The shallowest aquifer is the Mississippi River Valley Alluvium (MRVA), the Quaternary sand and gravel aquifer which covers the entire extent of the Mississippi Delta region. This very prolific aquifer contains significant amounts of iron, and is used primarily for agriculture and fish farming. It was more widely used in the past for human consumption but currently is used only by the City of Vicksburg for public water supply. Total thickness of the alluvial formation in the study area ranges from 75 to 210 feet with a mean thickness of 117'.

The alluvial formation is not homogenous. Shifting rivers and streams deposited varied bodies of sands, gravels, silts, clay plugs, and lignites. However, a few tendencies hold true. Normally an upper clay layer about 10'-30' thick directly underlies the soil. Below this are mostly sands and gravels, some extremely coarse. In general, the formation's coarsest and most transmissive sediments tend to occur in the lowest portion of the alluvium. Another fairly consistent trend is a thickening eastward from Greenville and from other marginal locations, toward the Sunflower and other central streams of the Mississippi Delta.

ALLUVIAL SUBCROP

Three formations directly underlie the Mississippi River Valley Alluvium (MRVA), the Cockfield, the Moodys Branch, and the Yazoo Formation. All deeper formations lie buried beneath these. In Washington County, only the lowest portion of the Yazoo Formation is present. The Yazoo Formation in the study area ranges from only 7' to 87' in thickness (mean 34') where drilled. Under its base, the Moody's Branch, a thin marl, ranges from 18' to 88' in thickness.

COCKFIELD AQUIFER

The major aquifer developed most for public use is the Cockfield. Brackish water occurs locally in this aquifer, and is the focus of this investigation. The Cockfield Formation is a Claiborne stage (about 52 to 40 mya), late Eocene unit of sands, clays, silts, and lignites. Normally it is overlain by the Moody's Branch Marl, but in the south and eastern portions of the county it directly underlies the MRVA gravels. From 153 to 466 feet of thickness has been drilled in study area wells.

The thickness and character of the Cockfield varies considerably across the state. Massive sands dominate in central Mississippi, where the formation is considered a deltaic deposit (Merrill, 1985, p. 43). Farther east and in Alabama, the Cockfield, like other Tertiary formations, thins and grades to a more marine unit. In Alabama and eastern Mississippi a distinctly marine sand member, the Gosport Sand, crops out in the lower Cockfield. This sand member is observed in the subsurface of Mississippi but is not reported in outcrop in central Mississippi. In central Mississippi, the Cockfield is a sand sequence of continental/deltaic origin, thickening to the west. Thomas (1942, p. 69) described the upper Cockfield, in its outcrop belt, as more silty and lignitic toward northern and western Mississippi.

As the Cockfield outcrop passes under the Delta's cover of later alluvium, little is known regarding the formation's character in western Mississippi. Descriptions from Washington County logs and samples are briefly summarized in the next section. Westward from Mississippi, the Cockfield continues into Arkansas, Louisiana and Texas as the Yegua Formation. A local characteristic of the Cockfield (Yegua) and Sparta outcrop belt in Texas is the presence of 'salines', which are topographic features related to saltwater springs and associated ephemeral lakes and shallow collapse features (Dumble, 1918, p. 309).

In Louisiana and Texas a marine sand has been recognized at the top of the formation (Treadwell, 1954, p.2307; Andersen, 1960, p. 92) and named the Creola Sand by Stenzel (Stenzel, 1940, p. 881). This member has been recognized in the subsurface of Hinds County in west-central Mississippi (Dockery, 1991, p. 27). In outcrop, Creola or equivalent beds in outcrop have also been reported by both Stenzel and Dockery in Clarke County at Mississippi's eastern border (Dockery, 1980, pp. 51-52).

LOCAL COCKFIELD LITHOLOGY

The state's archive of drill cores contains no examples of the aquifers studied, but cuttings have been preserved from the drilling of several water wells, and these were used to characterize the formations studied (Figure 14) The materials are by no means a comprehensive or exhaustive sampling and much more subsurface lithologic sample collection, particularly in the Wayside area, would be useful in any future work in this area.

