for predicting the distribution and quality of hydrocarbon reservoirs and for developing exploration and production strategies. Time and Events in Basin History Even though it is often incomplete and difficult to discern, the stratigraphic record contains the only available information about the timing and locations of geological events in the evolution of a basin. So, in order to establish a time framework, we must extract relative and absolute age information from strata. These statements may seem counterintuitive. However, regardless of the methods and goals of a basin analysis, whether we are concerned with climatic and paleogeographic aspects of deposition, tectonic and structural events, thermal and maturation history, or diagenesis and petroleum migration, strata contain the record of these events in a temporal and spatial framework. For example, consider the requirements of structural analysis that, at first glance, may seem unrelated to stratigraphic analysis. To construct any type of structural or isopach map, or to identify the position and type of a fault, we must first identify time-correlative surfaces and strata. Clearly, correlating strata is essential to basin analysis. We use many methods to extract time information from strata. Fossils and fossil assemblages are the most commonly used tools for establishing a biostratigraphic correlation. We can also calibrate biostratigraphic divisions of the geological time scale with absolute geochronologic ages using techniques that analyze mineral species. Since the 1960s, isotopic age-dating techniques of primary grains, cements and authigenic minerals have been increasingly important sources of information about absolute ages of source materials, deposition, and diagenetic changes in strata. Other methods -- for example, fission-track dating used since the 1970s, and thermoluminescence and electron spin resonance used since the 1980s -- have also been used to determine the ages and histories of different components of strata. Magnetostratigraphy, the analysis of ages of polarity reversals of Earth’s magnetic field, has become a common method for developing a high-resolution time framework, and is especially important in strata that lack fossils. Ocean floor magnetic reversals are effective in dating the formation of the ocean floor, and for determining the ages of continental rifting. Until the increased interest in sequence stratigraphy in the 1980s, however, relatively little attention was given to calibrating lithostratigraphic units to absolute ages of the geological time scale. This oversight contrasts starkly with two substantive, but largely neglected, merits of physical stratigraphy in basin analysis. First, lithostratigraphic information is the most complete, accessible and commonly available data type of any used in basin analysis; it is inexpensive and can be used by geoscientists without special training or equipment. Second, we can use physical stratigraphy to establish chronostratigraphic correlations with a temporal resolution that is generally greater than that we could achieve using biostratigraphy alone. Dividing stratigraphic successions into unconformity-bounded or tectonostratigraphic units is often the most natural division of sedimentary basins and the most logical choice for selecting mapping horizons. Walther's Law of Facies Successions (adsbygoogle = window.adsbygoogle || []).push({}); FIGURE 1 Walther's Law of Facies Successions Walther’s Law of Facies Successions Figure 1, expresses the relationships that must exist between a vertical succession of facies observed in the stratigraphic record and the contemporaneous, laterally connected, depositional environments through time. In this respect, Walther’s Law states which facies can succeed each other in time and space, and which can not. Only those geomorphological environments that existed side-by-side along a depositional profile can leave their imprint as the sediments of a continuous stratigraphic record. The vertical succession of facies at a particular geographic location is the same as the lateral succession of geomorphological environments along a single depositional profile, as if the sediments within the laterally linked environments were turned vertically on end to form a succession of strata. This traditional way of discussing Walther’s Law emphasizes the lateral connections of depositional environments and the sedimentary facies successions they produce, rather than time relationships among facies and the continuity or discontinuity of vertical facies successions. Alternatively, we can consider Walther’s Law as an extension of, or a special case of, Steno’s Law of Superposition, which states that sediment can be added to depositional surfaces in only one way -- from the top. Time equivalents of multiple facies that compose a vertical succession must have existed along a series of depositional profiles, each superimposed upon the previous one. Thus, older facies are replaced by younger facies in the vertical succession as sediments along successive depositional profiles are added incrementally to successive depositional surfaces. Through time, as facies succeed one another vertically, the more distant geomorphological environments along the depositional profiles move toward the reference site of the vertical profile. Thus, Walther’s Law not only describes which facies can succeed each other laterally and vertically, but dictates the temporal relationships that must exist in any lateral or vertical succession of facies. Walther’s Law is the fundamental concept for dividing stratigraphic successions into natural divisions of time-bounded stratigraphic units. In analyzing well logs and cores, we must pick the Walther’s Law divisions, represented by progradational/aggradational units, within stratigraphic successions. These time units form the basis for making high-resolution correlations, for establishing geometric stacking patterns, and for dividing the stratigraphic record into large-scale, unconformity-bounded depositional sequences. In some environments, for example shelf-slope breaks, submarine fans, and delta fronts, the inclination of the depositional surface is so pronounced that we can notice it by eye. In most other environments, we measure the inclination of the depositional surface in tenths to thousandths of a degree, and we can visualize this inclination only on a map or on a vertically exaggerated cross section. Whether the inclination angle is large or small, virtually all environments possess some depositional topography, given that they occur on a sloping surface that descends from mountains to ocean basins. If