Miocene of the S.E. United States: A model for chemical sedimentation in a peri-marine environment

Abstract

The Miocene sediments of the southeastern United States contain commercial deposits of palygorskite-sepiolite and phosphate. These minerals, in addition to carbonates, opal-cristobalite, zeolite, and some of the smectites, had an orthochemical origin. Many later formed allochemical sediments. Sediments were deposited in shallow water in a mildly tectonically active hinge area separating the Atlantic Ocean and the Gulf of Mexico. Montmorillonite is the dominant clay mineral in the Tertiary of the Atlantic and Gulf Coastal Plains except for the Upper Oligocene and Lower and Middle Miocene of northern Florida, Georgia, southern South Carolina, Georgia Shelf and Blake Plateau, where palygorskite and sepiolite are commonly dominant. A marine channel or trough extended through southern Georgia and connected the Atlantic Ocean and the Gulf of Mexico from Cretaceous through Oligocene time. The Florida Platform was an island. During the Miocene the platform was joined with the mainland. During the Late Oligocene (Tampa) the sea transgressed over the eroded karst land surface and tidal and lacustrine carbonates (largely dolomite) were deposited. This is the oldest occurrence of palygorskite and sepiolite in the area. General transgression continued during much of the Early Miocene (Torreya and Chipola) followed by a regression culminating in the development of an extensive soil and reworked (clay, phosphate and quartz pebbles) horizon near the end of Early Miocene and the beginning of the Middle Miocene. Concentrations of phosphate were developed during this period of reworking, though in some areas concentration (by winnowing) continued through Middle and Late Miocene. During the Early Miocene palygorskite and sepiolite formed throughout the region in brackish lagoon and tidal environments. Formation of these clays ceased at the end of the Early Miocene (at the soil-reworked horizon). During the Late Oligocene a large island existed near the present shore line in SE Georgia and NW Florida. The tectonic high area (Ocala High) migrated westward during the Early Miocene, closing the SW end of the Trough. The eastern portion tilted below sea level. By Middle Miocene time much of the area was slightly above or near sea level except for the NE-SW aligned Trough opening to the Atlantic and terminating near the Georgia-Florida border. The Middle Miocene Trough sediments (largely clays) contain an abundance of diatoms and sponge spicules. Reworked palygorskite and sepiolite are concentrated in the restricted SW portion of the Trough. Limestones and dolomites are abundant in the Upper Oligocene and Lower Miocene. Dolomite is present in the SW portion of the Trough and coquina, and along with sand and clays, in the NE portion. Tan tidal and lagoonal dolomite occur in the area east of the Trough. All dolomite is protodolomite. A NW-SE aligned high-energy estuarine facies is present in the center of the Atlantic Embayment. This is fringed by low-energy tidal-lagoonal sediments containing palygorskite and sepiolite. The younger Miocene sediments are largely clastic and were supplied by Atlantic Ocean currents (phosphate and Na-feldspar) from the east and by streams from the west (K-feldspar). The latter overlaps the former. The clays are predominantly montomorillonite, though kaolinite becomes relatively abundant in the detritus from the west. Two commercial palygorskite clay beds (less than 10% smectite) 0.9 – 4.5 m thick occur in SW Georgia and NW peninsular Florida. The lower clay beds contain sepiolite, the upper does not. They are separated by a paleosoil which has well-developed peds and argillans and concentrations of secondary sepiolite. The clay beds commonly contain clay peds and clasts. The texture and sedimentary structures closely reflect variations in the depositional environment. Montmorillonite occurs in the marine and continental facies. Palygorskite, sepiolite (with minor stevensite) and dolomite were formed in lagoonal and tidal-flat environments. Where reworking has occurred, both by currents and burrowing animals, both varieties of clay are present. Lateral and vertical changes indicate the lower clay bed was deposited during a regressive phase that was climaxed by the deposition of a thin organic-rich flood-plain deposit on top the lagoon-tidal sediments. A soil developed on the flood plain. This was followed by a transgression and deposition of a shallow brackish-water sand. A second regression occurred during which the upper clay bed was deposited. A ped and burrowed structure was developed on top of this clay bed. A final transgression deposited montmorillonite, nearshore brackish-water clays, sands and coquina near the end of the Early Miocene. The sea shortly withdrew from the area and the sequence is topped by thin continental sands. The dolomite, palygorskite and sepiolite were formed in brackish water, probably under schizohaline conditions. Sepiolite was deposited in the fresher-water environment. Twenty-five miles to the NE the commercial clays