Getting our feet wet: ancient Alabama reefs.
Most of Alabama is underlain by strata that Conned in marine or coastal environments. Many of Alabama's ancient rocks contain fossil reefs and mounds. Two examples of these ancicnt buildups are described. These are Mississippian reefs and mounds exposcd at the surface in central and north Alabama and Jurassic mounds in the subsurface of southwest Alabama.
The Upper Mississippian Bangor Limestone was deposited on a broad platform that stretched across north Alabama. Small carbonate buildups grew in the central part of the platform. A mound in Lawrence County is dominated by echinoderms and fenestrate bryozoans. In situ colonies of the rugose coral Caninia flaccida compose about 8 percent of the mound by volume. The exposed portion of the mound is approximately 25 meters wide, 1.6 meters thick at the thickest point and roughly circular in plan. The mound possessed about 45 cm of synoptic relief. Strong currents within the mound are indicated by preferred orientation of corals and by coarse debris in channels between coral colonies. Corals are most abundant on the windward side of the mound, and scarcest to leeward. Other Bangor buildups in Alabama are coral biostromes, microbe-coral reefs, and bryozoan-microbial mounds.
Carbonate mounds flourished in the Upper Jurassic Smackover Formation on a 65-kilometer-long ridge. The ridge supported distinct communities of mound builders that constructed different kinds of mounds. On the southeastern ridge flank, a mound at least 18 meters thick is dominated by locally derived debris. The mound incorporated microherms (small bioherms) up to I meter thick, which account for 16 percent of the mound. The microherms were constructed by a previously unknown microorganism. On the northern part of the ridge crest a similar microhermal mound 8.5 meters thick is directly overlain by a microbial mound 9 meters thick. The microbial mound consists of stromatolite (laminated microbialite) and thrombolite (clotted microbialite) with diverse microstructures.
The reefs and mounds described in this paper represent only a small part of the known diversity of ancient buildups in Alabama.
This paper summarizes research on two ancient reef/mound trends in Alabama (Mississippian of north and central Alabama and Jurassic of southwest Alabama), which are presented as examples of the diversity, abundance, and complexity of ancient organosedimentary buildups in the state.
METHODS AND TERMINOLOGY
Analytical methods were described in Kopaska-Merkel and Haywick (2001) and in Kopaska-Merkel (2000). Bed thicknesses are classified following McKee and Weir (1953); particle and crystal sizes are classified using the standard Udden-Wentworth scale (e.g., Friedman and others 1992). Carbonate rock names follow the system of Dunham (1962), modified for kinds of boundstone following current usage. Particle types are listed in rock names in order of increasing abundance. Both metric and English units are used in this paper, because the U.S. petroleum industry uses English units. Carbonate buildups (bioherms) are reefs or mounds. Both exhibit synoptic relief, are constructed (at least in part) by autochthonous organisms, contain distinct biotic communities, and may exhibit ecological zonation. Larger buildups influence water and sediment movement. Reefs are held up (at least in part) by a rigid skeletal framework whereas mounds are not. Biostromes (life layers) are bioconstructions like mounds, but with a tabular f orm. Biodetrital mounds have a biotic component but no discernible framework of any kind. Microbial mounds have a microbial framework. Microherms (microbioherms) are small bioherms on the order of tens of centimeters across. Microherms in the Smackover consist of concentrically laminated (stromatolitic), clotted (thrombolitic), calcimicrobial, or structureless (leiolitic) carbonate, and texturally resemble associated oncoids (algal balls). Microherms contain fossil assemblages different from those of enclosing strata. Thrombolites described here are composed of mesoscopic clots ranging from about 1 mm to about 1 cm across; these mesoclots exhibit a variety of shapes and microstructures. Calcimicrobes generated well-calcified deposits that preserved microbe morphology, commonly in conjunction with abundant synsedimentary calcium-carbonate cement (Webb, 1996).
MISSISSIPPIAN OF NORTH ALABAMA
Limestone and shale of the Upper Mississippian (Chesterian) Bangor Limestone crop out over a large area in north Alabama (Fig. 1). Most of the formation is composed of skeletal wackestone-packstone and oolitic grainstone. Uncommon carbonate buildups are small isolated reefs, mounds, and biostromes (Kopaska-Merkel and Haywick, 2001). The best exposed bioherm is a biodetrital mound located in a road cut on Highway 157 in Lawrence County, Alabama (Site H157, latitude 30[degrees] 24' 38" N, longitude 87[degrees] 49' 51" W; Fig. 1). The exposed part of the mound is approximately 25 m wide and up to 1.6 m thick and is composed of rugose corals in skeletal debris. Strata comprising this outcrop (Fig. 2) are assigned to the middle part of the Bangor because the exposure is centrally located within the Bangor outcrop belt (Szabo and others, 1988; Fig. 1). The excellent exposure of the Lawrence County mound permitted a detailed examination of a unique bioherm as well as the strata that enclose it. Previously reported B angor bioherms are few, and no other biodetrital mounds have been noted in the unit. Objectives of this study were to document the characteristics of the mound and to establish the mechanism of mound development.
Stratigraphic and sedimentologic framework
Chesterian sedimentation in northwest Alabama was strongly influenced by development of the Black Warrior foreland basin (Thomas, 1974, 1985; Mars and Thomas, 1999). Bangor deposition in north Alabama recorded deposition on a distally steepened ramp (Miesfeldt, 1985; Mars and Thomas, 1999). At the line of flexure of the ramp, in a belt trending NW-SE across western north Alabama, a major aggradational oolitic shoal complex developed in the Bangor (Thomas, 1972). Deposition seaward (southwest) of the shoal complex took place on the ramp-like margin of the Black Warrior foreland basin (Thomas, 1972), whereas deposition landward (northeast) of the shoal complex took place on the stable and gently inclined proximal ramp (Thomas, 1972; Fig. 1). Proximal Bangor carbonates on the Warrior platform contain minor siliciclastic material that becomes more abundant to the northeast, recording input of allogenic detritus from the craton (Thomas, 1972; Fig. 3). Bangor carbonate deposits grade northeastward into marginal ma rine and terrestrial deposits assigned to the Pennington Formation (Thomas, 1972). The Bangor ranges in thickness between 100 and 130 m throughout much of the Warrior platform (Pashin, 1993).
