Changes

[[Dip|Downdip]] from alluvial fans, rivers typically grade first into braided channels then, farther down the alluvial valley toward the coastal plain, into meandering channels. These different channel types can occur in the same river system and produce distinctly different kinds of sandstone bodies.

[[Dip|Downdip]] from alluvial fans, rivers typically grade first into braided channels then, farther down the alluvial valley toward the coastal plain, into meandering channels. These different channel types can occur in the same river system and produce distinctly different kinds of sandstone bodies.

''Braided rivers'' and ''braidplains'' form elongate, tabular, sandy and gravelly deposits composed of braided, sand-filled channels and sand and gravel bars (Figure 3c). They typically consist of coarse sand and gravel with relatively minor amounts of clay. Vertical sequences are composed of stacked, upward-fining channel sands and sand and gravel bars. Lateral trends in these deposits are dominated by an overall tabular geometry bounded by floodplain muds with an internally complex geometry of cross-cutting sands and gravels with subordinate mud-rich beds of varying thickness and dimension. Bar and channel deposits are typically elongate in the paleocurrent direction.

''[[Braided rivers]]'' and ''braidplains'' form elongate, tabular, sandy and gravelly deposits composed of braided, sand-filled channels and sand and gravel bars (Figure 3c). They typically consist of coarse sand and gravel with relatively minor amounts of clay. Vertical sequences are composed of stacked, upward-fining channel sands and sand and gravel bars. Lateral trends in these deposits are dominated by an overall tabular geometry bounded by [[floodplain]] muds with an internally complex geometry of cross-cutting sands and gravels with subordinate mud-rich beds of varying thickness and dimension. Bar and channel deposits are typically elongate in the paleocurrent direction.

''Meandering rivers'' are different in that sand is restricted to a single channel and surrounded by fine-grained sediments (Figure 3d). Sand is concentrated mainly in the channel bottoms and point bars. A vertical sequence through such a channel system frequently has an upward-fining character, starting from the channel lag at the bottom and grading upward into deposits of the adjacent levee and floodplain. Individual meander belts are built of cross-cutting and stacked individual upward-fining sequences often separated laterally by meander loop cutoffs and clay plugs. Multiple meander belts are built by abandonment of an entire river segment (''avulsion'') and by establishment of a new section in another position on the floodplain.

''[[Meandering rivers]]'' are different in that sand is restricted to a single channel and surrounded by fine-grained sediments (Figure 3d). Sand is concentrated mainly in the channel bottoms and point bars. A vertical sequence through such a channel system frequently has an upward-fining character, starting from the [[channel lag]] at the bottom and grading upward into deposits of the adjacent [[levee]] and floodplain. Individual meander belts are built of cross-cutting and stacked individual upward-fining sequences often separated laterally by [[meander loop cutoff]]s and clay plugs. Multiple [[meander belts]] are built by abandonment of an entire river segment (''[[avulsion]]'') and by establishment of a new section in another position on the floodplain.

[[Basic open hole tools#Gamma ray|Gamma ray]], [[Basic open hole tools#Spontaneous potential|SP]], and [[Basic open hole tools#Resistivity|resistivity]] logs through braided channel complexes generally have a blocky character, whereas individual meandering channels have an upward-fining signature except where stacked and cross-cut, where they may exhibit more complex wireline log signatures.

[[Basic open hole tools#Gamma ray|Gamma ray]], [[Basic open hole tools#Spontaneous potential|SP]], and [[Basic open hole tools#Resistivity|resistivity]] logs through braided channel complexes generally have a blocky character, whereas individual meandering channels have an upward-fining signature except where stacked and cross-cut, where they may exhibit more complex wireline log signatures.