publications > papers > significance of fresh-water limestones in marine carbonate successions of pleistocene and cretaceous age > lower cretaceous fresh-water limestones, central texas
|Holocene Fresh-Water Marls on Carbonate Platforms|
|Pleistocene Marlstones of South Florida|
Distinctive nodules and discontinuous layers of dark-gray lime mudstone, here interpreted to be fresh-water limestone (or marlstone), occur in association with a persistent and widespread key bed near the top of the Edwards Group (Lower Cretaceous) in the Edwards Plateau region of west-central Texas. The key bed was first mentioned by Calvert (1928), who identified the unique ledge-and-slope interval in Val Verde County near the top of the otherwise indivisible Devils River Formation, a lateral equivalent of the Edwards Group. This marker, called informally the "Calvert Slope," lent itself readily to surface structural mapping, and the subsequent discovery of distinctive black limestone nodules In the "Calvert Slope" by Humble Oil and Refining Company geologists during the late 1950's made it a reliable mapping horizon now known informally as the "Black Bed." Rose (1972) described features suggesting that the Black Bed sequence represented regional subaerial exposure. Subsequent investigations by Halley and Rose have now resulted in a more complete description and interpretation based on studies of rocks exposed at localities (listed in table 1) of the depositional and diagenetic attributes of the Black Bed succession.
The characteristic gray (N3-N6) lime mudstones that occur in this succession are distinctly darker than the enclosing Iight-gray to cream-colored shallow-marine limestones or yellowish-gray caliche. Apparently the dark color is not caused by large concentrations of organic material, inasmuch as analyses indicate that less than 0.3-percent organic carbon is present, but very fine disseminated algal debris and minor amounts of ferrous iron and manganese throughout the sediment may account for the dark color.
Fossils are rare in the dark lime mudstones, but indeterminate microfossils (fig. 5c), fine molluscan fragments, whole gastropod shells, and ostracod carapaces are present in very small numbers.
Fresh-water limestones of the Black Bed sequence are almost invariably lime mudstone, following the classification of Dunham (1962). Generally they are featureless and homogeneous, but many nodules show a clotted texture and many small spar-filled cracks (figs. 2c, 2d). Their fabric is identical to that of Pleistocene marlstone from Florida. The only known exception is at locality 15 in northeastern Edwards County, where a dark-gray calcisiltite limestone occurs as laminated internal sediment-filling solution holes and molds after aragonitic fossils dissolved away from within the underlying marine limestone (fig. 6). After emplacement of the dark laminated silt-sized lime sediment, incompletely filled coral molds were then filled by sparry mosaic calcite cement.
|Figure 6||Figure 7|
|Figure 6. a. (top) Outcrop photograph of Cretaceous limestone underlying marlstone. Mold of large gastropod has been filled with dark fresh-water limestone. b. (bottom) Rock-slab photograph of same rock containing partially and completely leached coral fragments (septa still visible near arrow) now filled with laminated fresh-water limestone and calcite spar. [Larger image]
Figure 7. a. (top) Rounded cobbles of fresh-water limestone weathering out of the "Calvert Slope." Weathering is thought to be Cretaceous, because unweathered limestone overlies the "Calvert Slope" at the quarry locality (locality 68). b. Rock slab through contact between Black Bed and overlying marine grainstone. Upper surface of Black Bed is bored by marine organisms, probably pholads (marine boring clams). [Larger image]
Dark limestones of the Black Bed sequence contain (or are associated with) at least three features that suggest early lithification:
(a) an underlying resistant light-gray skeletal marine limestone that forms a prominent bench;
(b) an intervening soft, "calichified" interval 1.5 to 2.4 m thick that forms a recessive slope; and
(c) an overlying light-gray to cream-colored, coarse-grained, porous skeletal marine limestone 1.8 to 2.4 m thick that forms a resistant ledge.
|Figure 8||Figure 9|
|Figure 8. Regional correlation of "Black Bed" interval from Edwards Plateau to subsurface at Central Texas platform. [Larger image]
Figure 9. Locality 15 illustrating (a) patches of fresh-water limestone overlying marine limestone in foreground, (b) soft recessive weathering "Calvert Slope," and (c) overlying skeletal marine limestone that forms resistant slope. [Larger image]
As described, dark-gray laminated limestone fills solution holes in the upper part of the underlying skeletal limestone, the upper surface of which is locally bored by pholad-type clams, suggesting early lithification but not necessarily subaerial exposure (Purser, 1969; Shinn, 1969; Rose, 1970).
The most common occurrence of the dark-qray limestone is as smooth, rounded cobbles and pebbles weathering out of the middle, recessive, calichified slope. Fresh quarry exposures of this interval show a deeply altered, massive, punky, fine-crystalline limestone or "caliche" with one or two hard, thin sucrosic limestone beds near the top. Dark lime mudstone cobbles are scattered through the massive altered or "calichified" zone.