The Bangor in the study area abruptly overlies the terrestrial, marginal marine and nearshore marine siliciclastic strata of the Hartselle Sandstone, which forms a series of barrier island complexes and associated strata on the Warrior platform (Thomas, 1972; Fig. 3). Sealevel rise following deposition of the Hartselle flooded the shelf and permitted accumulation of transgressive carbonate and shale of the basal Bangor Limestone (Thomas, 1972; Andronaco, 1986; Pashin, 1993). For the most part, Bangor deposition on the Warrior platform appears to have been progradational or aggradational (Pashin, 1993; Mars and Thomas, 1999).
In the study area, the Bangor is unconformably overlain by marginal marine and terrestrial deposits of the upper Carboniferous Pottsville Formation (Thomas, 1972; Pashin, 1993; Fig. 3). Paleotopography on the Bangor surface influenced facies distribution in overlying strata (Pashin, 1993). Similarly the upper surface of the Hartselle Sandstone, uneven because of development of parallel barrier-island trends in the Hartselle (Thomas, 1972), probably influenced Bangor sedimentation pathways and facies distribution as well (KopaskaMerkel and Haywick, 2001).
Strata exposed at the study site were assigned to seven lithofacies based upon sedimentologic, paleontologic and petrographic characteristics: (A) Oolite, (B) Skeletal, (C) in situ Coral, (D) Coral Floatstone, (E) Cross-stratified Skeletal Grainstone, (F) Fenestral, and (G) Breccia. Vertical relationships among lithofacies are illustrated in Figure 4. The lithofacies were described by Kopaska-Merkel and Haywick (2001); those descriptions are summarized here.
(A) Oolite lithofacies
Oolitic grainstone and packstone are restricted to the lowest portion of the outcrop (Fig. 4). This lithofacies comprises approximately 20 percent of the exposure.
Most of the oolite is well sorted grainstone, but some intervals contain up to 20 percent microcrystalline carbonate. Intact fossils are uncommon, but skeletal debris is locally abundant. Beds are generally internally structureless. The exposed upper bounding surface of the Oolite Lithofacies is sharp and planar. Lateral continuity of the facies is unknown.
In thin section, ooids exhibit well preserved cortices with both concentric and radial structure (Fig. 5). Ooid nuclei are dominated by bryozoan fragments, echinoderm ossicles, peloids, intraclasts, and gastropods, in order of decreasing abundance. Ooids account for 78 percent of allochems in mud-free samples of the lithofacies and 66 percent of allochems in the lithofacies overall. Relative abundances of non-coated allochems are similar to those of ooid nuclei. Bryozoan fragments, echinoderm ossicles, and intraclasts, in order of decreasing abundance, account for most noncoated particles. Some ooid cortices are moderately to heavily centripetally bored. Noncoated allochems and ooid nuclei display similar degrees and ranges of syndepositional particle alteration such as centripetal boring, micritization, and recrystallization. Particles range in degree of alteration from nearly pristine to highly degraded.
Most oolitic grainstone in this lithofacies was little compacted prior to cementation. Interparticle cements can exceed 30 percent by volume of oolitic grainstone and include very early isopachous marine spar, hematitic coats of uncertain origin, vadose meniscus microcrystalline spar, and pore-filling meteoric spar (Haywick and Kopaska-Merkel, in prep).
The muddy matrix filling interstitial space in packstone commonly appears clotted or micropeloidal (20-50 [micro]m in diameter; Fig. 5; cf., structure grumeleuse; Cayeux, 1935). Hence, some, and perhaps most, of the oolitic packstone at this location was deposited as a grainstone, and its muddy matrix may consist of peloids that were homogenized to lime mud by physical compaction or by diagenetic modification (Kopaska-Merkel and Haywick, 2001).
(B) Skeletal lithofactes
This is the dominant lithofacies, comprising approximately 40 percent of the total outcrop. Individual beds measure from 13 to 45 cm in thickness (medium- to thick-bedded), and vary in texture from grainstone to sparse packstone. Particles are generally fine to coarse sand, size, but locally reach a few millimeters in diameter. Sorting ranges from fair to good. All beds are internally structureless. Bounding surfaces range from sharp (grainstone) to gradational (packstone). Separate beds are laterally continuous across the exposure, but their thicknesses vary owing to the uneven surfaces of some underlying coral-rich intervals.
Fossils are diverse and abundant and include scattered large bivalves, gastropods and rugose corals, but crinoids and bryozoans predominate. The Skeletal Lithofacies contains articulated bivalves in life position as well as patches of wackestone, indicating considerable heterogeneity of fabric. Point counting indicates varied proportions of primary constituents (echinoderms, bryozoans and ooids; Kopaska-Merkel and Haywick, 2001); however, ooids are commonly less abundant than either echinoderms or bryozoans. Most ooids are better described as superficially coated grains in contrast to those in the Oolite Lithofacies. Many of the bryozoans consist of large fragments or entire specimens of fenestrate forms and some echinoderm specimens are stalk fragments consisting of multiple ossicles. This lithofacies also contains fine skeletal material, much of it identified as bryozoan (Kopaska-Merkel and Haywick, 2001).
The abundance of microcrystalline carbonate matrix in this lithofacies decreases from north to south within single beds. In a series of six thin sections across the outcrop, the microcrystalline matrix decreased from approximately 45 to 30 percent; the implications are discussed in a later section. As in the Oolite Lithofacies, much of the muddy matrix within packstone components of this lithofacies is clotted. Percentages of cement are highly variable (1 to 25 percent, average 8 percent).