the depositional surface is inclined, layers of sediment accumulating parallel to that surface prograde, and facies climb seaward through time. On the scale of sedimentary basins, the Law of Superposition requires that virtually all sediments accumulate by progradation. If the depositional surface is horizontal, then sediments accumulating parallel with that surface aggrade, and facies climb vertically through time. We can recognize episodes of progradation and aggradation as co-eval and as different manifestations of identical processes by viewing the geometry of strata in a foreshortened perspective. We do this using squeeze plots of reflection seismic profiles, or by constructing regional stratigraphic cross sections with considerable vertical exaggeration. In this foreshortened perspective, beds and other time surfaces that would appear horizontal over distances of tens to hundreds of kilometers in normal plots will now appear inclined. The geometries along time surfaces also change in these perspectives; for example, a zone of high aggradation changes into a zone of limited progradation, and a zone of limited aggradation changes into a zone of pronounced progradation. (adsbygoogle = window.adsbygoogle || []).push({}); Geohistory, Backstrip Analyses https://youtu.be/4akxUlvMLXM (adsbygoogle = window.adsbygoogle || []).push({}); We use geohistory and backstrip analyses to reconstruct movements and elevations of stratigraphic units and surfaces through time. Geohistory analyses depict the geologic history of stratigraphic units, and we display the elevation and thickness changes of several stratigraphic units on a single diagram. Backstrip analyses determine the causes of tectonic subsidence of a basin -- that is, subsidence in excess of that induced by sediment and water loads by matching observed tectonic subsidence curves with subsidence curves from geodynamic models. Consequently, we display only the movements of the surface of the basement or of a stratigraphic surface deep within the stratigraphic column. Traditionally, geohistory and backstrip analyses include different sets of corrections and display options, but their philosophical bases and calculation processes are essentially the same. Geohistory analyses, developed and illustrated by van Hinte (1978) following earlier work by Lemoine (1911), include corrections for paleowater depth variations and sediment compaction. Backstrip analyses (Steckler and Watts, 1978) include corrections for lithospheric compensation of sediment and water loads, in addition to paleowater depth variations and sediment compaction. We start with lithologic and age data from a measured stratigraphic column, divided into stratigraphic units whose upper and lower age limits we know or assume. We remove the youngest stratigraphic unit from the column, and then restore subjacent units within this "new" stratigraphic column to their elevations and thicknesses as they existed just before the youngest unit was deposited. We perform restorations by correcting for water depth during deposition, timing and amount of compaction of units, eustatic changes, and lithospheric compensation of the removed stratigraphic unit and water loads Figure 1, Correctinos used in constructing a geohistory or backstrip diagrams. FIGURE 1 Correctinos used in constructing a geohistory or backstrip diagrams Units 1, 2 and 3 were deposited at times T1, T2 and T3 respectively. Starting with a measured stratigraphic column, the youngest unit, Unit 3, is "removed" and a calculated stratigraphic column for time T2 is determined. Potential causes and results of elevation and thickness changes of units between times T2 and T3 are: water depth at deposition of Units 3 and 2 [2 was deposited in greater water depth than 3]; eustatic change [there was a eustatic rise between T2 and T3]; compaction [Units 1 and 2 are thinner at time T3 due to mechanical compaction]; subsidence [the basement at time T3 is at a lower elevation because of tectonic subsidence, lithospheric compensation of Unit 3 and the eustatic rise.). Continuing progressively with each youngest unit, we remove the top stratigraphic layers and restore the "new" stratigraphic column to its conditions just prior to deposition of the unit just removed. We plot each calculated stratigraphic column on a time-depth graph Figure 2 , Schematic geohistory or backstrip diagrams constructed from stratigraphic columns shown in Figure 1. FIGURE 2 Schematic geohistory or backstrip diagrams The diagram on the left was constructed without a correction for eustatic changes and has a relative reference frame. For each time [T1, T2 and T3], the depth to the top of the stratigraphic column is equal to the water depth at the time. The diagram on the right was constructed with a correction for eustatic changes and has an absolute reference frame. In this example, the absolute reference frame is sea level at time T3. For each time, the depth to the top of the stratigraphic column is equal to the water depth at that time plus any changes in eustasy relative to eustasy at time T3. Assuming that these diagrams were constructed using corrections for compensation of surface loads, the heavy line at the base of Unit 1 is the tectonic subsidence curve.). We construct geohistory and backstrip diagrams using different reference frames and dimensions. If we do not apply eustatic corrections, we calculate depths to stratigraphic units relative to the elevation of sea level at each time interval. The diagram has a relative reference frame because eustatic variations cause the absolute elevation of the reference frame to vary for each interval. In this case, the depth to the top of each stratigraphic column is equal to the water depth at that time (Figure 2, left diagram). If we apply eustatic corrections, then we calculate depths to stratigraphic units relative to a chosen absolute reference horizon, such as sea level at a particular past time or at present (Figure 2, right diagram). Commonly, geohistory and backstrip diagrams are one-dimensional, although two- and three-dimensional diagrams are possible. Two- or three-dimensional diagrams may provide greater understanding of the history and shape of basin subsidence, and the elevation and thickness changes of stratigraphic units. We do not always apply all of the corrections we have discussed. Which corrections we apply and how we calculate them depends on the purpose of our analysis, available data, desired accuracy, and computing capabilities. To use all these corrections in an analysis, we must know or estimate paleowater depths, compaction histories, and eustatic changes. We must also