The dark lime mudstone also occurs as an intermittent bed up to 1 ft thick in the basal part of the upper resistant limestone. This bed thins to zero from the top by erosion. The rock becomes darker gray upward, suggesting that the darker color may be related to length of weathering. The upper surface of this dark limestone bed is locally irregular and bored by pholad-type clams (Purser, 1969; Shinn, 1969; Rose, 1970). Large dark-gray limestone fragments, some bored, are incorporated into the overlying light-gray coarse-grained marine limestone, which also contains lithoclasts composed of other limestone types different from the darker Iime mudstone.
All the evidence indicates that two exposure surfaces are present in the Black Bed succession: one at the base of the calichified recessive middle slope and another at the base of the overlying resistant limestone ledge. The middle recessive interval (the "Calvert Slope") is therefore interpreted to represent a Cretaceous weathering unit or "paleo-caliche" containing clasts and lenses of dark-gray, fresh-water limestone or marlstone that formed in lakes and ponds on an exposed carbonate terrane analogous to present-day South Florida.
Although strong evidence was lacking, Rose (1972, p. 48) interpreted the Black Bed as a disconformity and correlated it to the southeast with the widespread disconformity between the Edwards Group and overlying Georgetown Formation in the Balcones Fault Zone and in the subsurface beneath the Gulf Coastal Plain. Compelling evidence exists to document this disconformity at the outcrop and particularly in the subsurface, such as: invariably sharp, irregular contact; truncated shells and spar-filled fractures; small solution caves just below the top of the Edwards that are filled with Edwards clasts and Georgetown matrix, soil breccia at the top of the Edwards; abrupt upward change from very shallow restricted marine to open marine fauna at the boundary, up to 30 m of stratigraphic truncation at the boundary (Rose, 1972).
The known area covered by this welI-documented disconformity between the Edwards and Georgetown includes approximately 13,000 km2 comprising parts or all of 11 counties and covering the crest of the Central Texas platform, the dominant structural positive that affected Lower Cretaceous shelf deposits of the region (fig. 11). Known area in which the Black Bed occurs (Brown, personal communication, 1975) covers the crest and flank of the Central Texas platform, covering an area of over 10,000 km2, comprising parts or all of eight counties.
|Figure 10||Figure 11|
|Figure 10. Detailed section of the "Black Bed" interval. [Larger image]
Figure 11. Major structural features of the Central Texas platform showing the distribution of the inferred total Black Bed subaerial exposure surface (vertical hatch), the known extent of the subsurface Georgetown-Edwards subaerial exposure surface (dots), and the known aerial extent of the Black Bed (pattern). Compare fresh-water limestone extent with that shown in figure 1. [Larger image]
In the Edwards Plateau no Georgetown Formation is present, and the Del Rio Clay rests directly upon the Edwards Group, whereas, to the Southeast, Del Rio rests upon Georgetown. Therefore, the interval from the Black Bed to the top of the Edwards is interpreted to be equivalent to the Georgetown Formation of the Balcones fault zone and the subsurface. If this correlation is correct, the open-marine Georgetown must change facies northwestward to shallow-shelf Edwards-type sediments. In those Balcones fault zone outcrops and shallow subsurface areas that are closest to the Black Bed outcrop localities and on strike with them, the Georgetown Formation is about 6 m thick, approximately the same thickness as the counterpart interval between the Black Bed and top of Edwards (Rose, 1972). Moreover, this correlation is entirely compatible with thickness patterns and correlation of more conventionally identified underlying and overlying stratigraphic units associated with this interval.
The area that was subaerially exposed before Georgetown deposition was an elongate carbonate terrane along the crest of the Central Texas platform that terminated to the southeast at the Stuart City reef trend (fig. 11). This exposed land surface must have been an extremely flat lowland not unlike present southern Florida, covering a total area perhaps 11 km wide and 320 km long, or 36,000 km2.
Subaerial exposure is one of the most important geologic events affecting diagenesis of carbonate rocks and in different situations may cause either preferential cementation (porosity occIusion) or solution (porosity enhancement). Accordingly, the recognition of previously unperceived subaerial exposure surfaces (disconformities) may allow the stratigrapher to understand existing patterns of porosity development and even to infer the existence of commercial reservoir rocks in undrilled areas. Thus, the identification of additional criteria indicating subaerial exposure, such as fresh-water limestone deposits that are contained within overall marine carbonate successions, may lead indirectly to the discovery of new petroleum accumulations. Evidence at hand indicates that fresh-water limestones or marlstones are indeed distinctive, that they occur in characteristic lithic successions, and that their occurrence is compatible with known regional stratigraphic patterns. In the example presented here, the Lower Cretaceous Edwards Group, the prevailing regional pattern of subsurface porosity development is that porosity and permeability are highest near the top of the Edwards and diminish downward progressively (if erratically) with depth, below the subaerial exposure surface at the top of the Edwards Group. In the Edwards Plateau, geophysical borehole measurements of porosity have not been carried out in numbers sufficient to make quantified generalizations. Moreover, the long Tertiary history of subaerial exposure and ground-water flow through exposed Edwards rocks in the Edwards Plateau area has doubtless severely altered previously developed porosity patterns. Accordingly, in the outcrop area where the recognizable fresh-water limestones are developed, we are unable to say whether or not the downward-diminishing porosity development is duplicated in the subsurface Edwards.