(C) In situ coral lithofacies
This lithofacies comprises colonies of Caninia flaccida (G. Webb, pers. commun., 1998) in growth or near growth position (subvertical to inclined orientation). It should be noted that the genus Caninia is in need of revision, and C. flaccida is a candidate for removal from the genus (G. Webb, pers. commun., 1998). Therefore, the generic assignment of corals at this outcrop is questionable. Two distinct horizons containing coral colonies have been identified in the outcrop at 1.25 to 1.5 m and 1.75 to 2.5 m above the bottom of the exposure (Fig. 4). Individual colonies are up to 0.6 m in thickness (averaging 0.2 m) and up to 2 m in lateral extent (Fig. 6). Most colonies are roughly equant and domal, with most individual corals either radiating upward from a central point (30 percent of the corals examined), or inclined in one direction (between 270 and 310 degrees; 55 percent of the total). In situ colonies contain both adult and juvenile corals in a numerical ratio of approximately 2:1. Some coral colonies ar e multilayered, and contain one or more individuals that bud additional corals, but most smaller colonies are essentially one coral thick. Colonies that are not equant tend to be sheet-like. Rugosan colonies account for between 2 and 13 percent of the volume of the lower portion of the outcrop and are especially abundant immediately above the Oolite Lithofacies at the base of the exposure. Here corals may have grown directly on top of the Oolitic grainstone. The percentage of coral colonies and average colony size increase steadily southward from the north end to the middle of the outcrop. Coral colonies are essentially monospecific, but bryozoans encrust some corals.
Interstitial material consists of fine skeletal packstone identical to the Skeletal Lithofacies from which it was derived. Detritus within coral intraskeletal pores is significantly finer, consisting mostly of mudstone with minor silt-size skeletal components (including ostracodes). The dimensions of intraskeletal pores presumably limited the size of sediment that could infiltrate coral skeletons.
(D) Coral floatstone lithofacies
This lithofacies is exposed only locally at the northern end of the outcrop; it comprises less than 5 percent of the exposure. Coral floatstone is primarily composed of adult rugose corals in an argillaceous matrix (Fig. 7). The lithofacies is lenticular (5 to 10 cm thick) and laterally interdigitates with the in situ Coral and Skeletal Lithofacies. The lower bounding surface is sharp to gradational and locally scoured. The upper contact is generally gradational, passing upward into skeletal grainstone or in situ coral colonies. Top and bottom surfaces of the Coral Floatstone Lithofacies are subparallel and the unit drapes over in situ coral colonies. Most corals in this lithofacies (71 percent) are broken and either horizontal or overtumed, without preferred orientation. These characteristics distinguish this "rubble zone" from the in situ Coral Lithofacics.
It is not possible to resolve matrix-coral relationships in this facies, because the matrix is too friable. The corals are petrographically and taxonomically identical to those in the in situ Coral Lithofacies.
(E) Cross-stratified skeletal lithafacies
The Cross-Stratified Skeletal Lithofacies comprises a single bed 15 cm thick exposed at the south end of the roadcut. The unit makes up less than 5 percent of the outcrop. This lithofacies resembles the Skeletal Lithofacies, but is composed of cross-laminated grainstone. Planar-tabular cross laminae pass laterally into, and are overlain by, low-angle cross laminac. Some parts of the Cross-Stratified Skeletal Lithofacies exhibit normal grading and scour. Top and bottom contacts are sharp. Scours contain pockets of echinoderm-dominated sand that varies from very coarse sand to very fine gravel. This lithofacies pinches out northward to a feather edge just south of the center of the outcrop between strata assigned to the Fenestral Lithofacies.
The lithofacies is dominated by echinoderms, with lesser amounts of fenestrate and ramose bryozoans. Other allochems include endothyroid foraminifera, brachiopods, peloids, algae, and ostracodes. Despite well-developed cross-stratification, in thin section this lithofacies appears to be an echinodermal packstone. As in the Oolitic and Skeletal Lithofacies, much of the "matrix" is clotted and in some places is clearly peloidal. Based on the cross lamination and peloidal "matrix," the lithofacies probably was deposited as a grainstone (Kopaska-Merkel and Haywick, 2001).
(F) Fenestral lithofacies
This lithofacies forms a single unit 46 cm thick near the top of the exposure and consists of yellow, fenestra-bearing, dolomitized pack-wackestone grading up to dolomudstone. The lithofacies is not internally stratified, but horizontal fenestrac are common. Calcite-filled vertical fenestrae which fork or bifurcate downward are especially common in the upper part of the lithofacies. The base of this lithofacies is sharp and planar but the upper contact is jagged. Macrofossils are limited to the lower part. Gastropods are the dominant fossils, but there are also bivalves (some in life position), and brachiopods with geopetal infilling.
(G) Breccia lithofacies
This lithofacies consists of a single unit with a maximum thickness of 22 cm, near the top of the exposure (Fig. 4). It contains abundant subangular to subrounded, yellow dolomudstone clasts up to 1 cm in diameter (Fig. 8). Pebble-size breccia in the lower 7 cm of the lithofacies grades up over 15 cm into a dark gray skeletal packstone containing pyrite, glauconite pellets, and phosphatic debris. The upper contact is the modem karstic surface. The Breccia Lithofacies is well exposed and laterally continuous over 20 m in the central portion of the outcrop, but it is poorly exposed elsewhere.
Breccia clasts consist of microcrystalline dolostone petrographically identical to the upper entirely dolomitized portion of the Fenestral Lithofacies. The surrounding matrix (in the coarse basal portion of the lithofacies) is a partially dolomitized carbonate that is too highly altered to permit unequivocal determination of depositional texture. By contrast, in the upper portion of the lithofacies where dolomite overprint was minor, the limestone is a fine- to very fine-grained packstone containing bivalves, echinoderm ossicles, bryozoans, trilobites, and ostracodes.
