Smackover Sedimentology in Alabama's
Conecuh and Manila Basins
 

David T. King, Jr.
Department of Geology, Auburn University
Auburn, AL 36849-5305 USA
 

"... from the changing ratio of algal and skeletal debris
one can decipher the life history of an algal deposit."
-- Johannes Walther (1885)


 



ABSTRACT

Excepting buildups and minor high-energy facies, Smackover facies of the Conecuh and Manila basins are dominated by carbonate mud, and there is generally a significant admixture of terrigenous material. In addition, the suite of Smackover allochemical grains is limited to low-diversity skeletal, fecal, and algal components. Most Smackover fine-grained facies are low photic zone deposits of relatively deep and(or) turbid water. In both basins, Smackover depositional facies complexity is high owing to a combination of paleogeographic and paleotopographic factors. And, in both basins, lower Smackover depositional facies generally differ strongly from upper Smackover depostional facies. There is a distinctive boundary, referred to herein as a maximum-flooding surface, between lower and upper Smackover. This surface represents a transition between depositional systems from an unrimmed platform to a carbonate ramp. Such a transition was brought about by a significant change in rate of relative sea-level rise during the Smackover's depositional cycle (spanning 150.5 to 144 Ma). Differences in depositional facies arrangement between the two basins show small, but significant differences in relative rate of sea-level change.

INTRODUCTION

Smackover, owing to its connection with petroleum production around the Gulf of Mexico rim, is one of the best known carbonate formation names in the world. Originally drilled in Union County, Arkansas, in the late 1930s and named for that county's Smackover field (Bingham, 1937), this unit has been extensively studied. Dozens of papers have been published on Smackover stratigraphy, petrology, and petroleum resources, many of them relating to Alabama basins. Among papers most commonly cited and relating to Alabama are ones by Mancini and Benson (1980; discussing Smackover's regional stratigraphy), Baria et al. (1982; discussing Smackover's reefs), Bradford (1984; discussing details of Smackover depositional facies), Tew et al. (1993; petroleum geology), and Benson (1988; presenting a review of Smackover's depositional history).

This paper's purpose is to summarize findings about Smackover sedimentology (i.e., petrology and stratigraphy) from research that I supervised. This research was focused on two Alabama basins, Conecuh and Manila (Figure 1). I draw from research done by three graduate students: Richard Esposito (who graduated in 1987), Glenn Hargrove (who produced a paper with me in 1991), and Daniel Moore (who graduated in 1991). Esposito (1987) worked on samples from 11 wells in the Conecuh basin. Hargrove worked on samples from 12 wells in the northern half of the Manila basin (King and Hargrove, 1991). Moore (1991) worked on samples from 8 wells in the southern half of the Manila basin. Subsequent published papers by Esposito and King (1987) and King and Moore (1992), cited herein, were based upon the two respective thesis projects.

This paper is dedicated to Johannes Walther (1860-1937), who pioneered the actualistic study of carbonate buildups in general, and algal carbonates in particular, beginning over 100 years ago. Most geologists know about Johannes Walther from basic geology and stratigraphy classes where they were introduced to "Walther's law" (or what Walther (1894) called "Law of Correlation of Facies"). Middleton (1973) has provided a translation of what Walther wrote on this Law in the third volume of his 1894 book Lithogenesis der Gegenwart (Modern Lithogenesis):

"... it is a statement of far-reaching significance that only those facies ... can be superimposed primarily which can be observed beside each other at the present time."

The last four words indicate an important aspect of this law, namely its actualistic basis. Walther was a widely traveled person and keen observer of processes and products, both modern and ancient. His actualistic overview of stratigraphy and paleontology is summed up in his 1893 statement: "From being (existence), we explain becoming (genesis)" (Middleton, 1973). Considering all his work with carbonate-mud dominated environments and algal carbonates (see Gischler, 1994), I think that Walther would have been fascinated by the Smackover Formation.

SMACKOVER RESEARCH

Smackover research from a sedimentological perspective can be divided into "before and after" based upon the scientific revolution produced by the publication of coastal onlap charts and eustatic sea-level curves. In 1977, publication of AAPG's now-classic Memoir 26, which contained the first systematic coastal onlap charts and eustatic sea-level curves, including representation of their interpretations for Jurassic (Vail et al., 1977), started this revolution. Later, Vail et al. (1984) presented an updated version of a Jurassic coastal onlap chart and eustatic cycle chart, which has since been updated by Haq et al. (1987; 1988). As Haq et al. (1988) point out, "usefulness of chronostratigraphic eustatic framework in exploration geology lies in the fact that it provides predrill estimates of geologic parameters ... and (leads) to more accurate stratigraphic, structural, and facies interpretations."

