Changes

Quiet-water deposition, reducing conditions, abundance of burrowing organisms (especially soft-bodied organisms such as polychaete worms), and occasional wave and current action are characteristic of this environment. Deposits within the lake bottoms consist of dark-gray to black organic-rich clays containing scattered silt lenses. In some lakes organic debris, such as large accumulations of freshwater shell, is present where overturning of the lake waters is a common process.

Quiet-water deposition, reducing conditions, abundance of burrowing organisms (especially soft-bodied organisms such as polychaete worms), and occasional wave and current action are characteristic of this environment. Deposits within the lake bottoms consist of dark-gray to black organic-rich clays containing scattered silt lenses. In some lakes organic debris, such as large accumulations of freshwater shell, is present where overturning of the lake waters is a common process.

Elsewhere near-anoxic conditions exist and deposits consist of extremely organic-rich, fine-grained clay. The most common types of stratification include parallel and lenticular laminations, intense bioturbation, and occasionally distorted primary structures.

Elsewhere near-anoxic conditions exist and deposits consist of extremely organic-rich, fine-grained clay. The most common types of stratification include parallel and lenticular laminations, intense [[bioturbation]], and occasionally distorted primary structures.

Although some of the parallel laminations result from alterations in textural properties, most are the product of alternating flocculated and nonflocculated layers. Within the flocculated layers there is commonly an abundance of small cracks and fractures. Most cracks are oriented perpendicular to bedding, but differing orientations are occasionally encountered. These are probably what has been termed syneresis cracks, having developed from expulsion of fluids when internal forces of attraction between particles are greater than internal forces of repulsion between solid phase particles. Microfaunal remains are usually abundant within lacustrine facies but consist of only a small number of ostracod species. Charophytes are encountered in some samples. Early diagenetic inclusions include vivianite, which is normally associated with drifted plant remains or burrow fills; and pyrite, which is extremely common and generally occurs as small cubes or isolated drusy masses.

Although some of the parallel laminations result from alterations in textural properties, most are the product of alternating flocculated and nonflocculated layers. Within the flocculated layers there is commonly an abundance of small cracks and fractures. Most cracks are oriented perpendicular to bedding, but differing orientations are occasionally encountered. These are probably what has been termed syneresis cracks, having developed from expulsion of fluids when internal forces of attraction between particles are greater than internal forces of repulsion between solid phase particles. Microfaunal remains are usually abundant within lacustrine facies but consist of only a small number of ostracod species. Charophytes are encountered in some samples. Early diagenetic inclusions include vivianite, which is normally associated with drifted plant remains or burrow fills; and pyrite, which is extremely common and generally occurs as small cubes or isolated drusy masses.

[[file:M31F7.jpg|thumb|300px|{{figure number|6}}Core photographs of bedding in lacustrine delta fill. core diameter is 13 cm (5 in.). A. Shell debris in lower portion of lacustrine deposits. B. Highly burrowed organic clays of lacustrine deposits. C. X-ray radiograph of core from the lower portions of the lacustrine delta fill showing alternating parallel lamination of silt and clay with abundant burrowing. D. Laminated silty clays in lower portion of lacustrine delta fill. Note the elliptical Fe2CO3 nodule. E. X-ray radiograph of clays in lower portion of lacustrine delta fill in which high sedimentation rates preclude burrowing organisms. F. Well-stratified silty and sandy deposits of the coarser sediments forming the bulk of the lacustrine delta fill. G. Parallel and lenticular laminations common in the upper portion of the delta fill. Quite often clay-filled burrows are common in the capping sequence over lacustrine delta fill. Note extremely large root burrow. H. High organic backswamp clays that accumulate in a poorly drained reducing swamp environment. Organic stringers and peat deposits are common in this environment. I. X-ray radiograph of core taken in backswamp deposit. Note the stringers of organic debris (dark layer) and the early formation of siderite nodules (Fe2CO3). Pyrite is abundant in this setting. J. Silty clays with abundant iron oxide and calcium carbonate nodules that form in well-drained oxidizing swamp environment.<ref name=Colemanandprior_1981 />]]