The seven lithofacies described above were deposited in distinct shallow marine and peritidal sedimentary environments. The Oolite Lithofacies is interpreted as a shallow marine shoal deposit. The Skeletal Lithofacies was laid down on an open shallow marine shelf. However, skeletal strata that enclose in situ Coral and Coral Floatstone Lithofacies constitute biodetrital carbonate bioherms (mounds; Fig. 9). The larger (higher) buildup measures 0.9 m thick at the core and thins toward the flanks. There is at least 45 cm of positive depositional relief at the top of this mound (Kopaska-Merkel and others, 1998) and this topography influenced the disposition of adjacent and overlying strata (Skeletal Lithofacies). Restriction of floatstone to the northern flank of the mound suggests it is a wash-over rubble bed, perhaps storm-generated, and that the northern flank is the leeward side of the mound. This inference is further supported by (1) greater abundance and larger sizes of coral colonies on the south end of th e outcrop, (2) onlap of cross-stratified skeletal grainstone on the south side of the mound, and (3) lesser concentrations of microcrystalline matrix in the Skeletal Lithofacies on the south side of the mound. Higher energy, more open marine conditions on the windward side of the mound would have winnowed out fine mud and encouraged vigorous coral growth.
The Cross-Stratified Skeletal Lithofacies is spatially associated with the Fenestral Lithofacies and was deposited in a shallow marine, current swept, environment. It is interpreted as a sand wave. Proximity to fenestral dolomudstone, diagnostic of peritidal deposition, suggests a tidal influence, but storms and/or ocean currents are also possible causes of the cross-stratification.
The contact between the Fenestral and Breccia Lithofacies is interpreted as a subaerial exposure surface based upon the rootlets in the underlying Fenestral Lithofacies and the irregular form of the surface. Dolomite content within the Fenestral Lithofacies is greatest just below the contact with the overlying breccia and diminishes rapidly below this horizon, suggesting that the dolomite was produced during subaerial exposure (cf. Folk and Land, 1975). The breccia that directly overlies the exposure surface is composed of dolostone clasts containing dolomite petrographically identical to that in the Fenestral Lithofacies. The breccia is interpreted as a transgressive lag that formed following a relative rise in sea level. The uppermost part of the Breccia Lithofacies (fine skeletal packstone) records a transition back into shallow marine deposition.
The oolitic strata at the base of the outcrop are interpreted as the uppermost part of an ooid shoal. The Skeletal and in situ Coral Lithofacies probably formed in deeper water than any of the other lithofacies (Kopaska-Merkel and Haywick, 2001). Hence, the transition from oolitic grainstone to skeletal grainstone plus coral colonies records minor deepening (probably less than 5 meters). Strata between the Oolite Lithofacies and the exposure surface at the top of the Fenestral Lithofacies record filling of accommodation space following the transgression. A relative sea-level fall exposed the upper portion of the succession to meteoric diagenesis and soil development (Haywick and Kopaska-Merkel, in prep.). The availability of a hard substrate (e.g., cemented ooid grainstone) probably was a key factor in the origin of the mound.
Rugose corals colonized the hard substrate at the top of the ooid shoal. The windward flank of the developing mound lay to the southwest, toward the shelf margin (Fig. 1). The oolite shoal underlying the mound at our study site is not part of the Bangor oolitic bank margin complex, which lay 70 km to the southwest. It is inferred to have developed on a local paleohigh created by an accumulation of sand in the underlying Hartselle Sandstone. Thomas (1972) mapped numerous sand thicks in the Hartselle that have been interpreted as barrier island or strandplain deposits. The southwest (seaward) flank of one of the Hartselle sand lobes underlies the study site.
The paleohigh formed by the top of the ooid shoal was colonized by rugose corals, stalked echinoderms, and fenestrate bryozoans. Deposits of this interval form the lowest of two mound horizons at the study site, assigned to the Skeletal and in situ Coral Lithofacies. Together, these organisms produced an effective baffle with a thickness of up to 15 cm (the height of the largest corals). The coral-crinoid-bryozoan thicket provided a congenial shelter for a wide variety of other organisms such as benthic foraminifera, ostracodes, trilobites, gastropods, small bryozoans, brachiopods, and sponges. Locally derived skeletal debris, as well as intraclasts, ooids, and skeletal material from the surrounding shelf was trapped by the baffling organisms. As the mound grew, the trapped sediment preserved or even enhanced the pro-existing relief on the ooid shoal. As a result, skeletal sediment is thickest at the mound core. Mound growth was terminated when skeletal sand (Skeletal Lithofacies) inundated and covered the mo und. This skeletal sediment thins over the crest of the mound, unlike the skeletal grainstone comprising the mound core, which forms beds that are relatively consistent in thickness (Kopaska-Merkel and Haywick, 2001).
A resumption of quiet-water conditions conducive to the growth of sessile benthos permitted corals, crinoids, and fenestrate bryozoans to recolonize the mound, forming the upper mound horizon. Because depositional relief of the mound had not been entirely obliterated, the mound resumed growth in the same location. The upper mound is similar to the lower. The major differences are a dearth of ooids and an increase in benthic foraminifera and bryozoans. The lack of ooids in the upper mound strata may record the disappearance from the vicinity of active ooid shoals. The upper mound was buried by skeletal debris, and conditions favorable for mound growth did not recur (Kopaska-Merkel and Haywick, 2001).
The Mississippian is well known for its abundance of bioherms, which have been well studied in Europe and western North America (e.g., Pray, 1961; Lees and Miller, 1995). However, no mounds had been reported from the Bangor Limestone in Alabama. The discovery of ten small carbonate buildups, one of which was large enough to affect paleocurrents during and after burial, demands revision of Chesterian paleoenvironmental interpretations in north Alabama. Small, inconspicuous mounds might have been relatively common on the Bangor shelf (Kopaska-Merkel and Haywick, 2001).
JURASSIC OF SOUTH ALABAMA
The Smackover Formation is a subsurface carbonate ramp deposit that lies beneath the U.S. Gulf Coastal Plain from Texas to Florida. Although the Smackover is dominated by nonskeletal detrital carbonate, a variety of carbonate buildups have been described (e.g., Baria and others, 1982). In this section, examples of a previously unknown mound trend are described. More detailed descriptions of these strata can be found in Kopaska-Merkel (2000).