identify all sediment and water loads, and calculate lithospheric compensation to these loads. For example, if a eustatic curve for the time period of interest is poorly known, or if eustatic variations are small relative to total subsidence, then we might not correct for eustatic variations. There may be different approaches to calculating a certain correction, and the approach we use may influence the outcome. For example, if we do not know the basin-wide extent of stratigraphic units, then we may not be able to calculate a flexural compensation of sediment loads, and we might use an Airy compensation model instead. Constructing Geohistory and Backstrip Diagrams (adsbygoogle = window.adsbygoogle || []).push({}); In this section, we discuss the steps we follow when constructing geohistory and backstrip diagrams. In this example, we backstrip a column of Cretaceous strata from the eastern Powder River Basin, Wyoming (original data courtesy of T.E. Jordan). Step 1. Select a stratigraphic column: We start with a measured stratigraphic column that usually has a time span of tens of millions of years. We divide the column into stratigraphic units, assigning absolute ages to the upper and lower boundaries of each unit Figure 3 , Stratigraphic column divided into stratigraphic units for calculating the backstrip and geohistory diagrams used as the example in the text and following figures. FIGURE 3 Stratigraphic column Biostratigraphic ages were calibrated with the absolute time scale to arrive at the age values of stratigraphic units [labeled from 1 to 9] shown. Paleobathymetry estimates are from topographic relief of clinoforms. Stratigraphic column is taken from a portion of a stratigraphic cross section.). The time span and stratigraphic thicknesses we select are a function of the accuracy, density and time/space distribution of the available data. The choice of time span and thickness of the stratigraphic column also depends on the accuracy and appropriateness of compaction and compensation models. The stratigraphic column we use to construct a backstrip or geohistory diagram should: (1) contain a sufficient amount of age-related geologic data so that the resulting curves are detailed and convey geologic reality, and (2) be thick enough so that the total elevation changes of units are large relative to estimated calculation errors. Properly dividing the column into stratigraphic units is crucial to the accuracy of the final diagram. Creating too few stratigraphic units results in large errors due to over-averaging, while too many units may unnecessarily increase computation time. First, select unit boundaries that represent major time breaks, such as unconformities and hiatuses. Second, select unit boundaries that have good age constraints. Third, to increase ease of computing decompaction and water-depth corrections, subdivide the major chronostratigraphic units into smaller intervals according to lithology. This approach is also useful for estimating thermal conductivities and for calculating temperatures and maturation through time. Sequence boundaries and surfaces associated with major geographic shifts of progradational events are ideal unit boundaries because these surfaces represent breaks in time, separate disparate lithologies, and provide a high-resolution correlation framework for age dating and for estimating paleowater depths using depositional systems or clinoform topography. Step 2. Plot the accumulation history: We plot the accumulation history using ages and present thicknesses of the units. In the example shown in Figure 4 (Elevation changes of base of Cretaceous stratigraphic column in Figure 3. FIGURE 4 Elevation changes of base of Cretaceous stratigraphic column "Accumulation" shows the accumulation history without any corrections. The "Decompacted" curve was calculated from the accumulation history by correcting for progressive and continuous compaction during burial. The lower, hachured curves were calculated by adding minimum [upper curve] and maximum [lower curve] water depth estimates. The accumulation- and decompaction- corrected curves show depth below the sediment-water or sediment-air interface. The water depth-corrected curves show depth below the water-air interface.), the stratigraphic column is 1735 m thick, and we will assume it had the same thickness at 68.4 Mya. Starting with this column, we remove the youngest unit, and calculate a new column thickness by subtracting the thickness of 80 m. Before this unit was deposited at approximately 69.3 Mya, the stratigraphic column was 1655m thick (1735m−80m). We continue removing stratigraphic units and calculating column thicknesses until we have removed all units. Table 1 shows the depths and thicknesses we calculate for this example. These units form the stratigraphic columns to which we next apply corrections. Because these corrections are applied linearly, we can apply them in any order -- it is not necessary to apply the corrections in the order given below. Age(MA)UnitTopDepth Uncorrected(meters)Depth Decompacted(meters)68.491735173569.381655169571.471415153377.3619951187805695888824525701903375508110213617911217088118Column Base00 (adsbygoogle = window.adsbygoogle || []).push({}); Paleo-Water DepthCorrection (min/max)Tectonic Subsidence(Water-Filled) (meters)Tectonic Subsidence(Air-Filled) (meters)1735/17558606001815/18959755891307/13877604821008/1088614381881/901579333708/758496267229/2791568988/10849340/2000 Table 1: Depths of base of the sediment column calculated in the Cretaceous example geohistory and backstrip diagrams. Units are described in Figure 3. Step 3. Decompact stratigraphic units: To restore stratigraphic units to their depths and thicknesses at specific times in the past, we must progressively decompact units to their former thicknesses at each time interval. To do this, we must know how much each unit has mechanically compacted, and when. We commonly assume that any reduction in porosity is due to progressive and continuous mechanical compaction with burial that is, we assume there has been no diagenetic changes in porosity due to stylolitization, dissolution, or cementation. Given this assumption, and the accumulation history we calculated in Step 2, we can model thickness changes through time using empirical or theoretical porosity/depth curves. First, we determine a relationship between porosity and thickness by assuming that the solid mass of a unit, or the mass of grains plus matrix, is constant during burial. The solid mass of a unit at depth z is equal to [latex]\textrm{Solid mass} = (1 - \phi_Z)T_Z[/latex] [latex]\quad[/latex] (1) where TZ and ϕZ are the thickness