ABSTRACT

The Choctawhatchee Formation of the Florida panhandle consists of fossiliferous marls and argillaceous sands which were deposited in an inner neritic environment undergoing progradation. The planktonic foraminiferal assemblages in this rock unit indicate that the sediments were deposited during Zone N 17 (Upper Tortonian-Messinian).

Detailed lithological and micropaleontological analyses of the Choctawhatchee indicates that a marked regression occurred near the end of the Late Miocene. This regression seems to be correlated with similar events recorded in the Pacific area and the Southeast U.S., with the Late Miocene period of glaciation recorded in Antarctic deep sea cores, and with the isolation of the Mediterranean at the onset of the Messinian Salinity Crisis. A glacio-eustatic lowering of sea level is the probable cause of the widespread regressions.

CHOCTAWHATCHEE FORMATION

Description

The Choctawhatchee Formation of north Florida (Fig. 1) is a fossiliferous marl and argillaceous sand which is known in outcrop only in the northern part of its extent. The formation can be divided into two members. The distinctive lower member is a tough, massive bedded, fossiliferous, slightly phosphatic and micaceous, argillaceous, fine calcarenite which grades into a quartzose foraminiferal calcarenite. This member corresponds to the typical Arca Zone of Mansfield (1930, 1932). It is this member that is exposed at the type locality at a spring head in a ravine on the western bank of the Choctawhatchee River (NW 1/4, NW 1/4, Sec. 15, T2N, R17W). The upper member, a micaceous agrillaceous arenite, is the sandy Permenter's Farm beds of Smith (1941).

The characteristics of these members as seen in outcrop are not always persistent in the downdip subsurface portion of the formation. In fact, the facies and faunal changes in the unit are rather complex and require close examination if their pattern is to be revealed (Fig. 2). The lower member shows a progressive change from a calcareous arenite in the north and west to a foraminiferal limestone in the south and eastern area. The amount of quartz diminishes in the downdip (offshore) direction. The upper member, which is generally more clastic than the lower member is an argillaceous, non-calcareous unit in all but the western area where its lower part is a calcareous, fossiliferous arenite. Only the finer grained, more argillaceous beds show any bedding structures.

Fine sand sized phosphorite grains are consistently present in the lower member, and are most abundant in the coarser grained intervals. The phosphorite diminishes in abundance downdip into the limestone facies of the lower member. It is also less abundant upsection and is rarely present in the upper member. The upper member does however, contain accessory minerals including angular and prismatic heavy minerals as well as biotite.

The Choctawhatchee Formation is a prism shaped unit whose greatest length in an east-west direction is at least 59 km and probably exceeds 100 km. It is 24-32 km wide in the west and tapers gradually to the east. Its greatest measured thickness is 70 m in the western area.

Age

The presence of a diverse planktonic foraminiferal fauna in the Choctawhatchee Formation was used to establish a Late Miocene age for the unit. The presence of Globorotalia tumida plesiotumida and the absence of G. tumida tumida in the upper member places that member in Zone N 17 of Blow (1969). G. acostaensis acostaensis occurs consistently throughout the unit. Its decendent, G. acostaensis humerosa, is absent in the lowest sediments and makes its first appearance higher in the section, an event which lies in the middle of Paleomagnetic Epoch 8 at 8.0 my near the base of Zone N 17 (Ryan and others, 1974). The lowest part of the Choctawhatchee therefore lies at the top of Zone N16 or the base of N 17. An maximum upper bound of approximately 5.0 my for the formation is indicated by the presence of Globoquadrina dehiscens (Saito and others, 1975).

Within the Choctawhatchee Formation itself, age relations are not so clearly understood. The distribution of

FIGURE 1. Distribution of Choctawhatchee Formation in northwest Florida

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some of the planktonic foraminiferal species however, suggests that a progradational event was occurring during the deposition of the Choctawhatchee. Table I shows the local distribution of these species. Although the local first occurrences and extinctions shown in the table do not allow correlations to be made with world wide datums, they do nevertheless give an indication of temporal sequences within the unit. The interpretations of the table are based on general aspects of the fauna such as species abundances, knowledge of the biostratigraphic ranges of the species, and morphological features of the species.

The progressive development of the Globorotalia acostaensis lineage from Association A to C is consistent with our understanding of this lineage (Blow, 1969). Globorotalia lenguaensis, a species which first occurs in Zone N 12 and has its last occurrence in N 17, is found in trace amount only in Association A, suggesting that this association occurs early in Choctawhatchee time. The occurrence of two additional taxa lends support to this interpretation. A form similar in appearance to Globorotalia continuosa occurs rarely in Association A. G. continuosa ranges from zones N 6 to N 16 and the specimens in the Choctawhatchee may be relict forms of this lineage. Also occurring in rare amounts only in Association A are specimens of Globigerinoides which probably evolved from Globigerinopsis aguasayensis and are very similar to Globigerinopsoides algeriana, a species described from Lower Tortonian (N 15) sediments in northeastern Algeria. The combination of these occurrences suggest that Association A is the oldest planktonic foraminiferal association in the formation. Association B overlies A and Association C overlies B in the downdip area and also has the most modern aspect of the three associations. Globoquadrina dehiscens, a species which became extinct near the Mio-Pliocene boundary, shows a declining abundance from Association A to B which is compatible with the temporal interpretation given in Table I. When these data are superimposed on the lithologic data of figure 2 a seawards progradation of the Choctawhatchee with time is indicated.