Critical to correct application of coastal onlap charts and eustatic cycle charts and their interpretation with respect to Smackover studies was acquisition of acceptably strong biostratigraphic control. This biostratigraphic control came from Imlay and Herman (1984) who studied ammonites recovered from Smackover wells in Texas, Louisiana, and Mississippi. On the strength of Imaly and Herman's (1984) age assessment as late early Oxfordian to early Kimmeridgian, researchers began to relate Smackover depositon to a specific Jurassic depositional cycle, namely J3.1, as identified globally by Vail et al. (1984). One of the first papers to do this for Alabama basins was written by Benson (1988). Esposito (1987) also remarked on the relation of cycle J3.1 to Smackover stratigraphy, and we wrote about these points in our paper based on his thesis (Esposito and King, 1987). Later, Mancini et al. (1990) connected total Jurassic sequence stratigraphy of Alabama's basins to Vail et al.'s (1984) Jurassic coastal onlap charts. In so doing, they debated validity of Haq et al.'s (1988) division of earlier designated cycle J3.1 into smaller cycles, designated by Haq et al. (1988) as LZA 4.1-4.4. Mancini et al. (1990) decided to refer to a single depositional cycle spanning an interval comprised of upper Norphlet marine sandstones, Smackover carbonates, and lower and middle Haynesville evaporites and clastics, calling it LZAGC4.1, a cycle meant to be specific to the Gulf Coastal region.

In this paper, instead of generalizing about Smackover sequence stratigraphy regionally or across Alabama, I intend to focus on the sequence stratigraphy within the Conecuh and Manila basins, including apparent similarities and differences. In the discussion that closes this paper, I will relate Alabama's Smackover sequence stratigraphy to global synthesis of coastal onlap and eustasy as viewed from the Conecuh and Manila basins.

SMACKOVER PETROLOGY

Smackover rocks have unusual original petrologic characteristics as compared with more common shelfal, platform, and ramp-derived carbonate rocks. Probably the two most striking aspects of its original, or pre-diagenetic, carbonate petrology are its large carbonate-mud component (with significant terrigenous admixture) and its low-diversity suite of constituent allochemical grains. Except for commonly minor oolitic and algal buildup facies, most Smackover rocks are have these key aspects. Depending upon depositional facies, Smackover samples usually contain only a minor percentage of skeletal and(or) algal grains, if any. In contrast, most Smackover rocks contain a significant component of fecal grains. Figure 2 shows the distribution of petrologic constituents within each Smackover depositional facies for which there is sufficient statistical data.

Skeletal grains - Esposito (1987) and Moore (1991) reported finding minor amounts of pelecypods, gastropods, brachiopods, pelmataozans, sponges, and foraminifera in thin sections from the Conecuh and Manila basins, respectively. Most of their thin sections contained minor or trace amounts of only one or two of these taxa. Regarding the Conecuh basin, these constituents were especially concentrated, averaging 4 percent by volume, only in lower Smackover clinoform turbidites (Esposito and King, 1987). Similarly, in the Manila basin these constituents were most common by far in lower Smackover clinoform facies (King and Moore, 1992).

Algal grains - Esposito (1987) and Moore (1991) reported finding significant amounts of algal grains, including oncolitic masses, masses of Tubiphytes, and other unclassified algal grains in thin sections from the Conecuh and Manila basins, respectively. Some of their thin sections contained 10 to 50 percent of these algal constituents. In addition, King and Hargrove (1991) and Moore (1991) recognized lower Smackover patch-reef and shelf-margin boundstones in the Mania basin, which were not seen by Esposito (1987) in the Conecuh basin, which had a greater-than-50 percent algal component. In the Conecuh basin, algal components (which averaged approximately 20 percent by volume) were restricted mainly to lower Smackover clinoform turbidites (Esposito and King, 1987). In the Manila basin, aside from boundstone facies, these constituents were mainly restricted to lower Smackover clinoform facies (King and Moore, 1992).

Fecal grains.- Esposito (1987) and Moore (1991) reported finding significant amounts of fecal material, i.e., pellets and peloids, in many thin sections from the Conecuh and Manila basins, respectively. Many of their thin sections contained 5 to 30 percent of these fecal constituents. Smackover pellets consist mainly of a petrographically distinct form called Fabreina, which has been attributed to a nektic crustacean. Pellets and peloids (any other micritic pellet-like form) are especially concentrated in lower Smackover clinoform facies in both the Conecuh and Manila basins (Esposito and King, 1987; King and Moore, 1992). However, fine-grained facies of both the upper and lower Smackover contain a small percentage of pellets and peloids.