Cockfield lithology varies significantly within Washington County. Only a few gross generalizations can be used to characterize the Cockfield. The most commonly drilled constituent is fine sand, and the next most common is silt. Many other constituents occur, including coarser sands, clays, shales, sulfides, lignite, shell fragments, and gravel. Cementation in the formation is highly variable, ranging from none to weak to very hard. Because hardness and rate of penetration are so important during the drilling process, driller's logs often log hard zones in detail. Of the 230 wells for which driller's logs were used in the study, 89 contained notations regarding very hard drilling or 'rock'. When these wells are mapped versus all logged wells, a distinct distribution of 'hard' intervals is seen, placing the overwhelming number of well-cemented zones west of a line near Highway 1, running through Avon, Wayside, and Greenville (Figure 15). Virtually every well near the Mississippi River contains some well-cemented zones, while very few in the eastern reaches of the county contain any 'hard' zones.

Lithologically, a very rough division of Cockfield beds into three sections was observed in the samples and logs: a lower sandy section, a middle shalier section, and an upper, partially marine, section.

Lower Cockfield

In the center of the basin ( well G-20), the Greenville riverside industrial area (well D-33), and northeast at Stoneville (well E-36), clean sands occur in the basal 180' of the formation. In G-20 the sands contain traces of marl, glauconite, shell fragments, gravel, and lignite. In E-36 there are instead silty shale interbeds scattered in the sands. In D-33 the basal 70' is lignitic, silty sandstone, some so well-cemented as to drill as 'rock', with carbonaceous clay and shale interbeds, overlain by a thin gravelly interval, in turn overlain by glauconitic and calcareous coarser sands and sandstones. In other northerly wells lower Cockfield material was not sampled.

Only selected intervals were sampled at G-132 in the riverside industrial area. Lower Cockfield samples in G-132 were loose, fine, silty lignitic and pyritic sand. Some intervals are well cemented, some with calcareous cement. In the southeastern quadrant of the study area, cuttings in well L-70 at Arcola indicated dominantly a hard, poorly sorted sandstone, sideritic and vuggy in its lowest portion, with considerable siltstone and claystone interbeds and some loose sands. In M-62 at Hollandale the lowest Cockfield beds are coarse-grained carbonaceous sandstones, coarse, containing some gravel and shell material, and some carbonaceous siltstone. In P-29, southwest of Hollandale, are brown poorly sorted sands, sulfides, carbonaceous shales, some micaceous, some bearing leaf prints, and gravels containing marl fragments.

While sands dominate, the accessory components and degrees of sorting vary across the study area. In the northeast, (sector E) silts and clays are the secondary components. Lignites occur in sectors D and G. Marine components (glauconite, shell) are minor but present in G-20 and D-33. Traces of marl fragments or shells are seen, in tandem with gravels, in sectors M and P to the southeast. Sulfides and carbonaceous materials are minor in sector D and G, but are significant in M and P. Sorting is best to the north, in sectors D, G, E. Sorting is poorest to the southeast (sectors L and P).

Middle Cockfield

In A-40 near Winterville, middle Cockfield sediments are fine-grained, loose sands, very soft and silty, with occasional thin lignitic beds. In D-33 loose medium sands prevail, interbedded with clayey calcareous siltstones. In G-198 samples are limited but mostly represent a mid-Cockfield sequence of coarsening downward, gravelly sand with some glauconite and shell debris, lying over a very fine, clean sand which appears to be part of the lower Cockfield. Elsewhere in northern wells, the basal sands are overlain by siltstone/shale sequences. In G-20 these contain some shales with leaf imprints, and interbedded sands, some calcareous. Well A-107 at Metcalfe encountered dense, silty sandstone with an interval of silty shale, and an interval containing some siderite concretions and gravels. In E-36 siltstones and shales make up the middle part of the formation.

The middle Cockfield in L-70 is sand and sandstone, fine, silty, carbonaceous, with significant dark shale and siltstone interbeds. In M-62 a coarse gravelly sequence begins about 100' above the Cockfield base, fining upward to loose clean sands, with some iron-stained rock fragments, alternating with dense, poorly sorted sandstones. P-29 has a basal iron-stained gravel and conglomeratic layer overlain by very clean white sand and sandstone, then by recurring gravels interbedded with sandstones and silty shales.

Figure 16 illustrates with graduated symbols the constituents observed in the middle Cockfield zone of the sampled wells, sized by their relative dominance in the samples. The highest percentages of 'fines' are to the far north, in A and E sectors, with strong shale and silt content; the next highest percentages in the carbonaceous sequence in L sector. The coarsest materials were found in M and P sectors. Clean, fine sands still dominate in sectors D and G. Marine materials are confined to the D and G sector wells.