Equivalent and adjacent units

Rock units of equivalent age are not well known in the area. The Charlton Formation of northeastern Florida and southeastern Georgia has been provisionally dated as Late Miocene on the basis of molluscan affinities with the Arca faunizone of the Florida panhandle (Waller, 1969). In addition, the molluscan fauna of a portion of the upper part of the Tamiami Formation of southern Florida is similar to that of the Arca faunizone (Hunter, 1968). The microfossils of this same interval of the Tamiami suggest an N 17 age (Peck and others, 1977).

The Choctawhatchee paraconformably overlies the Shoal River Formation and its downdip limestone equivalent. These Middle Miocene units contain a planktonic foraminiferal fauna which is typical of Zones N12 and N13 and equivalent to the middle Serravalian Stage. The upper surface of the Choctawhatchee Formation is an erosional surface which is overlain by several different rock units. In the northern part of its extent it is overlain by the Citronelle Formation of Pliocene age. In the southern and eastern area it is overlain by undifferentiated marine Pliocene deposits whose base contains a planktonic foraminiferal fauna indicative of late Early Pliocene; N19 of Blow (1969), P1-1 of Berggren (1973) and M P1-3 of Cita (1975).

Paleoenvironments

Benthonic Foraminifera

With rare exceptions the benthonic foraminifera of the Choctawhatchee Formation have been studied only in

FIGURE 2. Lithologic, faunal and temporal relationships in the Choctawhatchee Formation. The bases of each section are not necessarily correlated with each other.

TABLE 1. Distribution of some planktonic foraminifera in the Choctawhatchee Formation

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outcrop sections. Cushman 1930) and Cushman and Ponton (1932) prepared the first descriptions of the fauna and suggested that Arca zone benthonic foraminifera were deposited in a shallow water (30-100 m) environment. In a comprehensive revision of this early work, Beem (1973ms) concluded that the environment of deposition lay in near-shore nutrient rich waters less than 50 m deep. He based his conclusions on the taxonomic composition of the fauna and on various measures of population structure. The following analysis of the benthonic paleoenvironment of the Choctawhatchee is based on data that comes from the work of Beem.

The benthonic foraminiferal fauna of the Arca faunizone is dominated by inner neritic species such as Buliminella elegantissima, Cibicides lobatulus, Hanzawaia americana, Nonionella miocenica and Uvigerina subperegrina. Additional inner neritic species such as Ammonia beccaril, Bulimina elongata, Elphidium hughesi, E. sagrum, Eponides repandus, Florilus grateloupi and Neoconorbina terquemi occur in the outcrop samples.

There is no significant change in species composition per se within the unit that gives any indication of changes that occurred within Choctawhatchee time. However, an analysis of various faunal characteristics does suggest that conditions at the close of Choctawhatchee time were more restrictive and shallower than at the outset. Benthonic foraminifera from 3 outcrops of the lower member and one of the upper member show interesting trends in population structure (Fig. 3). The abundance distribution of individual specimens among the species in a population appears to be a function of environmental stability (Buzas, 1972). Populations with an even distribution of specimens among the taxa are typical of stable environments whereas unstable environments are dominated by a few opportunistic species. A simple measure of this population trait is dominance, the proportional contribution of the most abundant taxon in a population. Figure 3 A shows the distribution of sample dominance in the 3 outcrop sections of the lower member of the formation. In general the dominance increases upsection. It must be noted that there is no intended correlation of the 3 outcrop sections even though they are superimposed on a single diagram. The diagram merely shows the upsection change in a population characteristic, i.e., the temporal trend in a trait. Based on the planktonic foraminiferal distribution in the updip sections of the formation (see discussion above and Fig. 2), it is reasonable that these 3 outcrop sections are approximately time equivalent, but not precisely so. There is only one sample from the upper member and its dominance is shown by the closed triangle above the graph. The upsection increase in dominance suggests increasingly unstable conditions near the end of Choctawhatchee time.

A more sophisticated measure of population dynamics is equitability which gives an indication of the evenness with which the specimens are distributed among the species (Buzas and Gibson, 1969). High values of equitability characterize populations in which the species are evenly distributed. This is generally considered a characteristic of stable environments. Figure 3 B shows that the equitability values tend to decrease upsection. This trend substantiates the trend of increasing dominance shown in figure 3 A.

The ratio between planktonic and benthonic foraminiferal tests in a sample has been used as a paleobathymetric indicator. The proportion decreases as water becomes shallower. The proportion is affected not only by water depth but also by proximity to the open ocean, intensity of oceanic circulation, degree of terrestrial runoff and other factors. Consequently, quantitative utilization of the proportion of planktonic tests is difficult. However, trends in the behavior of the proportion may give an indication of water depths. The declining values of the proportion with time are shown in figure 3 C. The coincidence of this trend with those of species dominance and equitibility give further support to a Choctawhatchee senario in which conditions became more unstable, restrictive and shallower with time.

