Print this page
Click to view article in PDF format.
Lineaments and the Separation of Sicily Island from the Chalk Hills by the Ouachita River Valley, Northern Catahoula Parish, Louisiana*
By
Richard P. McCulloh1
Search and Discovery Article #50005 (2003)
*Adapted from Louisiana Geological Survey News, v. 13, no. 1, June, 2003, p. 1-2. Appreciation is expressed to the author and the Louisiana Geological Survey, Chacko J. John, Director and State Geologist.
1Louisiana Geological Survey ([email protected])
Background and General Statement
In a brief section entitled, “Diversion of the Ouachita River near Harrisonburg, La.,” of the Report of 1905 of the Geological Survey of Louisiana (p. 303-304; includes “Fig. 24” on facing page 302), A.C. Veatch (1906) addressed the origin and history of the current course of the Ouachita River flood plain relative to Sicily Island and the Chalk Hills. He set forth the case that the present course was a result of constructional depositional dynamics at the confluence of the Ouachita and Mississippi flood plains in early Quaternary time. During glaciation, he argued, outwash deposition raised the level of the flood plain of the Mississippi and the distal reaches of the flood plains of its tributaries at their confluences with it. At this time the southward-flowing Ouachita ran north and east of Sicily Island. As a result of the voluminous outwash deposition in the Mississippi flood plain, the gap between Sicily Island and the mainland encompassing the Chalk Hills to the west became buried, and was occupied by a flood plain contiguous with that of the Mississippi. This gap was raised as much as 18 m (60 ft) above present stream bottoms, yet was likely somewhat lower than the Mississippi flood plain proper because the depositional cone advancing down the Mississippi course must have been quite large compared to the volume of sediment being transported by the Ouachita system. This difference in elevation made possible the shifting of the Ouachita course into the gap before the onset of the succeeding period of downcutting that sculpted the outwash deposits into the terraces now preserved to the east and north of Sicily Island. The above history inferred by Veatch was his way of explaining the presence of the Catahoula Shoals in the Ouachita River to the west of Sicily Island—after the river shifted and had begun cutting down (Figure 1), it encountered a preexisting low drainage divide between minor north-flowing and south-flowing drainages that had formerly occupied the gap (Figure 2), at the position marked by the shoals. The question, however, remains: what, if anything, originally accounted for the gap itself between Sicily Island and the main region of hills to the west?
|
|
In his extensive monograph on the Lower Mississippi Valley, Fisk (1944) included a short section on interpreted lineaments, which he referred to individually as fault zones and collectively as a regional fracture pattern. It appears that Fisk and his team were working with small-scale black-and-white aerial photography, and traced drainage lineaments discernible on that imagery to come up with the trends. The lineaments resolve as a single pair of nearly orthogonal sets, oriented NE-SW and NW-SE, forming a rectilinear grid in the Mississippi embayment. This appears to have been the earliest such interpretation in Louisiana. A plot of Fisk’s data by Gay (1973) showed two very tight trends with mean orientations of N39oW and N49oE. A digital shaded-relief rendering of Louisiana topography (Figure 3) gives a clear suggestion of two intersecting lineaments, oriented approximately N37oW and N51oE, defining the large eastward-projecting promontory that encompasses the Chalk Hills west of Sicily Island. The apparent lineaments follow the pre-Holocene/Holocene contact along the western valley wall of the Ouachita River flood plain (on the north) and of the Mississippi River flood plain (on the south), and correspond essentially to two of the lineaments interpreted previously by Fisk. Many of the Tertiary geologic contacts mapped by Huner (1939), Chawner (1936), and Fisk (1938) in the area enclosed by these two trends show an overall parallelism to them in the recompilation of state geology at 1:500,000 scale (Snead and McCulloh, 1984). The NW-trending segment suggests that one aspect of the separation of Sicily Island by the Ouachita River valley from the mainland to the west may relate to a fracture trend coincident with this topographic lineament.
ReferencesChawner, W.D., 1936, Geology of Catahoula and Concordia parishes: Department of Conservation, Louisiana Geological Survey, Geological bulletin no. 9, 232 p. plus plates (includes one 1:62,500-scale geologic map). Fisk, H.N., 1938, Geology of Grant and La Salle parishes: Department of Conservation, Louisiana Geological Survey, Geological bulletin no. 10, 246 p. plus plates (includes two 1:62,500-scale geologic maps). Fisk, H.N., 1944, Geological investigation of the alluvial valley of the lower Mississippi River: Vicksburg, Mississippi, U.S. Army Corps of Engineers, 78 p. plus plates. Gay, S.P., Jr., 1973, Pervasive orthogonal fracturing in earth’s continental crust: Tech. Publication No. 2, Salt Lake City, American Stereo Map Company, 121 p. plus appendices. Huner, J., Jr., 1939, Geology of Caldwell and Winn parishes: Department of Conservation, Louisiana Geological Survey, Geological bulletin no. 15, 356 p. plus plates (includes two 1:62,500-scale geologic maps). Snead, J.I., and R.P. McCulloh (compilers), 1984, Geologic map of Louisiana: Baton Rouge, Louisiana Department of Natural Resources, Louisiana Geological Survey, scale 1:500,000. Veatch, A.C., 1906, Geology and underground water resources of northern Louisiana, with notes on adjoining districts, in Geological Survey of Louisiana, Report of 1905: Louisiana State Experiment Station, Geological Survey of Louisiana, Bulletin 4, p. 249–457. |
Figure
1. “Change in Ouachita River drainage near Harrisonburg, La.,”
from Veatch (1906, p. 302, his figure 24).
Figure
2. “Diagram illustrating the deflection of Ouachita River and the cause
of the formation of the Catahoula Shoals near Harrisonburg, La.,”
redrawn from Veatch (1906, p. 303, his figure 25).
Figure3.
Louisiana digital topography, annotated to show features discussed in
text (heavier lines = interpreted lineaments; modified from Chalk Butte
Inc., 1994).