Oolites.- Esposito (1987) and Moore (1991) reported finding oolitic rocks from the upper Smackover in Conecuh and Manila basins, respectively. In the Manila basin, King and Hargove (1991) and Moore (1991) reported oolitic rocks in the lower Smackover as well. Generally, dolomitization strongly affects Smackover oolitic rocks and thus their original texture is largely obscured. Moore (1991) found some oolitic rocks in the Manila basin that were relatively unaltered and determined that they consist of 65 to 75 percent oolites, which are well sorted and generally have small carbonate nuclei.

Terrigenous grains.- Esposito (1987) and Moore (1991) reported finding a small but significant terrigenous component across almost all Smackover depositional facies in the Conech and Manila basins, respectively. Terrigenous components, which averaged about 10 percent in the Conecuh basin and 15 percent in the Manila basin, consisted of angular quartz silt, silt-sized muscovite flakes, and clay. Esposito and King (1987) noted that terrigenous components were an order of magnitude less abundant in lower Smackover undaform basinal facies.

Carbonate mud.- Esposito (1987) and Moore (1991) reported finding a majority of carbonate mud in most thin sections from the Conecuh and Manila basins, respectively. In both basins, carbonate mud averaged 60 to 80 volume percent in facies other than oolitic grainstones and some beds of lower Smackover clinoform facies. Esposito and King (1987) noted that carbonate mud is least common in the Conech basin within lower Smackover clinoform turbidites.

Other constituents.- Esposito (1987) and Moore (1991) reported finding minor to significant amounts of other constituents in rocks from the Conecuh and Manila basins, respectively. Dolomite is the most common other constituent, ranging from a few volume percent to nearly all the rock in some instances. In both basins, Smackover dolomite is constituent- and facies-selective. Minor amounts of pyrite, anhydrite, and pore space were found in both basins. Intraclasts, relatively rare allochems, were noted in only a very few samples from each basin. The rarity of intraclasts as allochems is an anomalous feature of Smackover petrology.

IMPLICATIONS OF PETROLOGIC CHARACTERISTICS

Skeletal grains.- The low taxonomic diversity among skeletal grains is indicative of an impoverished fauna. The state of the fauna may have been due to factors like relatively high salinities, turbidity of sea water, and(or) inhospitable substrates. I think it likely that all these factors and perhaps others played a part. From direct evidence provided by thin section, I am inclined to think first of turbidity because of the pervasive presence of a significant clastic component in the rock. The turbidity may have been intermittent or episodic, something that algae could better deal with than the associated fauna.

Johannes Walther, now recognized as the father of modern actualistic investigations of sedimentology and paleontology, was one of the first to study carbonate depositional systems in detail. His observations from studies of buildups on Taubenbank, Gulf of Naples, (Walther, 1910, translated in Ginsberg et al., 1994) are applicable to our Smackover problem. About turbidity and soft substrates, he wrote:

"What holds true for floating organisms ... is true for the benthos of the sea floor to a much greater degree. All bottom dwelling animals and ... plants, show a distribution with strict dependence upon light ... . The poverty of life in the soft mud ... is caused not only by the mobility of the substrate, but to a much greater degree by the poverty of light in the water above."

Cause(s) for the "poverty of light," as Walther puts it, may have been turbidity (as noted above), but also could be due to relatively deep-water conditions prevailing during deposition of lower Smackover, in particular. In this view, the Smackover shelf would have been a place of "catch-up" carbonate deposition during a rather rapid rise in relative sea-level.

The skeletal component in Smackover rocks is preferentially concentrated in clinoform facies, especially turbidite beds. How this came to be probably tells us something about the function of Smackover algal buildups: they were probably centers of benthic productivity on the Smackover's soupy substrate. Intermittent or episodic degradation of Smackover buildups is likely connected with generation of resedimented debris, and thus turbidites, present in the clinoform.

