Surface-geologic mapping projects conducted by the Louisiana Geological Survey (LGS) during the past 20 years comprise 1:100,000-scale compilations of 30 × 60 minute geologic quadrangles and 1:24,000-scale field-mapped 7.5-minute geologic quadrangles. The vast majority of these mapping efforts were funded under the STATEMAP component of the National Cooperative Geologic Mapping Program (NCGMP), begun in 1993 and administered by the U.S. Geological Survey (USGS).
The principal goal of this program of geologic mapping for LGS initially was to prepare statewide surface geology coverage at 1:100,000 scale in 30 × 60 minute quadrangle format. This scale was emphasized because it is at the large end of the range of intermediate scales, and preserves abundant detail from source mapping done at larger scales (principally 1:62,500 and 1:24,000) while yet covering relatively large areas. By the close of FY 2013, LGS had completed 30 × 60 minute geologic quadrangle coverage of the entire state (30 sheets total) with a mix of published lithographs and draft open-file compilations.
Since the late 1990s LGS also has prepared 7.5-minute geologic quadrangles at 1:24,000 scale totaling 53 sheets. Forty-three were prepared with STATEMAP support, and the other ten were prepared for the U.S. Army Corps of Engineers within the Fort Polk region, southcentral Louisiana.
State map 2015-2016 deliverables completed and submitted included geological maps and pamphlets covering four 7.5 minute quadrangles in two study areas (Poverty Point area in northeastern Louisiana and the Amite River Valley north-northeast of Denham Springs).
New surface-geologic mapping and digital recompilation of Quaternary geologic units in two 30 × 60 minute quadrangles in northeastern Louisiana. This project covers the remaining area required to complete 30 × 60 minute geologic-quadrangle coverage of the state with a mix of published lithographs and draft open-file maps.
The investigation focuses on the stratigraphic succession across the unconformable contact between Midway and Wilcox groups exposed in the Hanson Brick pit southeast of Mooringsport in Caddo Parish. It includes testing of lignitic samples across the Midway–Wilcox contact for palynologic content and age significance (Paul Heinrich). The surface exposure of the Midway within which the pit is situated lies entirely within the extent of the Mooringsport, LA 7.5 minute quadrangle, which was mapped at 1:24,000 scale with STATEMAP support in fiscal year 2005. The upper 4.2 m (14 ft) of Midway in the pit is a distinctive whitish, leached kaolinitic clay. A few smaller exposures of the Midway lie directly to the west in the southeastern portion of the Potters Point, TX–LA 7.5 minute quadrangle. The Mooringsport and Potters Point surface exposures constitute the entirety of known Midway exposures in Caddo Parish and, apart from small exposures on Prothro and Rayburns salt domes in Bienville Parish some 100 km (60 mi) to the southeast, in the state. A field trip to the Hanson Brick pit exposures was organized by Gary M. Hanson and David Ray Williamson for the 2009 annual convention of the Gulf Coast Association of Geological Societies held in Shreveport.
The investigation includes measurement of section, description, and interpretation of Wilcox Group sediment containing numerous large concretions (Marty Horn), and investigation of mineralogy of Grindstone Bluff concretions (Paul Heinrich, Marty Horn), exposed at Grindstone Bluff northwest of Shreveport. The bluff shows relief of 24 m (80 ft), and marks the southwestern valley wall of Twelvemile Bayou, a tributary of the Red River. The strata exposed in the bluff are mapped as undifferentiable Wilcox Group. The bluff exposure consists of two distinct faces, the lowermost exposing mudstones and conglomeratic sandstones adjacent to the active Twelvemile Bayou channel, the upper exposing sandstone and mudstone and set back farther from the channel. Both exposure faces host ellipsoidal concretions up to 3 m (10 ft) across. Concretions comprise two principal types: those with muddy texture, in which the main cement mineral is siderite; and those with sandy texture, in which the main cement mineral is calcite.
Soils information was explored as a proxy for parent-material attributes correlative
with geologic map units in the compilation of a surface-
geologic map at 1:100,000 scale, to establish a natural boundary between two phases of a prominent stratigraphic unit. The map sheet compiled was a draft of the Shreveport South, LA 30 x 60 Minute Geologic Quadrangle in northwestern Louisiana; the stratigraphic unit in question was the Wilcox Group (Paleocene–Eocene). The surface Wilcox in this area undergoes a transition from a comparatively thin updip formation-rank unit that geologists have mapped as “undifferentiated” to a much thicker, downdip group-rank unit that geologists have differentiated into formation-rank members. Because of the parish format of previous investigations, however, there is a need to establish a natural boundary between these two areas to eliminate the parish-line change in classification that is an artifact of available source maps. An initial rendering of such a boundary was approximated from previous soils mapping as reflected in mosaicked General Soil Maps of Bienville, Caddo, De Soto, and Red River parishes; as it turned out, the only substantial reach coursing through continuous Wilcox outcrop was that in southern Caddo and northern De Soto parishes, whereas elsewhere the undifferentiated and differentiated areas are separated at the surface by overlying younger units.
Samples of the subsoil across the likely transition zone in this area were analyzed for surface-soil and subsoil particle sizes, pH, salinity, organic-matter content, and elemental composition ICP spectroscopy. Results were inconclusive relative to patterns that plausibly could correlate with undifferentiable and differentiable Wilcox Group strata. Consequently, for the first 1:100,000-scale draft compilation of the surface geology of the Shreveport South 30 x 60 minute quadrangle, the boundary traced in southern Caddo and northern De Soto parishes was approximated from the previous soils mapping summarized in the General Soil Maps.