[[file:M31F7.jpg|thumb|300px|{{figure number|6}}Core photographs of bedding in lacustrine delta fill. core diameter is 13 cm (5 in.). A. Shell debris in lower portion of lacustrine deposits. B. Highly burrowed organic clays of lacustrine deposits. C. X-ray radiograph of core from the lower portions of the lacustrine delta fill showing alternating parallel lamination of silt and clay with abundant burrowing. D. Laminated silty clays in lower portion of lacustrine delta fill. Note the elliptical Fe2CO3 nodule. E. X-ray radiograph of clays in lower portion of lacustrine delta fill in which high sedimentation rates preclude burrowing organisms. F. Well-stratified silty and sandy deposits of the coarser sediments forming the bulk of the lacustrine delta fill. G. Parallel and lenticular laminations common in the upper portion of the delta fill. Quite often clay-filled burrows are common in the capping sequence over lacustrine delta fill. Note extremely large root burrow. H. High organic backswamp clays that accumulate in a poorly drained reducing swamp environment. Organic stringers and peat deposits are common in this environment. I. X-ray radiograph of core taken in backswamp deposit. Note the stringers of organic debris (dark layer) and the early formation of siderite nodules (Fe2CO3). Pyrite is abundant in this setting. J. Silty clays with abundant iron oxide and calcium carbonate nodules that form in well-drained oxidizing swamp environment.<ref name=Colemanandprior_1981 />]]

[[:file:M31F6.jpg|Figure 5]] illustrates the major characteristics of the lacustrine delta-fill sequence. The upper left diagram is a schematic representation of a small distributary that has been diverted into a shallow freshwater lake. The schematic shows lateral relationships of the various facies, indicating that the bulk of the delta-fill forms a wedge of coarse clastics within an overall deposit consisting of fine-grained organic-rich clays from lacustrine and backswamp deposits. The upper right diagram of Figure 6 illustrates a typical vertical sequence, which normally displays a coarsening-upward trend. The lowermost units consist of lacustrine deposits. Quite often these lowermost units of the lacustrine delta consist of large accumulations or biomasses of shell ([[:file:M31F7.jpg|Figure 6A]]). Normally the shell deb is is encased in a matrix of fine-grained organic clays containing abundant pyrite inclusions. As the sedimentation rate within the delta increases, there is generally a decrease in the amount of coarse faunal remains. Organic activity, however, does not normally cease, and burrowing and abundant bioturbation of soft-bodied organisms can totally obliterate the primary structures ([[:file:M31F7.jpg|Figure 6B]]).

[[:file:M31F6.jpg|Figure 5]] illustrates the major characteristics of the lacustrine delta-fill sequence. The upper left diagram is a schematic representation of a small distributary that has been diverted into a shallow freshwater lake. The schematic shows lateral relationships of the various facies, indicating that the bulk of the delta-fill forms a wedge of coarse clastics within an overall deposit consisting of fine-grained organic-rich clays from lacustrine and backswamp deposits. The upper right diagram of Figure 6 illustrates a typical vertical sequence, which normally displays a coarsening-upward trend. The lowermost units consist of lacustrine deposits. Quite often these lowermost units of the lacustrine delta consist of large accumulations or biomasses of shell ([[:file:M31F7.jpg|Figure 6A]]). Normally the shell deb is is encased in a matrix of fine-grained organic clays containing abundant pyrite inclusions. As the sedimentation rate within the delta increases, there is generally a decrease in the amount of coarse faunal remains. Organic activity, however, does not normally cease, and burrowing and abundant [[bioturbation]] of soft-bodied organisms can totally obliterate the primary structures ([[:file:M31F7.jpg|Figure 6B]]).

In most fill deposits, the burrows are sand filled due to the physiological processes of polychaete worms, which burrow through the muds, concentrating sands within their bodies and leaving behind essentially a sand- or silt-filled burrow. As the sedimentation rate continues to increase, laminations of thicker silt and sandy silt layers alternate with thinly laminated organic clays ([[:file:M31F7.jpg|Figure 6C]]). Burrowing persists upward within the deposits, but not to the point of masking primary stratification. Quite often the parallel-laminated clay deposits consist of extremely organic-rich clays, and leaf remains are common along the bedding planes. Color laminations and inclusions of iron carbonate nodules are often common within this part of the delta fill ([[:file:M31F7.jpg|Figure 6D]]).

In most fill deposits, the burrows are sand filled due to the physiological processes of polychaete worms, which burrow through the muds, concentrating sands within their bodies and leaving behind essentially a sand- or silt-filled burrow. As the sedimentation rate continues to increase, laminations of thicker silt and sandy silt layers alternate with thinly laminated organic clays ([[:file:M31F7.jpg|Figure 6C]]). Burrowing persists upward within the deposits, but not to the point of masking primary stratification. Quite often the parallel-laminated clay deposits consist of extremely organic-rich clays, and leaf remains are common along the bedding planes. Color laminations and inclusions of iron carbonate nodules are often common within this part of the delta fill ([[:file:M31F7.jpg|Figure 6D]]).