Johannes Walther was the first to study algal reefs and related fauna in the field and in the sea. His observations from studies of algal dominated sediments forming in the Gulf of Naples (Walther, 1885, translated in Ginsberg et al., 1994) are useful today in appreciating Smackover sedimentology. Of those algal sediments, he wrote:

"Growth of an algal deposit is dependent upon various living conditions, and fluctuations in these, in turn, have an effect on the growth of the algae. If the environment changes only slightly, the weaker plants will die off and, in contrast, stronger algae will continue to grow. In this way local gaps develop in the upper surface of the algal deposit, which become filled with detritus. The greater the vitality of the algae, the rarer the accumulations of the detritus; the more the vitality declines, the larger the areas occupied by skeletal debris. If the environmental limits for the algae change to a larger degree, such gaps become more frequent and larger; and correspondingly the participation of skeletal debris in the construction of the limestone deposit becomes greater. If one wishes to assess a fossil (algal) deposit, the enclosed lenses or intervening layers of detritus will give a standard for the vitality of the deposit concerned, and from the changing ratio of algal and skeletal debris one can decipher the life history of an algal deposit."

I think that Walther's observations are useful today in understanding Smackover sedimentology: it makes perfect sense, judging from what Walther is saying, that the fauna (which comprises less than 5 percent of the rock in algal buildups (Moore, 1991)) ended up being slightly more concentrated in resulting buildup detritus, i.e., clinoform turbidites. In this process, fauna flourish in times of buildup decline. And, detritus are more often swept into deeper water due to the decimated condition of algal buildups. When buildups florish, the opposite is true and relatively few skeletal grains are produced.

Algal grains.- It is notable that algal grains are strongly concentrated in lower Smackover facies, except in buildups surrounding paleotopographic highs that influenced upper Smackover deposition as well.

As inferred from the quote and discussion above, occurrence of algal grains in detritus is likely connected to the productivity and physical state of algal buildups extant at any given time. Thus, there was an apparent algal buildup connection to the texture of most facies throughout Smackover deposition. As noted below, algal buildups may have contributed much to Smackover carbonate-mud production. Perhaps it is fair to call the whole Smackover an "algal deposit."

Fecal grains.- Fecal grains are well preserved in Smackover rocks, which is probably a testament to rapid sedimentation and perhaps poorly oxygenated bottom conditions. Fecal grains are most common in rocks that lack significant skeletal and algal component. Indeed, petrographically determined skeletal-algal-fecal ratios (or simply algal-fecal ratios) of consitituents grains can be used as paleoenvironmentally sensitive indicators. Figure 3 shows how such ratios relate to lower Smackover paleoenvironmental conditions.

Carbonate mud.- Carbonate mud (micrite) in Smackover facies likely had multiple sources. The easiest source to envision is biogenic debris from pelagic organisms, however the turbidity argument above makes that contribution somewhat questionable. Benthic sources could contributed greatly to Smackover production of fines, and primary productivity within algal buildups is a likely benthic source for carbonate mud. Discussion at the end of this paper suggests a much higher sedmentation rate in lower Smackover versus upper Smackover. Carbonate mud obviously played a strong part in determining sedimentation rate. Because algal buildups play a much stronger role in lower Smackover facies than in upper Smackover, we have a point in favor of benthic production of carbonate mud.

FACIES STRATIGRAPHY

One of many peculiar aspects about Smackover facies stratigraphy that I noticed when I began to read about it and look at core is the high level of complexity in what looks superficially like a simple unit. Complexity here results from several factors, including coarse- and fine-scale basinal paleogeography, variable intrabasinal paleotopography and paleobathymetry, local peculiarities of paleogeomorphology (i.e., slight differences in slope and resulting diffuse facies belts), and variable paleooceanographic conditions.

In order to try to see broader depositional facies relationships, and thus see sequence stratigraphic relationships, beyond this complexity, I selected for study several series of wells that seemed to me to illustrate either cross-sectional strike or dip relationships upon or adjacent to basinal margins of both Conecuh and Manila.

Genetic packages.- In previous work by myself and my graduate students, we used a generic term, genetic packages, in reference to parts of Smackover stratigraphy. Namely, we recognized a lower genetic package (GP-1) and an upper genetic package (GP-2). By genetic package, we meant 'a suite of rocks that had interpreted closely related origins within a single coeval depostional system,' e.g., a prograding ramp. From the outset of our studies (i.e., in my early intial work with Esposito on the Conecuh basin beginning in 1986), it was evident to us that there was a distinctive difference in depositional facies and facies relationships within lower versus upper Smackover. When Moore, Hargove, and I began to look at Manila basin rocks in 1987, the same lower versus upper Smackover division seemed to apply. While we were never able to see the lower-upper boundary in a piece of core, it certainly looked sharp and distinctive on some electric logs.