The Smackover was deposited in southwest Alabama on a system of preexisting ridges and basins (Fig. 10). Lithologic characteristics of the Smackover differ among the basins and also with paleotopographic setting (for example, basin margin versus basin interior). The Smackover Formation (Oxfordian, Late Jurassic) conformably overlies the Norphlet Formation, a predominantly continental siliciclastic deposit formed in an arid climate (Wilkerson, 1981). The contact is commonly abrupt or gradational over an interval of a meter or less (Tolson and others, 1983; Kopaska-Merkel and others, 1992). Strata that formed under high-energy conditions (such as grainstone) were deposited in nearshore areas rimming exposed paleohighs and near the updip limit of Smackover deposition; muddy strata accumulated in basin centers.
Basal Smackover strata in Alabama contain bioherms and biostromes that formed in shallow water, especially in the southeastern Manila and northern Conecuh embayments (Kopaska-Merkel, 1994, 1998a). Middle Smackover strata, especially in basinal areas, are dominated by lime mudstone and pelletal or fossiliferous wackestone, deposited in relatively deep water. Prolific production of grainy nearshore carbonate sediment on the flanks of paleohighs initiated progradation of shallow-water strata. Upper Smackover strata comprise a succession of stacked, upward-shallowing cycles (Kopaska-Merkel and Mann, 1993, and references therein). Bioherms are widespread in upper Smackover strata in Alabama (Baria and others, 1982; Benson, 1988; Kopaska-Merkel and others, 1992; Don Fish, verbal commun., 1992; Benson and others, 1996). All known Smackover bioherms in Alabama are microbially dominated, though some contain the remains of foraminifera, sponges, skeletal algae, and metazoans (Baria and others, 1982; Crevello and Harris , 1984; Markland, 1992; Kopaska-Merkel, 1994, 1998a; Kopaska-Merkel and Schmid, 1999).
The Smackover Formation underlies the Buckner Anhydrite Member of the Haynesville Formation. The formation boundary corresponds to a brief hiatus and period of subaerial exposure of the top of the Smackover in much of southwest Alabama (KopaskaMerkel and others, 1992; Mann and Kopaska-Merkel, 1992). This was followed by inundation with hypersaline waters. The basal Buckner is dominated by subaqueous evaporites in depositional basins and by peritidal strata on the flanks and crests of paleohighs (Mann, 1988; Mann and Kopaska-Merkel, 1992).
Biodetrital mound, southeastern ridge flank, mound petrography
The core from well Permit No. 2943 (IJAMS core), located on the southeastern flank of the Saint Stephens Ridge (Fig. 10), penetrated 18.6 meters of microhermal wacke-packgrainstone that has been interpreted as a biodetrital mound (Kopaska-Merkel and Schmid, 1999). The core consists of dolomitic mixed-particle peloid wacke-pack-grainstone containing microherms. The microherms average 3.9 cm thick (n=84), but range up to 0.32 meter thick; many exceed the width of the core (9 cm), and microherms that appear to be separate may be connected in three dimensions. Microherms account for 16 percent of the core by volume. Microherms consist of one or more of (I) the remains of tubular calcimicrobes (renalcid species A; Fig. II) encased in radial fibrous calcium-carbonate cement, (2) thrombolite, and (3) ovoid or spheroidal sand-size peloids, in order of decreasing abundance. Matrix is dominated by peloids, but contains tuberoids, other intraclasts, oncoids, thin-shelled bivalves, benthic foraminifera, smooth-walled ost racodes, echinoid spines, and gastropods, in approximate order of decreasing abundance. Locally, the matrix contains burrows about 1 cm in diameter. Recognizable fragments of microherms account for only a small fraction of the matrix.
Microherms grew by expansion of grounded oncoids and tuberoids. Some larger microherms exhibit shapes suggesting retrenchment and rejuvenation, and in some cases contain almost as much enveloped matrix material as they do microherm material (KopaskaMerkel and Schmid, 1999). Strong waves or currents eroded the tops of microherms, generating tuberoids that became nuclei for oncoids. When water energy was lower, oncoids came to rest and new microherms commonly grew upon the microbial "seeds" (KopaskaMerkel, 1994; Kopaska-Merkel and Schmid, 1999). The tops of microherms were bored at particular levels in the core (Kopaska-Merkel and Schmid, 1999). Borers may have been controlled by changes in water chemistry, such as fluctuations in oxygenation of bottom waters (Kopaska-Merkel, 1998b). Overall shoaling of the mound is suggested by upwardly increasing particle size, decreasing abundance of microherms, and increasing particle support in the matrix. Interaction between sediment supply, water chemistry, and water ene rgy governed the interplay between formation of oncoids, conversion of mobile oncoids to sedentary microherms, and destruction of microherms with subsequent formation of new oncoids (Fig. 12).
The inferred mound-constructing action of the microherms involves biocementing, binding, and baffling. Biocementing and binding occurred within microherms through the formation of early marine cement coats on renalcid species A and the encrusting growth of thrombolite in layers a few 100 pm to about 1 mm thick (Kopaska-Merkel and Schmid, 1999). Baffling occurred within and among microherms. Large microherms were efficient bafflers because they have complex, deeply invaginated shapes and envelop considerable amounts of rock matrix.
Comparison to other mounds
The sediment body containing the microherms is known from a single core; it is not known whether the putative mound exhibited synoptic relief or affected water circulation. However, the fabric of the bioherm-bearing succession resembles that of the Bangor mound in Lawrence County. Mounds with a packstone fabric also have been reported from the Fort Payne Formation of Kentucky (Ausich and Meyer, 1990) and Visean strata of Ireland and England (Lees and Miller, 1995). The IJAMS core contains 16 percent microherms, which compares favorably with the percentage of inferred framework material in the Bangor mound and in Waulsortian mud mounds (J. L. Wilson, 1975). Finally, microherms in the IJAMS core (chiefly constructed by renalcid species A) record the former presence of a distinctive mound biota.
Grainy microhermal mounds are a kind of bioherm previously unrecognized in Jurassic rocks. Although Jurassic mounds are widespread in outer ramp settings, they consist mainly of thrombolite, siliceous sponges, and carbonate mud (Leinfelder and others, 1994), and the only "rock-forming" microorganism in these mounds is Tubiphytes morronensis (e.g., Schmid, 1996).