and porosity of the unit at depth Z. Assuming constant solid mass during burial, we can equate the solid mass of a particular unit at two different depths, Z1 and Z2, using Equation 1 : [latex]\left( 1-\phi _{z_{1}}\right) T_{z_{1}}=\left( 1-\phi _{Z_{2}}\right) T_{z_{2}}[/latex] [latex]\quad[/latex] (2) By rearranging terms, we can calculate the thickness of a unit at depth Z2, if we know its porosity at depth Z2, and its thickness and porosity at another depth, Z1: [latex]T_{z_{2}}=\dfrac {\left( 1-\phi _{z_{1}}\right) }{\left( 1-\phi _{Z_{2}}\right) }T_{z_{1}}[/latex] [latex]\quad[/latex] (3) If depth Z1 equals the present depth of the unit, we can measure or estimate its thickness, [latex]T_{z_{1}}[/latex], and porosity, [latex]\phi_{z_{1}}[/latex]. To calculate the thickness of a unit when it was at another depth, we need a relationship between porosity and paleoburial depth. Commonly, we estimate this relationship by plotting observed values of porosity versus depth for specific lithologies, and then fitting a generalized curve to these data. This approach assumes that the generalized curve is applicable to past conditions. Sclater and Christie (1980) fit exponential curves to lithology-dependent, porosity/depth data using the following expression: [latex]\phi =\phi _{0}e^{-cz}[/latex] [latex]\quad[/latex] (4) where ϕ = the porosity at burial depth Z, ϕO = the initial porosity (i.e., at Z = 0), and c = a constant The curves and data values are given in Table 2 and Figure 5 (Lithology-dependent exponential porosity-depth curves.). FIGURE 5 Lithology-dependent exponential porosity-depth curves (adsbygoogle = window.adsbygoogle || []).push({}); We can use these generalized curves, or we can fit a curve to our own porosity/depth data. For this example, we used Equation 4 with the constants listed in Table 2. Lithologyφounitlessc(m−1)ρg(gm/cm3)Sandstone0.403.0⋅10−42.65Shale0.505.0⋅10−42.72Limestone0.507.0⋅10−42.71Table 2. Lithology-dependent compaction coefficients and grain densities used in Cretaceous example geohistory and backstrip. For convenience, we display calculated values in a diagram showing the progression of porosities and thicknesses as we remove successive units and decompact the column. Table 3 shows a sample calculation corresponding to the following steps. Calculated changes in elevation of the base of the stratigraphic column are shown in Figure 4. Unit = 4Time = 77.3 Ma Step 1: New Depth=Depth to Unit 4 at 71.4 Ma (Thickness of youngest unit) =1141m−472m =669m Step 2: Midpoint=New Depth−1/2 Thickness of Unit 4 at 71.4 Ma =669m−1/2 ⋅ 160m =589m [latex]\varphi \left( 77,3Ma\right) =0,50e^{\left( -7\times 10^{-4}m^{-1}\times 589m\right) }[/latex] =0,33 Step 3: [latex]Thickness=\dfrac {1-\phi \left( 68,4Ma\right) }{1-\phi \left( 77,3Ma\right) }\times Thickness\left( 68,4Ma\right) [/latex] [latex]\dfrac {1-0,20}{1-0,33}\times 150m=179m[/latex] Depth to Base =Depth to Top+Thickness Depth to Base =584m+179m=763m Table 3: Example decompaction calculation for Unit 4 Formation at 77.3 Mya. Steps correspond to the list of steps for decompaction given in the text. We take the following steps to progressively decompact units: Step 3a: Starting with the complete measured stratigraphic column, remove the youngest unit, shift the column up so that the top of the next youngest unit is at depth zero, and calculate new unit depths (Figure 6, Shifted Column, Example decompaction of units). FIGURE 6 Shifted Column Unit 3 was removed, and all other units were shifted such that the new column top is at a zero depth [z=0]. The heavy tic marks on the right side of this "shifted column" mark the midpoints of the remaining units. These midpoints can be used in calculating the average porosity of the unit, as discussed in the text. We obtain the "decompacted column" by correcting thicknesses of remaining units for the compaction effect caused by the deposition os Unit 3.). This step is equivalent to calculating the accumulation history, but we calculate elevations for each unit. Step 3b: Calculate unit porosities at the new shifted depths using Equation 4. A simple approach is to assume that the average porosity of the unit is equal to the porosity at the midpoint of the unit; this approach is more valid for thin units than for thick ones. Using values for φ and c from Table 2, and letting Z equal the midpoint depth of each unit, calculate a new porosity for each unit. Step 3c: Using the porosities calculated in Step 2, and present unit thicknesses and porosities (i.e., the column at 68.4 Mya), use Equation 3 to calculate new unit thicknesses. To reduce progressive errors in applying Equation 3, let Z1 equal the original depth for each calculation, rather than the depth of the unit from the previously calculated column. Arrange units using these calculated thicknesses to obtain a decompacted column (Figure 6). Plot the new positions of units on the geohistory or backstrip diagrams (Figure 4). We repeat these three steps by removing successively younger units until we have removed all units. Step 4: Correct for paleowater depth variations and construct a geohistory diagram: We use geohistory diagrams to show as accurately as possible the changes in elevations of stratigraphic units and surfaces. The accumulation history and decompacted curves we have calculated to this point show elevation changes of stratigraphic units and surfaces relative to the sea floor or land surface at each time interval (i.e., the top of the stratigraphic column; Figure 4 Because tectonic movement, sedimentation, erosion and eustasy cause elevation changes of the land surface and sea floor relative to a fixed absolute elevation, we must apply corrections to translate the calculated columns into an absolute reference frame. To do this, we begin with a relative reference frame. We use paleobathymetric estimates from fossils, lithofacies, stratigraphic sequences, clinoform topography, or depositional systems to translate column elevations to a reference frame relative to sea level at each time interval. To translate the columns, we add the water depth at the time of deposition of the top of the youngest unit to the stratigraphic unit depths. This correction shifts columns down an amount equal to the paleowater depth. Geohistory diagrams show actual elevation changes of stratigraphic units and surfaces, and are commonly constructed by applying corrections only for sediment compaction and paleowater depth variations. To construct diagrams with an absolute reference frame, we must apply corrections for eustatic variations. Figure 7 shows a typical geohistory diagram for the Cretaceous units. FIGURE 7 Typical