Maximum-flooding surface.- In this paper, I will depart from previously used nomenclature and refer to this genetic package (lower-upper) boundary as a maximum-flooding surface, because the latter is a more accurate interpretative term according to the tenets of sequence stratigraphy. I think this intra-formational flooding surface is the turning point in Smackover deposition, representing the time when Smackover depositonal systems changed from those of an unrimmed platform (i.e., what we formerly called a shelf) to a true carbonate ramp. I have also referred to this surface as a "parasequence boundary" (King and Moore, 1992).

Platform versus ramp.- Workers in Alabama, beginning with Mancini and Benson (1980), have invoked Ahr's (1973) concept of a carbonate ramp to explain Smackover carbonate facies relations and pointed to Persian Gulf sedimentation (described by Purser, 1973) as a modern analogue. When I started looking at Smackover depositional facies relationships during Esposito's project, it seemed to me that a true ramp (sensu Ahr, 1973) would explain what we were seeing in the upper Smackover (genetic package 2), but not in the lower Smackover. In the lower Smackover, what we interpreted as carbonate turbidites (Esposito and King, 1987) within a clinoform carbonate unit could be found interfingered with basinal deposits. The apparent turbidite origin of these beds and their stratigraphic relations suggested to us a distal steepening or what we called then a shelfal depositional system. In this paper, I will not refer to this as a shelfal system as before, but prefer to call it now a unrimmed platform (sensu James and Kendall, 1992).

James and Kendall (1992) describe this unrimmed platform as "one in which there is no barrier." They go on to say that "what sets (them) apart is the continual in-place production of carbonate sediment ... which constantly alters the nature of the sea floor ... (as well as) absence of restrictive barriers (which) limits evaporites." I like this concept as a model for lower Smackover because I do not think that Smackover algal buildups (I prefer not to address them as "reefs", except perhaps about paleotopographic highs) were barriers to wave action, circulation, or much of anything, but did generally occupy a more distal position on the unrimmed platform and did continually modify the platform's sea floor.

In discussing our interpretation of clinoform sedimentation and turbidites (Esposito and King, 1987), Benson (1988) concluded that "there is little evidence ... to support the existence of a shelf margin during middle Smackover time." Other workers, including Benson (1988), generally favor a shallow-water, storm-generated (tempestite) origin for the same graded beds that we interpreted as turbidite deposits (or, more specifically, fluid debris-flow deposits). Thus, there is a key difference of opinion on lower Smackover graded beds and what they mean.

Turbidite versus tempsetite.- Obviously, much hinges upon interpreting Smackover graded beds. Normal grading alone does not prove a unique mechanism of origin, nor water depth at time of deposition. Careful study of Smackover graded beds shows that there is a critical lack of key indicators of shallow-water tempestite deposition, perhaps suggesting deeper water deposition by default. These missing key indicators include hummocky lamination, wave-ripple structures, repopulation burrows, infiltration fabrics, and mud-coated shells (criteria taken from Sellwood, 1986). Further, there is no obvious systematic decrease in grain-size and bed-thickness in an offshore direction. Negative evidence like this needs some additional support for their proposed clinoform origin and that comes from stratigraphic relationships: there is intercalation of graded units with basinal carbonate facies within lower Smackover. Figure 4 compares characteristics of the average Smackover turbidite bed, according to Esposito (1987), with characterisitics of idealized examples of tempestite and turbidite beds according to Sellwood (1986) and Stow (1986).

Lower Smackover depositional facies.- Lower Smackover depositional facies are interpreted in this paper as being related to unrimmed platforms in both basins. Results from studies cited below show that there are undaform, clinoform, and(or) fondoform depositional facies. In the brief paragraphs which follow, I signal terminological changes within the present discussion by presenting the now-favored term in brackets, e.g., [clinoform].

Esposito and King (1987) recognized three main facies within lower Smackover of the Conecuh basin: intertidal; slope [which I will call herein clinoform]; and basinal [fondoform]. Within the clinoform facies, they recognized a "fourth facies" of turdidite or debris-flow beds that has too small a scale to be dealt with here, so I include it within clinoform.

King and Hargrove (1991) recognized four facies within lower Smackover of the the Manila basin's southern part: shallow clastics; oolitic shoals; shelf [unrimmed platform]; and shelf-margin and patch-reef [unrimmed platform buildups].

King and Moore (1992) recognized three facies within lower Smackover of the Manila basin's northern part: oolitic shoals; shelf-margin [unrimmed platform buildups]; basinal [fondoform]; and slope [clinoform].

