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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