INTRODUCTION

The recent acquisition of both manned and unmanned satellite imagery not only has allowed rapid identification of a variety of geologic phenomena but also has provided new information about the structural history of the Coastal Plain of southern Alabama. Representative of the types of satellite imagery now available are three photographs that are described in this paper. These photographs were provided by the National Aeronautics and Space Administration (NASA), Houston, Texas, and the Earth Resources Observational Satellite (EROS) Data Center, Sioux Falls, South Dakota. They consist of both Apollo 7 photography and imagery obtained from the Earth Resources Technology Satellite (ERTS-1), which is presently in sun-synchronous, circular, nearly polar orbit around the earth. Descriptions of the three photographs discussed in this paper are as follows.

Figure 1:
NASA photograph no. AS7-8-1918; lat. 30° 28^primeN, long. 90° 14^primeW (principal point). This photograph was taken during the Apollo 7 flight and is to an approximate scale of 1:2,000,000. The original photography is in color and was shot using a 70-mm, handheld camera. The photo shown in Figure 1 is a black and white enlargement of the southern half of the original photo.

Figure 2:
NASA ERTS-1 photograph no. E-1194-15564-5; lat. 30° 20^prime N to 30° 19^prime N, long. 88° 03^primeW to 88° 09^prime W; approximate scale, 1:3,369,000; elevation approximately 900,000 m; image sensor, multispectral scanner, band 5 (lower red, wavelength 0.6-0.7 µ).

Figure 3:
NASA ERTS-1 photograph no. E-1194-15564-6. Same as Figure 2 except shot using multispectral scanner, band 6 (upper red-lower infrared, wavelength 0.7-0.8 µ).(FOOTNOTE 4)

DISCUSSION

The areas shown on the three photographs include Mobile, Baldwin, and parts of Washington, Clarke, Monroe, and Escambia Counties in Alabama. Also visible are parts of the western Florida panhandle and southeastern Mississippi. Miocene, Pliocene, and Pleistocene sediments crop out throughout the region and consist chiefly of nearly flat-lying sands and clays of fluvial, marine, and transitional-marine origin (Riccio et al., 1972).

Initial examination of the photographs revealed several prominent lineations which subsequently were verified by field work as probable faults. Recently published geologic maps of Mobile and Baldwin Counties, Alabama, make no mention of faults in this part of the Coastal Plain and, until recently, it was believed that no surface expression of faulting existed in this area. Admittedly, the field identification of faults in late Tertiary and Pleistocene

FOOTNOTE 4. In addition to the two bands described above, the multispectral scanner aboard ERTS-1 produces two other images. Band 4 (green) has a wavelength of 0.5-0.6 µ, and band 7 (infrared) has a wavelength of 0.8-1.1 µ.

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sediments commonly is difficult because of the lithologic similarity of the sediments and the difficulty of identifying marker units that can be used to determine the magnitude of throw. For this reason, many abrupt lithologic changes in younger Coastal Plain sediments are attributed to facies variations. Regardless of this, the absence of mapped surface faults in southern Alabama is somewhat of an enigma in view of the fact that many faults have been mapped in strata of the same age in the neighboring states of Louisiana, Mississippi, and Florida (Saucier, 1963; Marsh, 1964; Otvos, 1973). The lineations on the photographs, therefore, may provide the first

Click to view image in GIF format. Fig. 1. [Grey Scale] Apollo 7 photograph of southern Alabama, southeastern Mississippi, and western Florida showing major lineaments and domal features. Oblique view.

Click to view image in GIF format. Fig. 2. [Grey Scale] Multispectral imagery (band 5) of southern Alabama, southeastern Mississippi, and western Florida. Dotted line shows location of 25-ft (Pamlico ?) shoreline. Area shown is approximately that seen in Figure 5.

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positive evidence of faults at the surface in this area. In addition, the imagery also has permitted identification of sediment-distribution trends, ancient shorelines and deltas, and other erosional and depositional features. Some of the more prominent structural and physiographic features visible on the photos are discussed in the following paragraphs.