Figure 5 summarizes approximate correlations among these depositional facies and their interpretations in the three studies cited herein.

Upper Smackover depositional facies.- Upper Smackover depositional facies are interpreted in this paper as being related to true carbonate ramps in both basins. Results from studies cited below show that there are two ramp facies groups: shallow-water (e.g., shoals) and subtidal.

Esposito and King (1987) recognized two facies within upper Smackover of the Conecuh basin: oolitic shoals and subtidal ramp.

King and Hargrove (1991) recognized four facies within upper Smackover of the Manila basin's northern part: shallow clastics; lagoonal; oolitic shoals; and subtidal ramp.

King and Moore (1992) recognized four facies within upper Smackover of the Manila basin's southern part: shallow (high-energy) carbonates; lagoonal; oolitic shoals; and subtidal ramp.

Figure 5 summarizes approximate correlations among these depositional facies and their interpretations in the three studies cited herein.

STRATIGRAPHIC CORRELATIONS

Conecuh basin.- Figure 6 shows cross-sectional correlations that are slightly modified from those presented by Esposito and King (1987). New terminology for depositional facies, introduced earlier in this paper, is included.

In both dip and strike sections, correlations A and B, respectively, we see lower Smackover stratigraphic relationships attributable to unrimmed platform sedimentation (Figure 6). In a basal position are intertidal (undaform) deposits, which are directly overlain by clinoform unrimmed platform sediments. At three levels in lower Smackover, basinal or fondform deposits intercalate with clinoform.

I interpret these stratigraphic relationships to indicate relatively rapid inundation of undaform intertidal deposits (cryptalgal laminated lime mudstone), thus producing a transgressive surface just above formation base. Clinoform deposition (graded packstone-wackestone with interbedded algal clast-bearing debris-flow beds), which is intercalated with basinal (fondoform) lime mudstone and calcareous shale, subsequently ensued for an interval characterized by rather rapid relative sea-level rise.

Across the Conecuh basin, there is an apparent maximum-flooding surface (indicated on Figure 6) where relatively rapid transgression has occurred, and equally importantly a reorganization of depositional systems was initiated spawning a ramp-type sedimentation. Significant water depth may have attended maximum flooding, as indicated by Esposito's (1987) slab x-radiography. This x-radiography revealed a Thalassinoides-Zoophycos trace-fossil assemblage that is indicative of water depths up to 180 m (according to models in Ekdale and Bromley, 1984).

In both dip and strike sections, correlations A and B, respectively, we see upper Smackover stratigraphic relationships attributable to ramp sedimentation (Figure 6). In a basal position with respect to upper Smackover (i.e., Smackover above the maximum-flooding surface) are subtidal ramp deposits. These subtidal ramp deposits are directly overlain by prograding oolitic grainstones. These oolitic rocks give way to prograding Buckner anhydritic sebkha facies.

Northern Manila basin.-Figure 7 shows cross-sectional correlations that are slightly modified from the one presented by King and Hargrove (1991). New terminology for depositional facies, introduced earlier in this paper, is included. Neither correlation A nor B should be regarded as representing strike or dip sections exclusively. Owing to paleogeographic and paleotopographic complexity within the Manila basin, it is difficult to present straightforward strike and dip sections using available wells.

In both stratigraphic cross-sections, correlations A and B, respectively, we see lower Smackover depositional facies relationships attributable to unrimmed platform sedimentation (Figure 7). In a basal position over most of the area studied are clinoform (graded packstone and wackestone) unrimmed platform sediments. Owing to paleotopographic controls, unrimmed platform-buildup facies and oolitic shoals occur in a basal position instead of clinoform deposits like those described above in Conecuh basin.

Absent in the Manila basin's northern part are any lower Smackover basinal or fondform deposits intercalating with clinoform as described above in Conecuh basin. Also absent are basal undaform (intertidal) deposits like those seen in Conecuh: these must be part of the upper Norphlet in this area, or are missing owing to stratigraphic omission along a basal transgressive surface.

I interpret these stratigraphic relationships to indicate relatively rapid inundation of underlying Norphlet clastics without significant reworking and notably without development of algal laminated intertidal deposits like those in the adjacent Conecuh basin. Clinoform deposition (graded packstone-wackestone) generally ensued after basal transgression, however in some reaches of the Manila basin's northern part, oolitic shoals and unrimmed platform buildup facies developed. Clinoform deposition continued over an interval characterized by rather rapid relative sea-level rise.

Throughout this lower Smackover interval, algal buildups of the Manila basin (southern part included, as noted below) played a game of "catch-up and keep-up" in their attempts to stay viable within the photic zone.