geohistory diagram for the Cretaceous units (adsbygoogle = window.adsbygoogle || []).push({}); Step 5: Correct for lithospheric compensation of surface loads and construct a backstrip diagram: On time scales of millions of years, total subsidence within basins is caused by lithospheric responses to tectonic and surface loads. The tectonic subsidence equals the total subsidence minus vertical movements caused by surface loads. To identify the response due to each type of load, we might ask "What would the subsidence of the basin have been had no surface loads been applied?" To answer this question, we build a compensation model, and, knowing total sediment and water loads, calculate the vertical movements due solely to surface loading. To remove the effect of surface loads, we must first determine their magnitudes. To do this, we measure or estimate stratigraphic unit thickness and bulk density, paleowater depth variations, and, if we use a flexural compensation model, the regional distribution of stratigraphic units and shoreline positions through time. The compensation model we choose depends on the degree to which surface loads were compensated isostatically or flexurally, and the available data and computer resources. If loads were compensated flexurally, there is a trade-off between the accuracy in determining responses to loads and the increased computing resources necessary for applying a flexural compensation model. Flexural models can introduce errors, if we have insufficient geological data to constrain the model. Often, incomplete or uncertain data may prevent us from using flexural models, and we apply Airy models instead. In the current example, we use an Airy compensation model. One approach to calculating tectonic subsidence is to ask, "How much would the basin have subsided if no sediment had been deposited, and the basin had been filled with water only?" To answer this question, we remove sediment loads, calculate the uplift caused by this reduced loading, and replace the remaining space below sea level with water (Figure 8; "Water-Filled Basin"). The uplift, w, caused by the removal of sediment with thickness ts, and bulk density rs, is equal to FIGURE 8 Water-Filled Basin [latex]W=\dfrac {\rho _{s}-\rho _{W}}{\rho _{m}-\rho _{W}}t_{s}[/latex] [latex]\quad[/latex] (5) where ρw= the density of water, and ρm= the density of the mantle The depth to "basement" (i.e., the surface at the base of the removed stratigraphic column), Z, is [latex]Z = W_d + t_s - w[/latex] [latex]\quad[/latex] (6) where Wd is the water depth. Substituting Equation 5 into Equation 6 gives an expression for calculating the depth to basement if the basin had been water-filled: [latex]Z=W_{d}+\dfrac {\rho _{m}-\rho _{s}}{\rho _{m}-\rho _{w}}\cdot t_{s}[/latex] [latex]\quad[/latex] (7) Values for depth to basement assuming a water-filled basin for the Cretaceous units are given in Table 1 and are plotted as a backstrip diagram Figure 9, Backstrip diagrams for the Cretaceous units of Figure 3. FIGURE 9 Backstrip diagrams for the Cretaceous units For both graphs, "Total Observed Subsidence" is equivalent to the "Water-Depth Corrected" curve in Figure 4 . The "Water-Filled Subsidence" in the Figure 9 was determined by calculating the isostatic adjustment of the total observed subsidence curve to removal of sediment loads.) and Figure 10, FIGURE 10 Tectonic Subsidence The "Tectonic Subsidence" curve in Figure 10 was determined by calculating the isostatic adjustment of the total observed subsidence curve to removal of sediment and water loads.). For these values, ρm equals 3.3 gm/cm3, ρw equals 1.0 gm/cm3, Wd equals the minimum water depth estimate for each column (Figure 3), and ts equals the decompacted thickness of each column (Table 1). We calculate values for ρs, or column bulk density, for each column using decompaction corrected unit porosities, and grain and water densities: [latex]\rho _{s}=\sum ^{n}_{i=1}\left[ \left( 1-\phi _{1}\right) \rho _{g}+\phi _{1}\rho _{W}\right] [/latex] [latex]\quad[/latex] (8) where n= the number of units in a column ϕi= each unit porosity, and ρg= the grain density Typical values for lithology-specific grain densities used in this example are listed in Table 2. Another approach is to ask, "How much would the basin have subsided if no sediment had been deposited, and the basin were drained of water?" To answer this question, we remove sediment and water loads, calculate the uplift caused by this reduced loading, and replace the remaining space with air (Figure 8; "Air-Filled Basin"). The uplift, w, caused by removal of sediment with thickness ts and bulk density ρs, using water depth Wd and density ρw is equal to: [latex]W=\dfrac {\rho _{w}W_{d}+\rho _{s}t_{s}}{\rho _{m}}[/latex] [latex]\quad[/latex] (9) where ρm is the density of the mantle. Substituting Equation 9 into Equation 6 gives the following expression for depth to basement, Z, assuming no sediment or water loads: [latex]Z=W_{d}+\dfrac {\rho _{w}W_{d}-\left( \rho _{m}-\rho _{s}\right) t_{s}}{\rho _{m}} [/latex] [latex]\quad[/latex] (10) Values for depth to basement assuming an air-filled basin for the Cretaceous units are given in Table 1 , and plotted as a backstrip diagram (Figure 9 and Figure 10). The parameter values used in calculating depths of an air-filled basin are the same as those we used in calculating depths of a water-filled basin. We can divide the total observed subsidence of a basin into three components: 1) subsidence caused by tectonic stresses; 2) subsidence caused by sediment loads; and 3) subsidence caused by water loads (Figure 11). (adsbygoogle = window.adsbygoogle || []).push({}); FIGURE 11 Subsidence caused by water loads FIGURE 11 Tectonic subsidence equals the total observed subsidence minus the subsidence caused by all surficial loads (Equation 9; Figure 10). We also use tectonic subsidence to represent the total observed subsidence minus the subsidence caused only by sediment loads (Equation 7; Figure 9). Which backstrip calculation we use depends on the purpose of our study: do we need actual values to identify tectonic responses, or are we comparing the backstripping results to other curves? For example, if we want to compare the backstrip calculation with the output of a geodynamic model that calculates subsidence curves of a water-filled basin, then we calculate the backstrip assuming a water-filled basin. Similarly, if we want to compare the backstrip calculation with other backstrip diagrams, then we calculate the backstrip using the same assumptions and procedures used in those studies. But, if we need actual values for the response of a basin to tectonic