Biodetrital mound, northern ridge crest, mound substrate
The 26.2 m core from well Permit No. 2769 (Wilson core) (Fig. 10) penetrated two stacked mounds in their entirety, as well as over- and underlying strata. The basal 6.1 m of core (the mound substrate) consists of very fine peloid grainstone. This unit contains oncoids, echinoderm ossicles (including spines), bivalve fragments, tuberoids containing renalcid species A, ostracode valves, intraclasts, Parafavreina, and quartz silt. A I-m-thick interval contains up to 40 percent poorly preserved small microherms.
The mound substrate grades up over a meter or less into a microhermal biodetrital mound. In the transition zone, 1 to 3 cm of oncoid packstone grade up into coarser microhermal packstone. The mound, which is 8.5 m thick, consists of small microherms embedded in a matrix of silt to very fine sand size oncoidal tuberoid peloid grainstone. Microherms account for 15 to 50 percent of the mound (averaging 25 to 30 percent). Fragments of microhenns are uncommon in the matrix. Oncoids contain peloids but no skeletal microfossils. Microherms are crudely laminated and clotted; their nuclei resemble associated oncoids. Microhenn cortices are dominated by fans or botryoids of coarse, inclusion-rich spar, but they also contain microspar, renalcid species A, well-defined peloids, and peloids with diffuse boundaries (grumose structure). Associated biota include bivalves, possible dasycladacean algae, and Parafavreina. Renalcid species A, which is found in microhcnns but not in oncoids in this core, may have preferred a phys ically stable microenvironment, growing only on particles that were at rest for significant periods. It is also possible that renalcid species A throve only under conditions hostile to the growth of other calcifying organisms. This is inferred from the observation that renalcid species A is common where other fossils are scarce (Kopaska-Merkel and Schmid, 1999).
Comparison to other mounds
This mound differs from that in the IJAMS core: microherms are more abundant, the mound deposits are thinner, and well preserved specimens of renalcid species A are scarce. Figure 13 schematically represents the morphology of microhermal biodetrital mounds at the time of formation.
The biodetrital mound is abruptly overlain by a layer of oncoid tuberoid peloid packstone 0.8 to 1.7 m thick (0.9 m of core are missing), which appears identical to the matrix of the biodetrital mound, but lacks recognizable microherms. This debris layer is overlain by a microbial mound (see below).
Microbial mound, northern ridge crest, mound petrography
A microbial mound nucleated on the top of a thin layer of debris mantling a biodetrital mound. The microbial mound consists of thinly laminated mierobialite (stromatolite) and crudely laminated to thin bedded microbialite composed of millimeter- to centimeter-scale irregular strata, blobs of various shapes, and uneven coatings, here all considered to be thrombolite. The microbial mound is a 9-m-thick thrombolite-stromatolite complex (Fig. 13) in which renalcid species A is rare, but other fossils arc abundant and diverse. Thrombolite (Fig. 14 A-B) accounts for well over half the volume of this interval. At least three distinct thrombolite microstructures are represented: diffuse clots (grumose structure; Fig. 14 A; cf., Turner and others, 2000), well-defined clots (Fig. 14 B, base), and homogeneous microspar (Fig. 14 B, top). Thrombolites with differing microstructures overlie one another directly (Fig. 14 B), in a constructional relationship that Wood (1999) called mutual encrustation. Microscopic elements ( e.g., Parafavreina) exhibit consistent preservational styles throughout the thrombolite-stromatolite complex, which suggests that the three distinct thrombolite microstructures do not result from variable taphonomic or diagenetic effects (cf., Turner and others 2000), but were formed by three different organisms or biotic associations. The thrombolite-stromatolite complex includes Parafavreina (Fig. 14 B); Helicerina (Fig. 15); intraclasts; ooids; oncoids; foraminifera; bivalve and gastropod mollusks; echinoderm ossicles; smooth-walled ostracodes; calcispheres; coccoid calcimicrobes embedded within the primary framework (Fig. 14 A, center); renalcid species A (an uncommon component of the primary framework); several distinct unidentified microfossils; mica and quartz silt within the framework, and microcrystalline clots, laminac, drapes, and irregular blobs that secondarily (Wood, 1999) encrusted the thrombolite or stromatolite framework (Fig. 16). Framework is used here to refer to the three-dimensional body (chiefly consisting of calcium carbonate), which was constructed by thrombolitic and stromatolitic microbes, and which does not necessarily contain calcified skeletons. The framework exhibits the following characteristics: (1) biologic encrustation, (2) in situ brecciation, and (3) near-vertical exterior surfaces. No evidence of biocrosion was recognized. The microbial framework probably was lithified at or just after the time of formation.
The thrombolite contains abundant fenestrae or crypts (Fig. 16), which may have formed as gas evolved during decay of microbial mats. Fenestrae in the lower two-thirds of the mound are filled with three generations of cement: silt-size clear pore-rimming bladed calcium carbonate, fine to medium textured planar-s dolomite, and medium textured clear blocky pore-filling calcium carbonate.
Fenestrae in the upper third of the mound contain a very different succession of phases (Fig. 17 B). Thrombolite surfaces lining fenestrac in the upper part of the mound are locally encrusted by microcrystalline stalagmitic and stalactitic deposits (Fig. 16) that are probably microbial. Medium textured inclusion-rich calcium-carbonate crystals with abundant nonplanar boundaries and few enfacial junctions form a pore lining that has an irregular surface; this phase overlies the microcrystalline stalagmitic and stalactitic carbonate.
Inclusion-rich spar contains inclusion-rich planar-e dolomite, which has been extensively calcitized and locally pyritized. Inclusion-rich spar is locally underlain and locally overlain by a thin discontinuous rind of silt-size limpid planar-e dolomite. Inclusion-rich spar is overlain by pore-filling medium textured clear blocky calcium carbonate with abundant enfacial junctions.
It is suggested that inclusion-rich spar records the former presence of a prolific microbial binder of unknown affinities, which induced precipitation of magnesian calcite that was later either neomorphosed to inclusion-rich calcite (c.f., Monty, 1967; Buczynski and Chafetz, 1991; Arp, 1995; Gonzilez-Muiioz and others, 2000) or replaced by dolomite. In either case, most of the dolomite was replaced by inclusion-rich spar. Following formation of inclusion-rich spar and local dolomite cement, medium textured blocky calcium-carbonate cement filled most of the remaining primary porosity in the upper part of the mound. A trace of primary porosity remains locally.