Across the Manila basin's northern part, there is an apparent maximum-flooding surface (indicated on Figure 7) where relatively rapid transgression has occurred, and equally importantly a reorganization of depositional systems was initiated spawning a ramp-type sedimentation as in the Conecuh basin. Regarding algal buildups, this surface represents a point where "catch-up and keep-up" became "give-up" as they were largely drowned (excepting some buildups encircling paleotopographic highs).

In both cross-sections, correlations A and B, respectively, we see upper Smackover stratigraphic relationships attributable to true carbonate ramp sedimentation (Figure 7). In a basal position with respect to upper Smackover are subtidal ramp deposits generally, but in one part of the area, shallow-marine clastics overlie this maximum-flooding surface. Subtidal ramp deposits of upper Smackover are directly overlain by lagoonal (wackestone and mudstone) facies or shallow-marine clastics. These deposits are in turn overlain by prograding oolitic grainstones or shallow-water clastics, in the northern and southern parts of the study area, respectively. These upper higher energy facies give way vertically to prograding Buckner anhydritic sebkha facies.

Southern Manila basin.-Figure 8 shows a cross-sectional correlation that is slightly modified from the one presented by King and Moore (1992). New terminology for depositional facies, introduced earlier in this paper, is included. This correlation can be regarded as approximating a dip section, even though paleogeographic and paleotopographic complexity within the Manila basin make it difficult to present a completely straightforward dip section using available wells.

In stratigraphic cross-section (Figure 8), we see lower Smackover depositional facies relationships attributable to unrimmed platform sedimentation. In a basal position over most of the area studied are undaform intertidal facies consisting of extensively dolomitized grainstones, which are directly overlain by clinoform facies (graded packstones and wackestones) and, in the southern part of the area, by fondoform facies (basinal wackestones and mudstones). Just below the maximum-flooding surface at the cross-section's northern end, a substantial thickness of unrimmed platform-buildup facies has developed at roughly the same level as similar buildup activity described by King and Hargrove (1992) who called them "patch reefs."

Absent in the Manila basin's southern part are any lower Smackover basinal or fondform deposits intercalating with clinoform like those in the Conecuh basin. Also absent are basal undaform (intertidal) deposits like those seen in Conecuh. Undaform deposits must be part of the upper Norphlet in this area, or are missing owing to stratigraphic omission at a basal transgressive surface.

I interpret these stratigraphic relationships to indicate relatively rapid inundation of underlying Norphlet clastics without significant reworking and notably without development of algal laminated intertidal deposits as those in the adjacent Conecuh basin. Clinoform deposition (graded packstone-wackestone) with inclusive unrimmmed platform-buildup sedimentation subsequently ensued for an interval characterized by rather rapid relative sea-level rise.

Across the Manila basin's southern part, there is an apparent maximum-flooding surface (indicated on Figure 8) where relatively rapid transgression has occurred, and equally importantly a reorganization of depositional systems was initiated thus spawning a ramp-type sedimentation (like that in the Conecuh basin and Manila basin's northern part).

In stratigraphic cross-section (Figure 8), we see upper Smackover facies relationships attributable to ramp sedimentation. In a basal position with respect to upper Smackover are subtidal ramp wackestones and mudstones across the area. At this lower level within upper Smackover of the Manila basin's northern part, there is a lack of complexity among depositional facies. In northern reaches of the cross-section correlation (Figure 8), we can see development of deeper ramp (mudstone) facies, which form a tongue within upper Smackover ramp wackestones and mudstones. This tongue suggests a transgressive-regressive facies arrangement not seen in the Manila basin's northern part nor in Conecuh. Up to this level, upper Smackover ramp facies in this area are finer grained than their counterparts in the other areas studied.

The fine-grained, upper Smackover ramp deposits are in turn overlain by shallow-ramp packstones and grainstones (within southern reaches) and generally across the area by dolomitic grainstones. Dolomitic rocks are mainly prograding oolitic facies seen elsewhere at this level. Absent in this area are shallow-marine clastics that characterize this level within the Manila basin's northern part. Upper Smackover higher energy facies subsequently give way to prograding Buckner anhydritic sebkha facies.