Upper Cockfield

Upper Cockfield beds in D-33 glauconitic and calcareous, consisting of siltstones and very fine sand and sandstones. The upper Cockfield at G-20 is a brown, poorly sorted sand which contains significant gravel, some lignite and glauconite, fining upward into gummy calcareous shales and carbonaceous shales. In E-36, the repetitive sequences of siltstones and shales continue, but with the addition of some siltstone conglomerates, containing clasts of lignites, marls, and fine gravels, but no sands. In E-45 the upper Cockfield consists of glauconitic sands, calcareous and some shelly, overlain by a thin sandstone, soft, glauconitic, and pebbly; a lignitic bed; and then alternating shales, gravels, and sand marls. In A-107 at Metcalfe the upper Cockfield consists of interbedded silty fine sands and siltstones, slightly shelly and gravelly, overlain by a fine glauconitic sand, clayey and calcareous. The upper Cockfield is thin at A-40, represented by a coarse glauconitic sand.

Upper Cockfield at L-70 begins with a gravelly, loose, porous, vuggy sand, fining upward to hard brown sandstone with siltstone interbeds. In M-62 the unit consists of coarse sands interbedded with carbonaceous siltstones; gravels and lignites are common. At P-29 the upper Cockfield is absent.

Sand dominates the upper Cockfield, but is virtually absent from one well in E-sector where siltstones and shales dominate. Five of the wells in sectors A, D, and G contain significant glauconite and calcareous materials. Southeasterly wells in L and M sectors contain coarse sands and thin gravels, but thin gravels occur in all sequences, except those farthest west (D-33, A-40), which are dominated by glauconitic sands.

Cockfield Deposition

As with other formations in the coastal plain, the sediments do not form continuous or uniform beds. Lithologies grade laterally and vertically, making up large transgressive and regressive wedges of sediment. Variability is to be expected as the result of shifting shorelines, distributaries, and varying environments of deposition. Within each formation the rock record may be the result of several pulses of rising and falling sea level; locally prograding sediments may build out while there is a net sea level rise overall.

In Washington County, deposition was in and adjacent to the Mississippi Embayment. Late Eocene was a time of maximum transgression of Gulf waters into a large north-trending arm of the ocean. At its maximum, the shoreline of the embayment reached into what is now Illinois. The depositional model used in central and south Mississippi, of a Cockfield continental system pouring sediments southward into the Gulf, would not be an accurate model for deposition in this locality during this time.

Local lithologies in the Cockfield confirm the highly variable and discontinuous nature of the formation that has been noted in previous studies. No continuity of beds from one well to another was discerned. Bockelman (1987, p. 102) and others attribute this to fluvial-deltaic deposition which created lenticular deposits of sands and shales.

The content of the Greenville area samples suggest that the Cockfield deposits are complex and include some marine materials. In the lower Cockfield beds, there may remain the record of a delta becoming increasingly tide-dominated by rising sea level from the west and northwest. An expanding Mississippi embayment, pushing its shoreline north toward the center of the continent likely sent a bay-like arm of the sea into this area following the downwarp axis of the Desha Basin. In the lower Cockfield, deltaic sands and silts, often poorly sorted, were being transported from the east and southeast. Offshore to the northwest, cleaner sands were being deposited, along with siltstones; perhaps marginal marine deposits dominated by bars and intertidal deposits.

As the sea rose, even in the southern deltaic area, more of the sands were reworked by shore processes, and pyritic and marcasitic zones reflected rapidly drowned organics. As the last of the Cockfield material was deposited, marls and glauconitic sands dominated in the northwest; whereas "dirty" coarse sands, lignites and gravels predominate in the southeast. This may reflect a deepening embayment in the northwest, while in the southeast a center of continental/deltaic deposition suffered increasing encroachment, evolving to an estuarine system.

Eventually, marine deposition overwhelmed the entire area, with the Moody's Branch marl transgressing over both the marine Cockfield Creola member beds and the estuarine beds to the south, followed in turn by the marine and estuarine clays of the Jackson Formation. The unique sub-basin structure environment here may have given rise to the thick development of the Moody's Branch and its sometimes 'doubled' log signature in this area.

COOK MOUNTAIN FORMATION

Below the Cockfield, and separating it from the Sparta formation and aquifer, is the Cook Mountain Formation. This formation also undergoes significant change along strike. In east-central Mississippi it is thin and has three distinct members, one lower fossiliferous marl overlain by two shales. In electric logs the Cook Mountain registers clearly as a shale. This is often not the case in the study area where the formation contains much more sand.