Recognizable remains of the organisms that actually constructed the mound are preserved only locally in this core (e.g., renalcid species A) and the nature and affinities of the major constructors are unknown.
Comparison to other mounds
The preserved biota of this mound is more diverse than those of most mounds previously reported from the Smackover of Alabama (e.g., Baria and others, 1982; KopaskaMerkel, 1998a). Clearly, environmental conditions on the Saint Stephens ridge during late Smackover time were conducive to microbial growth.
The mound is abruptly overlain by tuberoid peloid packstone and this in turn is overlain by oolitic grainstone with keystone vugs. These two units are interpreted to record rapid development of a beach on top of the mound, perhaps by progradation from a nearby island or by a drop in sea level. Platy tuberoids up to 2 cm across in the tuberoid peloid packstone resemble the upper part of the underlying mound and also resemble tuberoids trapped within large fenestrae in the uppermost 0.5 m of the mound.
Paleoenvironmental setting of mounds
The biodetrital mound at the south end of the ridge consists of calcimicrobial microherms in a wacke-pack-grainstone matrix. This mound is dominated by a single taxon, the calcimicrobe renalcid sp. A, and faunal diversity is very low. The base and top of the mound were not cored. Water depth during mound growth probably was less than about 35 m, for the base of the core is 35.6 m below the top of the Smackover, inferred to consist of peritidal deposits as it does in nearly every core in this area. Thickness of upper Smackover strata is considered a good predictor of palcowater depth. Upper Smackover strata were deposited during a single sea-level highstand and there is no evidence for major sea-level fluctuations during late Smackover time in southwest Alabama (Mancini and Benson, 1980; C. H. Moore, 1984; Benson, 1988; Mancini and others, 1990; King and Moore, 1992; Kopaska-Merkel and others, 1992; Mann and Kopaska-Merkel, 1992; Benson and others, 1996). Subsidence on this passive margin during the brief time represented by the upper Smackover (part of the late Oxfordian) probably was negligible compared to the sediment accumulation rate.
A similar microhermal biodetrital mound was cored in its entirety in the Wilson core. The mound is underlain by peloid grainstone interpreted as a lagoonal or subtidal shelf deposit. The biota in the mound substrate is more diverse than that of the mound itself. The base of the mound is 19.4 m below peritidal (beach) deposits at the top of the core and therefore the mound probably grew in water less than 20 meters deep (see previous paragraph). The mound is mantled by a debris layer generated by storm reworking of the upper part of the mound. The two biodetrital mounds appear to have grown in essentially the same way (Fig. 13). Mound growth was initiated within the euphotic zone when water energy decreased to a point at which oncoids ceased to roll frequently and became microherms. Fluctuation in water energy and oxygenation during mound growth affected rates of microherm growth and destruction by waves and boring organisms. These fluctuations could have been caused by small-scale sea-level changes. The mound s were killed when water energy increased once more and mobile sediment buried the mounds.
A microbial mound, 9 meters thick and cored in its entirety, overlies the biodetrital mound in the Wilson core. Although the framework of the mound is entirely microbial, the preserved community is unusually diverse for the Alabama Smackover. Framework elements include three distinctly different thrombolite microstructures as well as stromatolites. tncorporated biogenic material is taxonomically diverse and a variety of noncalcitied microbes inhabited crypts. There is little evidence of bioerosion. In contrast to the biodetrital mound lower in the core, the microbial mound in the Wilson core contains no recognizable remains of the framework constructing organisms, whose nature and affinities are unknown. The diverse biota, including two genera of thalassinideans and probable photosynthesizers (noncalcified and calcified cyanobacteria), indicates that the site was home to a thriving community during late Smackover time (Fig. 17) and that waters were probably well ventilated and shallow. The base of the mound i s 11 meters below inferred beach deposits at the top of the core, and therefore the mound probably grew in water no deeper than 11 meters (see previous discussion of sea-level history). The microbial mound began to grow when water energy decreased, stabilizing mobile sea floor sediment. Mound growth continued while moderate levels of water energy maintained circulation but did not smother or break up the mound. Mound growth ended when increased wave agitation, possibly associated with storm activity, caused scouring of the "crest," filled open cavities in the upper part of the complex with debris, and buried the mound (Fig. 18).
The Saint Stephens ridge was shallowly submerged and probably locally subaerially exposed during late Smackover time. Water overlying the ridge during late Smackover time probably departed significantly from normal marine conditions and may have been moderately hypersaline. Abnormal water characteristics in southwest Alabama during the late Oxfordian may have excluded members of the Jurassic coral-calcified sponge-solenoporacean algalmierobialite community, which formed many shallow-marine Jurassic bioherms (Leinfelder, 1994, cited in Wood, 1999). Environmental exclusion of typical European Oxfordian shallowmarine communities from the Alabama Smackover could explain the importance of thrombolites, which have been interpreted as indicating deep-water deposition in the Oxfordian of Europe (Leinfelder, 1994). Biodetrital and microbial mounds in the Smackover Formation on the Saint Stephens ridge may be appropriate models for Mesozoic mounds in restricted, shallow-water settings.
Most of Alabama is underlain by strata that formed in marine or coastal environments. These units range in age from more than half a billion years old to deposits forming today on the coast and in Mobile Bay. Many of Alabama's ancient rocks and sediments contain fossil reefs or mounds, ecologically similar to living oyster reefs in modern Mobile Bay. Reefs and mounds (collectively called bioherms) are organosedimentary buildups: positive features made by, or influenced by, organisms. Reefs are held up (at least in part) by rigid frameworks; mounds lack such frameworks.
Two examples of these ancient buildups are described. These include Mississippian bioherms and biostromes exposed at the surface in central and north Alabama and Jurassic bioherms buried more than a mile beneath the surface in southwest Alabama.