stresses only, then we should calculate subsidence for an air-filled basin. In the Cretaceous example above, after we apply corrections for paleowater depth, the elevations of stratigraphic units and surfaces shown in the geohistory and backstrip diagrams are in a relative reference frame. That is, the zero depth of each diagram is set to the elevation of the water/air interface for marine sediments or the elevation of the sediment/air interface for non-marine sediments. To determine paleo-elevations in an absolute reference frame, we must add eustatic variations to water depths at each time interval. Geohistory and backstrip diagrams that have not been corrected for eustatic variations contain perturbations in shape due to these variations. To increase the accuracy of representing paleo-elevations of stratigraphic units or amounts and timing of tectonic subsidence, we must correct for eustatic variations. (adsbygoogle = window.adsbygoogle || []).push({}); Geohistory, Backstrip Diagram Corrections (adsbygoogle = window.adsbygoogle || []).push({}); To apply eustatic corrections we need a curve that shows the elevations of sea level relative to a fixed reference elevation during the time span of the analysis. We can use the elevation of sea level at present or at a particular time in the past as a reference elevation. Eustatic curves showing absolute sea level through time are not available for much of geologic time, because absolute magnitudes of eustatic variations are poorly known. Instead, we use relative sea-level curves, such as the Haq et al. (1987) curve. Until an absolute eustatic curve becomes available, relative sea-level curves that have been calibrated to a fixed reference elevation provide the most adequate substitute. To correct a geohistory diagram for eustatic variations, we subtract the magnitude of eustatic rise (or add the magnitude of eustatic fall) to the waterdepth corrected depth of each stratigraphic unit (Figure 1, Correction of units in a geohistory diagram for eustatic variations. FIGURE 1 Correction of units in a geohistory diagram for eustatic variations Wd is water depth at time T2 during deposition of Unit 2. ΔSL is the magnitude of eustatic rise at time T2 measured form a fixed reference elevation. ΔSL is positve for a eustatic rise and negative for a eustatic fall. On the left column, depth to Unit 1 at time T2 in a relative reference frame, Zr, is measured from the elevation of eustasy at time T2. In an absolute reference frame, Zr is equal to the sum of ΔSL and the depth to Unit 1 in an absolute reference frame, Za, using Equation 1. [latex]Z_a = Z_r - \Delta SL[/latex] [latex]\quad[/latex] (1) where: Za= the depth to a unit in an absolute reference frame Zr= the depth to a unit in a relative reference frame, and ΔSL = the magnitude of eustatic rise Zr equals the water depth corrected depths of stratigraphic units. ΔSL is positive for a eustatic rise and negative for a eustatic fall. To correct a water-filled backstrip diagram for eustatic variations, we must remove the eustatic component of loading, and we must calculate the lithospheric response to the change in load using an appropriate compensation model. In the case of eustatic rise, we subtract the thickness of the water column due to the rise, and then calculate uplift due to removal of this water load Figure 2, Correction of water-filled backstrip diagram for eustatic variations using an Airy compensation model. FIGURE 2 Correction of water-filled backstrip diagram for eustatic variations using an Airy compensation model Zr is the depth to basement for a water-filled backstrip diagram with a relative reference frame. Za is the eustasy-corrected depth to basement for a water-filled basin with an absolute reference frame.). In the case of an eustatic fall, we add the thickness of the water column lost due to the fall, and then calculate the depression due to addition of this water load. Depth to basement, in an absolute reference frame, Za, equals [latex]Z_a = Z_r - \Delta SL - w[/latex] [latex]\quad[/latex] (2) where: Zr = depth to basement in a relative reference frame, and w = uplift due to removal of the water load For calculating a eustasy-corrected subsidence curve, Zr equals depth to basement corrected for removal of sediment for a water-filled curve. The uplift due to removal of the load of a eustatic rise assuming a water-filled basin is: [latex]w = \dfrac{\rho_w}{\rho_m - \rho_w}\Delta SL[/latex] [latex]\quad[/latex] (3) Substituting Equation 3 into Equation 2 gives an expression for depth to basement for a water-filled basin: [latex]Z_a = Z_r - \dfrac{\rho_w}{\rho_m - \rho_w}\Delta SL[/latex] [latex]\quad[/latex] (4) Substituting Equation 7 into Equation 4 gives a more complete expression for depth to basement in a water-filled basin: [latex]Z = W_d + \dfrac{\rho_m - \rho_s}{\rho_m - \rho_w}\cdot t_s - \dfrac{\rho_w}{\rho_m - \rho_w}\Delta SL[/latex] [latex]\quad[/latex] (5) Geohistory diagrams illustrate the stratigraphic and tectonic history of a section of sedimentary rock, and show process/response histories in a continuous time frame, rather than only in the physical dimension of depth or stratigraphic thickness. In contrast, unconformities and depositional hiatuses are represented by intrastratal surfaces rather than by strata. Therefore, where an unconformity is present within a stratigraphic column, we must model the rise and fall of earlier stratigraphic units and surfaces during the time not represented by rock. To reconstruct the stratigraphic history during the time span represented by an unconformity, we must first infer the processes that formed the unconformity, and second, determine the rates, magnitudes, and timing of the responses to those processes. Unconformities are formed by three general processes: 1) non-accumulation of sediment by sediment bypass; 2) non-accumulation by sediment starvation; or 3) loss of previously deposited sediment mass by subaerial or subaqueous erosion. The different types of unconformities show different thickness and elevation changes of stratigraphic units and surfaces on geohistory diagrams as seen in (Figure 3). FIGURE 3 The different types of unconformities (adsbygoogle = window.adsbygoogle || []).push({}); Each diagram has an absolute reference frame, and thickness changes due to compaction are not illustrated. Time increases to the right, equivalent to previous geohistory displays with age before present increasing to the left. The dashed lines delineate the time span of section not represented by rock, Δt. Tb is the time of deposition of the youngest preserved sediment below the unconformity, Tt is the time of