In D-33 an upper dark silty shale, with some sand interbeds, overlies a hard pyritic sandstone. In P-29, gray and black platy shale alternates with micaceous sands in the upper two-thirds of the formation, and the basal beds are brown lignitic, sulfidic sandstones. In three other wells, this formation, normally defined in the subsurface as a shale "break" between the Sparta and Cockfield sands, exhibits a greater sand content. In G-20 the upper Cook Mountain includes brown shales interbedded with silty sandstones containing clay clasts and gravels; in the lower a fine well-sorted sand with some shale interbeds, and minor sulfides and traces of glauconite and lignite. M-62 samples were more dominantly clean sands with some sulfides, and a thin sideritic sandstone occurring both under its base and about 30' above its base. In this well, shales were essentially absent. No samples contained marls such as are seen in the Cook Mountain of eastern Mississippi.

SPARTA SAND AQUIFER

Like the Cockfield, the Sparta is a thick, variable sand and sandstone formation and aquifer, which contains significant intervals of shales, silts, and some minor sulfides, lignites, and rarely marls. It is thicker than the Cockfield. In study area wells the average thickness drilled was 595 feet. The minimum thickness measured was 467' in A-503, maximum thickness 720' in D-33. Few samples were available, but those logged indicated generally thicker, cleaner sands than those found in the Cockfield, particularly in the lower half. Sulfide-bearing layers are common in the southeast area.

Below the Sparta sand, the Zilpha, a marine clay, also contains some well-developed sands in its upper portions. This study follows convention and correlates the first strong sand above the Zilpha shale profile as the base of the Sparta since they are hydraulically part of the Sparta aquifer, but it should be acknowledged that stratigraphically some of these sands may belong to the Zilpha formation. As a source of water, the Sparta is utilized less in the study area than are the higher aquifers, yet it has been an important alternative for water associations confronted with brackish Cockfield water. Currently seven public-supply wells draw from it.

LOWER AQUIFERS

Very few wells in the study area draw from two deeper aquifers, the Tallahatta, and the Meridian-Upper Wilcox. These are separated from the overlying Sparta by the Zilpha clay, and the Winona greensand.

The Tallahatta was used for public wells early in the 20th century, but is little used today. Directly below the Tallahatta is the deepest aquifer tapped, the Meridian/upper Wilcox. Three public supply wells currently draw from this aquifer, all in the O, P, or S sectors in the southeast part of the study area.

DEEP FORMATIONS

Deeper formations underlie the aquifers. Immediately below are the Lower Wilcox clastics, including the thick Midway shale, a major barrier to fluid flow. Below the Tertiary section lies a thick sequence of carbonates. To the west, near the center of the Monroe Uplift, these carbonates constitute productive Cretaceous and Jurassic petroleum reservoirs.

Within the county, various lithologies including carbonates, volcanics, chalks, and anhydrite are present at depth. Igneous intervals have been logged in the Cretaceous carbonate section below 3,900' of drilled depth. The deepest exploration well in Washington County stopped in the Jurassic Smackover Formation (10,176' in a well in sector L).

Little is known of the geology at greater depths. Less than 30 miles northeast in Sunflower County, a deep test (Amoco Word #1) encountered Jurassic basalt at 9,613', directly overlying igneous materials dated Ordovician in an interval 11,210' to 11,984'. These rocks are immediately underlain by Precambrian syenite. (Harrelson et al, 1992, p. 485).

Within the deep formations are significant sources of saline waters at depth. In Washington County several wildcat exploration wells were drilled in the 1940s and 1950s, but no fields were developed. Drilling reports recorded that saltwater was produced during drill stem tests in at least three of the wells, at depths ranging from 3,700 to 4,400 feet. In one well in sector K near Wayside, saltwater was a surging flow. "Salty" shales and sands were logged in cores from a well in sector G, in the Swiftwater area, in the interval 4,515' to 4,637'. Completion problems and casing breaks were recorded in some of the old wildcat wells, and more may have gone unrecorded.

Salt domes or beds are not known in Washington County. However, more than 40 miles south lies the Kings Dome field of Warren County, a faulted salt dome near the north margin of the Mississippi Salt Basin, where gas was produced from a Zilpha sand, and oil was produced from Wilcox sediments. Some wells in that area reported bottoming in domed salt in the area of 3,800 to 4,100 feet of depth. In the 1940s explorationists sought similar conditions in Washington County, but no production resulted.

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