The Chesterian (Upper Mississippian) Bangor Limestone was deposited on a broad platform that stretched across north Alabama. Small carbonate buildups grew on local topographic highs in the central part of the Bangor marine shelf. A mound near Moulton in Lawrence County consists chiefly of packstone and grainstone dominated by echinoderm ossicles and fragments of fenestrate bryozoans. In situ colonies of the rugose coral Caninia flaccida compose about 8 percent of the mound by volume. The exposed portion of the mound is approximately 25 m wide, 1.6 m thick at the thickest point and roughly circular in plan. The mound possessed about 45 cm of synoptic relief when fully developed. Strong currents within the mound are indicated by preferred orientation of corals and by coarse, commonly crossstratified grainstone in channels between neighboring coral colonies. Corals are most abundant on the windward side of the mound, where they account for about 13 percent of the mound, compared to 6 to 10 percent in the central part of the mound, and 2 to 4 percent on the leeward flank. Other(smaller) Bangor buildups in Alabama are rugose-coral biostromes, microbially bound rugose-coral reefs, or bryozoan-microbial mounds.
Carbonate mounds flourished in the Oxfordian (Upper Jurassic) Smackover Formation on the 65-kilometer-long Saint Stephens ridge. The ridge originated as an accumulation of eolian sand in the Norphiet Formation, which in turn formed upon a positive erosional feature on the pre-Mesozoic basement. The ridge crest, up to 15 kilometers wide, supported distinct communities of mound builders and associated organisms that constructed fundamentally different kinds of mounds.
On the southeastern ridge flank, a biodetrital mound at least 18 meters thick is dominated by locally derived nonskeletal packstone. The mound incorporated microherms (small bioherms) up to I meter thick, which account for 16 percent of the mound. The microberms were constructed by a renalcid calcimicrobe (microorganism that induced the precipitation of a calcified "skeleton"). On the northern part of the ridge crest a similar microherm-bearing biodetrital mound 8.5 meters thick is directly overlain by a microbial mound 9 meters thick. The microbial mound consists of stromatolite (laminated microbialite) and of thrombolite (clotted microbialite) with three different microstructures: (1) diffuse clots (grumose structure), (2) well-defined clots, or (3) homogeneous microspar. Fenestrac within thrombolite contain the remains of a low-diversity cryptic microbial community.
Alabama has been home to many different kinds of mound-building organisms over the past half a gigayear. When environmental conditions were right, these organisms built mounds and reefs in the nearshore marine environments that so often occupied Alabama. Much of our state was deposited in these environments because they facilitate rapid sediment accumulation. Terrestrial environments are disproportionately underrepresented in the rock record, even though they represent nearly all of what we experience in our daily lives, because they are predominantly erosional.
Portions of the manuscript were critically reviewed by Christophe Dupraz and Douglas W. Haywick. Several of the figures and all of the thin sections were made by Douglas W. Haywick. Donald F. Oltz, State Geologist of Alabama, gave permission to publish this paper.
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Allochem. A particle in a carbonate rock; i.e., not mud or cement.
Biodetrital. Pertaining to carbonate mounds. Indicates a mound that is essentially a pile of debris, albeit formed (at least in part) by the action of organisms.
Bioherm. A positive-relief feature built (at least in part) by organisms. Bioherm biotas commonly differ from surrounding flat-bottom communities. Bioherms are large enough to affect water circulation and may show ecological zonation.
Biostrome. A planar feature built (at least in part) by organisms.
Bryozoan. "Moss animals." Relatives of brachiopods that resemble miniature colonial corals. Habits are upright, encrusting, or (less commonly) boring.
Buildup. See bioherm.
Calcimicrobe. A microscopic organism that induces precipitation of calcium carbonate in such a way as to preserve some aspects of its form when alive. Not shells or skeletons per se, calcimicrobial precipitates tend to vary in their fidelity of preservation.
Dolomite. Calcium-magnesium carbonate. A mineral most commonly formed by the replacement of calcium carbonate (calcite or aragonite).
Fenestra. Window-like opening.
Floatstone. A carbonate rock type in which large particles "float" in a finer matrix.
Grainstone. A carbonate rock type in which particles support one another and mud matrix is minor or absent. A lithified carbonate sand.
Intraclast. A particle (clast) that was deposited, lithified, broken loose, and reworked into the same deposit in which it was originally laid down. This is a common process in carbonate rocks.
Micrite. Lime mud; microcrystalline limestone.
Microbialite. A carbonate rock formed by the action of microbes.
Microherm. A small bioherm (which see).
Mound. A positive-relief feature made (at least in part) by organisms, but lacking a rigid skeletal framework. Mound biotas commonly differ from surrounding flat-bottom communities. Mounds are large enough to affect water circulation and may show ecological zonation.
Oncoid. "Algal ball." A spheroidal particle, millimeters to centimeters in diameter, made by the concentric encrustation of a nucleus by algae, cyanobacteria, or other microbes.
Oolite. A rock composed chiefly of ooids.
Ooid. A spheroidal particle, up to about 1 mm in diameter, evenly concentrically laminated, that has a smooth exterior.
Packstone. A carbonate rock consisting of particles that are in contact but contain some lime mud in its interstices.
Peloid. A spheroidal microcrystalline carbonate particle of indeterminate origin. Most peloids are either fecal pellets or particles that were converted to microcrystalline carbonate through the action of microborers.
Reef. Like a mound (which see) but with a rigid skeletal framework supporting at least part of the structure.
Rugose Coral. A group of Paleozic corals, mostly solitary, only distantly related to modern corals. Solitary rugose corals are conical or banana shaped.
Stromatolite. A laminated rock that was formed as a result of the influence of algae, cyanobactena, or other microbes.
Thrombolite A clotted rock that was formed as a result of the influence of algae, cyanobacteria, or other microbes.
Vadose. Formed in the zone of partial water saturation (e.g., the soil zone). Refers to calciumcarbonate cements.
Wackestone. A carbonate rock in which particles are floating in a matrix of fine material.
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|Author:||Kopaska-Merkel, David C.|
|Publication:||Journal of the Alabama Academy of Science|
|Date:||Apr 1, 2002|
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