The dominant benthonic foraminifer in the Choctawhatchee Formation is Buliminella elegantissima. This species, not well known from the Gulf today, is common in the Caribbean region and along the southern California and Baja California coasts. The species seems to thrive in areas of high nutrient supply (Boltovskoy and Wright, 1976, p. 50). In all three of the studied Choctawhatchee outcrop areas the relative abundance of B. elegantissima increases upsection. This may be a response to increased proximity of terrestrial runoff and supplies of organic carbon caused by shoaling conditions and a prograding shoreline as suggested by the population dynamics discussed above.

Depositional History

The facies distribution within the Choctawhatchee shown in figure 2 indicates that there is a decrease in the clastic component toward the south and east (seaward). This is not surprising in view of the observation that the source areas for Neogene sediments in this part of the Gulf Coast lay generally to the north. The formation also exhibits a coarsening upward sequence with increasing proportions of heavy minerals and decreasing carbonate content. The unit is wedge shaped in the direction of the depositional axis and is capped by an erosional surface.

The lithologic trends and characteristics coupled with the upsection reduction in environmental stability indicated by the benthonic foraminifera suggest a

FIGURE 3. Upsection changes in foraminiferal population dynamics, Choctawhatchee Formation

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progradational/regressive model for Choctawhatchee sedimentation. We postulate that although neritic conditions prevailed throughout the deposition of the unit, the continued influx of terrestrial sediments from the northwest combined with the gradual withdrawal of the sea to the southeast resulted in restrictive, unstable, increasingly organic rich, shallow water environments by the end of Choctawhatchee deposition. The sediments were finally exposed to erosion and the deltaic, lagoonal and/or estuarine sediments that might have capped this sequence were removed. Structural contours drawn On the top of the Miocene (Fig. 4) show the dissection that resulted from this erosion. No such irregularities are present on the upper surfaces of the pre-Choctawhatchee Miocene units.

GLOBAL IMPLICATIONS

The presence of this nearshore wedge of mixed clastic and carbonate sediments near the Mio-Pliocene boundary provides low latitude evidence for a Late Miocene regression. Rock sequences recording similar sorts of regressive events can be found in Late Miocene rocks in several parts of the world including Japan (Burckle and Akiba, 1976), Spain (Berggren and Haq, 1976), California (Ingle, personal communication, 1977) and south Florida (Peck and others, 1977). Erosional intervals spanning Zone N 17 are also rather common in the rock record and have been documented by Adams and others (1977). The Choctawhatchee Formation appears to fit into the first category of these observations.

The relationships between these regressive and erosional events and the Messinian Salinity Crisis can not be overlooked. The results of two DSDP legs and numerous land based studies now seem to indicate that tectonic closure of the Betic and Rif Straits in the Early Messinian resulted in restriction or even complete isolation of the Mediterranean and the initiation of an evaporitic cycle (Hsu and others, 1977). The production of a 1-2 km thick evaporate body over the floor of the Mediterranean (10⁶ kM³ of salts) during the evaporitic phase of the Messinian would have had significant effects on the world ocean (Ryan, 1973) and might even have induced the onset of an episode of polar glaciation (Ryan et al., 1974). The presence of Late Miocene climatic cooling and glaciation has been well documented in the antarctic region (Shackelton and Kennett, 1975; Hayes and Frankes, 1975) and may have occurred in arctic regions as well (Wolfe and Hopkins, 1967; Bandy, Butler and Wright, 1969; Denton and Armstrong, 1969). An expansion of polar ice would have resulted in an eustatic drop in sea level.

Whether or not Mediterranean desiccation proves to be a result of (Van Couvering and others, 1976) or a cause of (Ryan and others, 1974) glacio-eustatic sea level changes, the record of this extraordinary event should have been preserved in rocks outside the Mediterranean area. The eustatic fall in sea level should have had a significant and detectable effect on the sediments and faunas of shelf areas.

The order of magnitude of sea level drop during the latest Miocene has been estimated at 40-100 m. This degree of oceanic drawdown on the shelf areas of continents would have resulted in changes in benthonic faunas and would have caused a seaward. migration of facies. Mid-Tertiary subtropical and tropical areas are characterized by carbonate sediments. A seaward migration of the shoreline toward the edges of the continental margins would have partially eliminated shallow water photic zones necessary for carbonate production. One would therefore expect to find replacement of carbonate rich sediments by clastic and terrigenous deposits and there should also be a progressive change to a benthonic faunal assemblage representative of shallower waters.

The Choctawhatchee Formation of the Florida panhandle shows lithologic and faunal characteristics which fit the pattern described above. Further documentation of the timing of the events recorded in the Choctawhatchee can only come from more detailed subsurface biostratigraphical studies of the unit. Perhaps these studies can provide the refinement needed to place this shoaling sequence of rocks more precisely within Zone N 17 and consequently to help answer the question of whether or not the closing of the Mediterranean proceeded or postdated the onset of high latitude glaciation.