deposition of oldest preserved sediment above the unconformity, and To is the time that sedimentation was initiated. The lines labeled "Base" and "Top" depict elevation changes of the base and top of the stratigraphic column through time. A dashed line marks the top of the stratigraphic column that has been eroded. Within the preserved and eroded stratigraphic column, several "time lines" show elevation changes of sedimentary units within the column. The area below the base of the column is shaded to depict basement movements of subsidence and uplift caused by tectonic and surface loading. The area above the top of the column is patterned to depict processes controlling changes in the sediment-air or sediment-water interface such as sedimentation, erosion, or sediment bypass. Each diagram has an equivalent preserved sediment record, but accumulation histories differ during the time span of unconformity formation due to different processes of unconformity genesis: [a] unconformity caused by base level fall; [b] unconformity caused by tectonic uplift; [c] hiatus caused by sedmient bypass [non-deposition] We must determine which processes occurred because the elevation changes and thicknesses of eroded sedimentary units can influence the results of thermal maturation, decompaction, and tectonic subsidence calculations. The processes that form an unconformity may act singularly or in combination. Previously deposited sediment may be eroded either when base level falls (Figure 3 [a] ), or when tectonic uplift occurs (Figure 3 [b] ). If there is neither erosion nor deposition, a hiatus forms due to either sediment bypass or sediment starvation (Figure 3 [c] ). As illustrated in Figure 3 , the base of a stratigraphic column changes elevation due to tectonic or surface-load induced subsidence or uplift; basal movements indicate long-term changes in accommodation space. The top of a stratigraphic column changes elevation due to the addition or removal of sediment in response to changes in base level or accommodation space. During the time span of the unconformity, Δt, elevation changes at the top and base of a stratigraphic column and the thicknesses of eroded sediment are depicted differently for different processes of unconformity formation. Because there is no preserved section associated with the time of unconformity formation, we have no direct measure of how the unconformity formed. The signature of preserved sediment encasing an unconformity on a geohistory diagram (e.g., To Tt) is non-unique with respect to the processes which formed the unconformity. Consequently, we must infer the processes of unconformity formation from other data such as stratigraphic sequences, depositional systems, successions of depositional systems, geothermometers, and tectonic histories. We then plot the sediment accumulation history on a geohistory diagram using estimates of timing and magnitudes of deposition, non-deposition and erosion of any strata missing at the unconformity surface. Which portions of the time span of the unconformity were depositional, erosional, and non-depositional? Did tectonic uplift occur, and if so, was erosion initiated synchronously with uplift? Was there a time lag between uplift and initiation of erosion? How much sediment was deposited and then eroded? We can best answer these questions by using a sequence-stratigraphic framework to predict which facies and succession of facies may have been deposited and later eroded. The portions of time represented by deposition, non-deposition and erosion depend on geographic position with respect to a base-level transit cycle (Figure 4, Wheeler diagram of a base level transit cycle, or a rise and fall of base level. FIGURE 4 Wheeler diagram of a base level transit cycle (adsbygoogle = window.adsbygoogle || []).push({}); Position of sediment column of Figure 5 is shown and Figure 5, Geohistory diagram including an unconformity caused by a fall in base level [Figure 4]. FIGURE 5 Geohistory diagram including an unconformity Key for shading is shown in Figure 3 [c]. ts is the total thickness of sediment previously deposited, but eroded during base level fall. The bypass line is the line of no net accumulation that is preserved. This line parallels movements of the base of the column, assuming no thickness changes due to compaction.). For example, as the position of the stratigraphic column moves further landward, the duration of time represented by erosion increases. A sequence-stratigraphic framework constrains estimates of depositional environments and duration of erosion and, therefore, constrains estimates of magnitudes of accumulation and erosion during the time span of the unconformity. The magnitude of accumulation during the unconformity equals the maximum thickness of eroded sediment above the bypass line (ts, Figure 5). The rate of accumulation during the time span of the unconformity equals the rate of increase of column thickness above the bypass line. Similarly, the magnitude of erosion equals the maximum thickness of eroded sediment above the bypass line, and the rate of erosion equals the rate of decrease of column thickness above the bypass line. (adsbygoogle = window.adsbygoogle || []).push({}); Alternative Decompaction Approaches (adsbygoogle = window.adsbygoogle || []).push({}); The compaction corrections we use in constructing geohistory and backstrip diagrams can be a substantial portion of the total corrections we make. The decompaction assumptions and methods we choose significantly influence the shape of resulting curves, particularly within the central portion of geohistory and backstrip diagrams. For example, in decompacting a stratigraphic column, we might use an exponential porosity/depth function with lithology-department constants. We might assume that compaction is progressive and continuous during burial, porosity reduction with depth is due to mechanical compaction only, and that the applied porosity/depth function adequately represents past porosity distributions. Alternative approaches that calculate paleo-porosities differently, or that assume diagenetic reduction of porosity during burial, discontinuous compaction, or a different timing of compaction based on depositional environments may yield more accurate representations of compaction histories. One approach is to calculate decompacted thicknesses using an exponential porosity/depth function and integrating, porosity within each unit. Angevine et al. (1990) discuss the application and derivation of this method in detail. We can calculate the solid mass of a unit by integrating the solid masses of columns with differential thicknesses, dZ: [latex]\textrm{Solid mass} = \displaystyle \int \limits _{Z}^{Z+T} (1 - \varphi )\, \textrm{d}Z[/latex] [latex]\quad[/latex] (1) where T is the thickness of the unit and Z is the depth to the top of the. Equating solid masses of the unit at two different depths, Z1 and Z2, gives an expression similar to the equation; [latex](1 - \phi _{Z_{1}}) \, T_{Z_{1}} = (1 - \phi _{Z_{2}}) \, T_{Z_{2}}[/latex] [latex]\displaystyle \int \limits _{Z_{1}}^{Z_{1}+T_{Z_{1}}} (1 - \varphi )\, \textrm{d}Z = \displaystyle \int \limits _{Z_{2}}^{Z_2+T_{Z_{2}}} (1 - \varphi )\, \textrm{d}Z[/latex] [latex]\quad[/latex] (2) Substituting the expression [latex]\Phi =\Phi _o \, e^{-cZ}[/latex] (where c is a constant) into Equation 2, integrating these expressions, and rearranging terms gives an expression for decompacted thickness, TZ2: [latex]T_{Z_{2}} = T_{Z_{1}} + \left( \dfrac{\phi_0}{c} \right)\left( e^{-cZ_{1}} \right) \left( e^{-cT_{Z_{1}}} \right) - \left( \dfrac{\phi_0}{c} \right)\left( e^{-cZ_{2}} \right)\left( e^{-cT_{Z_{2}}} -1 \right)[/latex] [latex]\quad[/latex] (3) Notice that TZ2 occurs on both sides of Equation 3 and cannot be isolated for easier calculation. We can solve this equation by guessing a value for TZ2, entering our guess on the right side of Equation 3 and calculating a new value for TZ2. Then, we take the calculated value and enter it into the right side of the equation, calculating another value. We repeat this process until the value for TZ2 does not change. Another approach is to assume a porosity/depth function other than an exponential curve. We may determine that another function fits the porosity/depth data for our study area better than an exponential curve. For example, Schmoker & Gautier (1989) concluded that there is less scatter on porosity-versus-TTI (time-temperature index) curves for some areas. Middleton (1981) argued that the following porosity/depth function fit data better, particularly in the shallower parts of sedimentary basins: (adsbygoogle = window.adsbygoogle || []).push({}); [latex]\dfrac{1}{\phi} =\dfrac{1}{\phi _0} + cZ[/latex] [latex]\quad[/latex] (4) where ϕ = porosity at depth z, ϕ0 = initial porosity, and c = a lithology-dependent constant Magara (1980) suggested that an exponential relationship is adequate for shales, but a linear relationship better matches sandstone porosity/depth data: [latex]\phi = \phi _0 - cZ[/latex] [latex]\quad[/latex] (5) We present these as examples of empirical functions that have been fit to porosity/depth data. We can use these empirical porosity/depth functions or any other functions that may best fit our data set in a manner similar to the exponential function discussed above to progressively decompact a stratigraphic column. The goal of correcting a stratigraphic section for compaction is to determine the thicknesses, and thus elevations, of stratigraphic units in the past. Because porosity data are the only estimates of thickness changes as strata are buried progressively deeper, we approximate past porosity/depth distributions using present porosity/depth distributions as proxies. We then apply empirical curves fit to present-day porosity/depth data to stratigraphic depths determined from geohistory and backstrip diagrams. Geoscientists have also attempted to determine past porosity distributions by relating present distributions to processes of porosity reduction, and then quantitatively separating processes that reduce unit thicknesses from those that do not. Processes that reduce porosity may include mechanical rearrangement of grains, grain deformation, dissolution, and cementation. Mechanical compaction results in porosity and thickness reduction, but cementation processes may result only in porosity reduction and not thickness reduction if the cement source is external. Porosity-reduction processes may be discontinuous, may occur at different times and stratigraphic positions within a basin, and may be strongly controlled by fluid-flow pressure gradients and pathways during the evolution of sedimentary basins. Moreover, the operation of different processes and the amount of compaction may be dependent upon lithology, depositional environment, and the timing of compaction. For example, a fluvial floodplain mudstone with low porosity may have compacted mechanically soon after deposition while still in its recognizable geomorphological environment, and later cemented during burial. In reality, the thickness of this unit would not change appreciably during burial, but normal application of porosity/depth functions would suggest cumulative compaction on the order of a few tens of percent. By contrast, a lithologically identical marine mudstone may have compacted mechanically, continuously and progressively during burial, and may not have been subjected to significant, externally sourced cementation. The thicknesses we calculate for the marine mudstone unit using an empirical porosity-depth curve might be a good approximation of the actual compaction history. For most studies, it is impossible to exactly reconstruct past porosity-reduction processes. For constructing geohistory and backstripping diagrams, however, determining average conditions and defining uncertainty limits may be sufficient. Gallagher (1989) discussed methods of determining uncertainties in decompaction studies for both mechanical compaction and cementation models. (adsbygoogle = window.adsbygoogle || []).push({}); Assessment Basin Histories and Properties (adsbygoogle = window.adsbygoogle || []).push({}); 1. __________, the analysis of ages of polarity reversals of Earth’s magnetic field, has become a common method for developing a high-resolution time framework. A .BiostratigraphyB .GeostratigraphyC .MagnetostratigraphyD .Stratigraphy 2. Walther’s Law emphasizes: A .time relationships among facies.B .lateral connections of depositional environments.C .continuity or discontinuity of vertical facies successions.D .A, B and C 3. The tectonic subsidence equals the total subsidence minus vertical movements caused by surface loads. (adsbygoogle = window.adsbygoogle || []).push({}); A .TrueB .False 4. Unconformities and depositional hiatuses are represented by: A .strata.B .intrastratal surfaces.C .backstrip diagrams.D .a sequence-stratigraphic framework. 5. _________ processes may result only in porosity reduction and not thickness reduction. A .Grain deformationB .DissolutionC .Mechanical compactionD .Cementation Recommended for You Structural Interpretation Braided Stream Environments Classification of Nonmarine Sandstones Alluvial Fan Environments Meandering Channel Alluvial Systems (adsbygoogle = window.adsbygoogle || []).push({});