Linear and domal features:
The three major northwest-trending curvilinear features in the northeast part of the Apollo 7 photograph (Fig. 1) are believed to represent faults that extend from Clarke and Monroe Counties, in Alabama, to the panhandle of western Florida. According to Causey and Newton (1972), the Jackson fault in Clarke County, Alabama, consists of a north-northeast trending segment and one that trends in a northwesterly direction (see Fig. 4). From a review of their geologic map and a study of the space photographs, the northwest-trending segment appears to be the extension of the most westerly of the three faults mapped from the photographs. Further, the generalized structure contour map by the same authors also confirms this interpretation.

Whether the two easterly faults represent extensions of the West Bend and Coffeeville faults (Fig. 4) or are alignments of previously unrecognized buried faults is unknown. If they are the former, a graben system may extend from Choctaw County, Alabama (Turner and Newton, 1971), to the panhandle of western Florida. More probably, these two faults may be related to the Bethel fault system (Moore, 1971). No proof, however, is advanced in either case.

Previously unmapped linear features in southern Baldwin County, Alabama, are believed to be the eastward extensions of lineaments or faults that, in part, extend from Mississippi into southern Mobile County, Alabama. The two faults, labeled #1 and #2, in south Baldwin County (Fig. 1) have not been verified in the field. Segments of the faults in south Mobile County were recognized in the field prior to the acquisition of the space photographs, however. These faults are considered to be down-to-the-basin type with throw increasing with depth. Because sediments as young as Pliocene-Pleistocene are affected, some movement along these faults must have occurred during later Pleistocene time.

The northwest-trending, relatively straight coast along the eastern shore of Mobile Bay

Click to view image in GIF format. Fig. 3. [Grey Scale] Multispectral imagery (band 6) of southern Alabama, southeastern Mississippi, and western Florida showing location of abandoned Escatawpa delta. Area shown is approximately that seen in Figure 5.

Fig. 4. Generalized structure map of Clarke County, Alabama (after Causey and Newton, 1972). Inset shows relation of Clarke County to other counties in south Alabama. Dotted line shows approximate area covered in Figure 1.

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is interpreted as a possible fault-line scarp. The possibility that it may be a wave-cut feature also exists but, if so, its development must have occurred prior to the deposition of the spit (Fort Morgan Peninsula). Inasmuch as Mobile Bay is a drowned river valley, the possibility that this feature is related only to marine processes appears remote.

Although no supporting subsurface data are available, the area bounded by the curvature of the Escatawpa River and two of its tributaries may indicate a domal structure. This is especially apparent on the 7½-minute, Wilmer, Alabama-Mississippi quadrangle (1943) where domal area A (Fig. 1) is shown as a topographic high with modified radial drainage. The Citronelle oil field in north Mobile County (Fig. 1) likewise is on a slightly elongated, northwest-trending topographic high with radial drainage (domal area B).

Physiographic features:
Of the four bands transmitting on the ERTS-1 satellite, the EROS Data Center (written commun.) states that band 7 (infrared) is best for land-water discrimination, band 5 (lower red) is best for showing topographic and cultural features (drainage patterns, towns, roads, etc.) whereas band 4 (green) commonly can discriminate, qualitatively, the depth and/or turbidity of standing bodies of water. Band 6 (upper red-lower infrared) also is useful for land-water discrimination and shows the best tonal contrasts that reflect various land-use practices.

Because the ERTS-1 satellite is in a circular, near-polar orbit, data can be collected from the same location every 18 days. Obviously, this provides an excellent means of obtaining information on sediment dispersal, coastal erosion problems, hurricane damage, and a variety of short-term geologic processes. An example recently noted has been the widening of Petit Bois Pass, between Dauphin Island and Petit Bois Island (Figs. 2, 5) by erosion of the western part of Dauphin Island to allow a heavy inflow of salt water from the Gulf. This has resulted in the migration of the oyster-boring conch Thais haemastoma into Mississippi Sound, causing depletion of the once plentiful oyster beds. This gastropod was previously absent because of the lower salinity of the Sound.