RELATIVE SEA-LEVEL CHANGES

As noted earlier, Haq et al. (1988) have established the currently accepted eustatic sea-level curve for Late Jurassic, including the interval of Smackover deposition. According to Imaly and Herman (1984) this interval is late early Oxfordian to early Kimmeridgian (approximately 144 to 150.5 Ma based upon Haq et al.'s (1988) biochronostratigraphic chart). Haq et al. (1988) subdivide this interval into four global coastal onlap sequences (Figure 9), but as Mancini et al. (1990) pointed out, there is really only one such sequence when the Gulf Coastal region is considered by itself.

Regarding Figure 9, I think it is reasonable to infer that the two "medium intensity" maximum-flooding surfaces, as defined by Haq et al. (1988), are the same as the two distinctive Smackover transgressive events. The 150 Ma transgressive event is probably the one which drowned thin, basal undaform facies such as the intertidal algal laminites of the Conecuh basin and dolomitic grainstones of the Manila basin's southern part. The 144.5 Ma transgressive event is probably the one which produced a maximum-flooding surface between my lower and upper Smackover. The alternate view, of course, would be that Smackover sequence stratigraphy does not relate to global sequence stratigraphy, and that position has been taken effectively by Mancini et al. (1990).

There is a point in favor of a global connection to Smackover carbonate deposition: Engebretson et al. (1985) showed that calculated relative linear velocity of the North American versus Farallon plate, a factor indicating a key change in plate tectonic interaction, declined abruptly by approximately 68 m/kyr at about 145 Ma. What kind of far-field effect this relative velocity may have had is speculative, but it is notable that this velocity decline was the largest regarding North American and Farallon plates over the last 175 million years (Engebretson et al., 1985). Further, the age (about 145 Ma) is close to our that inferred for the Smackover's maximum-flooding surface.

Figure 10 shows my interpretation of the sequence of events brought on by relative sea-level changes within the basins studied. Regarding the Conecuh basin, the apparent sequence of events is: 1) a rapid relative sea-level rise followed by an interval of progressive, comparatively slow relative sea-level rise punctuated by three transgressive episodes; then 2) a relatively rapid transgressive event (at maximum-flooding surface) followed by an interval of progressively more rapid relative sea-level fall. Regarding the Manila basin's northern part, the apparent sequence of events is: 1) a rapid relative sea-level rise followed by an interval of progressive, comparatively slow relative sea-level rise; then 2) a relatively rapid transgressive event (at maximum-flooding surface) followed by an interval of progressively more rapid relative sea-level fall. Regarding the Manila basin's southern part, the apparent sequence of events is: 1) a rapid relative sea-level rise followed by an interval of progressive, comparatively slow relative sea-level rise; then 2) a relatively rapid (maximum-flooding surface) transgressive event followed by an interval of progressively more rapid relative sea-level fall that is perturbed in its midst by a transgressive pulse.

Evident differences in depositional facies stratigraphy between Conech and Manila basins and within the Manila basin suggest that local or intrabasinal tectonics played a strong part in producing observed differences within side-by-side Alabama basins.

I think it is reasonable to assume that the basal transgressive events in both basins and the maximum-flooding events in both basins are coeval. As explained above, these events could be Haq et al.'s (1988) "medium intensity" flooding events of 150 and 144.5 Ma, respectively.

If my assumptions above are correct, the lower Smackover would have accumulated during a span of 6.0 million years and upper Smackover during 0.5 million years. Such a difference in temporal spans would produce vastly different computed sedimentation rates for lower versus upper Smackover. Assuming a reasonable, average Manila-basin thickness for lower Smackover of 150 m, average sedimentation rate for that interval would have been 25m/myr. Similarly considered, upper Smackover (thickness = 50 m) would have been 100 m/myr. Perhaps the differences in rates are due to the different depositional systems, unrimmed platform versus carbonate ramp, extant during Smackover deposition. Recently, Mancini et al. (1998) have computed facies-specific, Smackover carbonate accumulation rates that are in the 25 to 30 m/myr range (except for algal reefs, 84 m/myr).

ACKNOWLEDGMENTS

Support for writing this paper came from the ADECA contract titled 'Partnership Program in Petroleum Technology Transfer' (1STE9703) awarded to Ernest A. Mancini, and subcontracted in part to me. I thank Ernie for asking me to participate in this program. My Smackover work in the past (1988-1992) was supported by a grant to me from Chevron USA, Inc. My graduate students' work, cited in this paper, was supported in part by grants from the Gulf Coast Association of Geological Societies, the Gulf Coast Section of SEPM, and the Alabama Academy of Science. I thank April Barnes for designing the original layout of this web page, especially the figures and tables.  Finally, I thank Rick Esposito, Danny Moore, and Glenn Hargrove for their many helpful comments on this paper.

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