Examination of the band 5 photo (Fig. 2) provided the most useful information on shoreline features, sediment dispersal, and water depth in Mississippi Sound. For example, the 25-ft (Pamlico ?) shoreline of late Pleistocene age is clearly visible on Figure 2. The shoreline can be traced easily in southern Mobile County but, on the east and west, is more difficult to see because of dissection of the terrace. Its position also can be seen on Figure 1 and on topographic maps of the area. None of the other nine terraces that Cooke (1966) believed present in south Alabama could be identified clearly on the photos, though several can be traced, at least in part, on existing topographic maps.

With regard to sediment dispersal, Upshaw et al. (1966) noted that, although water moves both north and south through inlets from the Gulf of Mexico into Mississippi Sound, southward movement is dominant. This is seen clearly in Figure 2, which shows large sediment plumes extending southward from the major inlets into the Gulf of Mexico. These sediment plumes are reported to consist largely of sand-sized material, and their distribution, seen on Figure 2, is in good agreement with the entropy-ratio map published by Upshaw et al. (1966). Particularly visible, both on the band 5 (Fig. 2) and band 6 (Fig. 3) photos because of their high reflectivity, are the medium to coarse sediments along the mainland beaches and along the barrier island chain from Santa Rosa Island, in western Florida to Cat Island, in Mississippi, and the Chandeleur Islands, in Louisiana (Fig. 4). The

Fig. 5. Generalized location map of central Gulf Coast area.

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finer sediments that Upshaw et al. (1966) reported along the mainland beaches east of Pascagoula and in east-central Mississippi are the darker areas of both Figures 2 and 3. This is also true of the elongate, mud-floored trench north of the eastern end of Horn Island and a similar trench north of the western end of Ship Island. The high concentration of sand near the Chandeleur Islands was considered by Russell (1936) to be the result of reworking of older deltaic sediments of the Mississippi River. The area with a high proportion of sand south and southeast of Mobile Bay is, in part, the result of recent deposition from the Mobile River, but consists largely of reworked older submerged offshore barrier island deposits of Pleistocene age that lie southeast of the Fort Morgan Peninsul .

Another feature that can be seen best on the band 6 imagery (Fig. 3) is the ancient delta of the Escatawpa River. The river has been the victim of stream capture by a small tributary of the Pascagoula River. Headward erosion of the tributary during the late Pleistocene diverted the Escatawpa to its present course and resulted in the abandonment of its delta.

Band 6 imagery also was found to be the most useful for the study of changes in coastal morphology. For example, the obvious westward longshore drift can be noted easily by the presence of the several barrier islands along the coast, particularly the Fort Morgan Peninsula, which has been developed by headland erosion coupled with longshore deposition. Also visible on this photo is the breach in Ship Island caused by Hurricane Betsy in 1965. Further, band 6 photography clearly permits observation of the seasonally inundated flood plains along the Pascagoula, Mobile, and Perdido Rivers, as well as oxbow lakes, distributary patterns and marsh areas. Upshaw et al. (1966) noted that, because of the relatively steep alluvial coast in the central Gulf Coast, the shore and coastal marshes in his region are narrow and relatively inconspicuous. Though coastal marshes were not visible on bands 4 or 5 imagery, they could be seen clearly on both infrared bands (bands 6, 7; Fig. 3).

SUMMARY

Satellite photographs and multispectral-scanner imagery provide the geologist with a rapid means of observing the effects of many short-duration geologic phenomena. In addition, major structural lineations, not visible on low-altitude aircraft photographs, commonly can be observed. The satellite photographs also were found useful for identifying sediment-dispersal patterns, ancient shorelines and deltas, pirated streams, high- and low-energy depositional environments, and a variety of other terrane features. Naturally, many of these features are seen best on the original photographs and can be verified on topographic maps and by field observation. Probably many other uses can be made of the multispectral-scanning imagery in the fields of petroleum geology, structural geology, geomorph logy, etc. The high resolution of the ERTS-1 imagery and its multiband scanning-capabilities thus provide the geologist with a valuable new tool.