were deposited in the narrow Middle Miocene Trough on top of the soil zone separating the Lower and Middle Miocene. Diatoms and sponge spicules, present in amounts up to 30%, indicate a restricted-marine environment becoming more marine to the NE (seaward). The palygorskite and sepiolite (20–70% of the clay suite) is detrital and derived from Lower Miocene clays on the flank of the uplifted Ocala High (to the east and southeast). Clay grains and pebbles containing appreciable apatite are abundant. This clay is overlain by brackish water (diatoms) montmorillonitic clay and sand (K-feldspar) derived from the west flank and deposited during the final marine regression in the area. The palygorskite-sepiolite clays occur as lenses (10–50 feet thick) and were deposited in sheltered depressions between Lower Miocene barrier island, beach, and chenier sand ridges when the area was transgressed by the Middle Miocene seas. The deposits are restricted to the SW portion of the Trough which existed as a sill. The sill effect was produced by the uplift of the flanking Ocala High; this uplift also afforded the palygorskite-sepiolite source. Farther to the NE the Trough deepens and montmorillonitic clays were deposited. Plots of textural data indicate the lagoonal palygorskite clays contain less than 10% sand and have a M z less than 3.0. Marine montmorillonite clays and clayey sands contain more than 30% sand and the M z increases as the percent sand increases. Reworked samples and soil samples have intermediate values. TEM and SEM pictures show a number of interesting features. Short, 1-μm fibers comprise the bulk of the palygorskite-sepiolite clay but long (greater than 10 μm) fibers are locally abundant. Long fibers occur in small areas with desiccation features, indicating they grew from residual fluids when dehydration was nearly complete. These occur in a matrix of short fibers. Long fibers occur in the soil samples where they form mats and are also aligned perpendicular to vein walls. Short fibers were observed forming from montmorillonite, replacing quartz and calcite fossils, and by the coalescing of small opaline spheres. Much of the clay occurs as thin, parallel laminae, suggesting a periodic supply of detritus (montmorillonite) to the lagoon. Many quartz and feldspar grains in the soil have been etched and many contain a clay skin (palygorskin). Coarse rice calcite occurs in vertical fractures and in horizontal bands in the clay. In both instances it was precipitated in desiccation voids. When the desiccated and fractured clay surface was invaded by water and detritus from the landward side, Mg was adsorbed by the palygorskite and fine calcite precipitated in the voids, eventually recrystallizing into spar calcite. The Ca Mg ratio was lowered to the extent that palygorskite (from montmorillonite) and dolomite (from solution) could form. Their formation was enhanced and continued as the lagoon was periodically invaded by brackish water from the seaward side. Shells in sands underlying the lagoonal clay have been replaced by palygorskite and dolomite. Seeping Mg-rich waters from the lagoon established a dolomitization gradient. The deepest shells are converted to dolomite (protodolomite) preserving the plate texture of the calcite. The outer surfaces are rich in Ca relative to the interior. This is true of most of the dolomites. Once the plates achieve a protodolomite composition, growth starts at the edges and two- and three-sided rhombs and eventually six-sided hollow rhombs are formed (vertical sequence). Plates arranged in various subspherical forms are present in some dolomite lenses and beds. Dolomite rhombs in the clay beds have overlapping, thin, flame-shaped layers suggesting a slow sheet-by-sheet growth rather than replacement or abrupt precipitation. Dolomite formation precedes palygorskite in some situations and follows in others. Both replacement and ‘primary’ limpid dolomite grow by slow sheet-by-sheet and plate-by-plate accretion in brackish and schizohaline environments that also allowed the formation of palygorskite. The competition for Mg may cause this type of growth or this may be the normal way most dolomites form. In some areas it appears that montmorillonite reacts with dolomite or Mg-calcite to form palygorskite and calcite (fine disks). There is a complete compositional graduation between clay pebbles and phosphate pebbles. Some apatite coprecipitated with the clay (high sepiolite content) in areas where diatoms provide a high P and Si concentrations. During periods of weathering apatite replaces additional clay and diatoms in the pebbles. Most of the apatite occurs as short rods but that replacing diatoms has shapes ranging from spherical to prismatic crystals. Opal-cristobalite formed from dissolved diatoms and sponge spicules is relatively abundant. It is commonly massive but occurs as bladed spherules and well rounded opaline spheres. Fe and Al oxides-hydroxides caused the precipitation of SiO 2 at fairly high pH values. In outcrops palygorskite alters to a chlorite and possibly montmorillonite but the latter was not demonstrated. In calcareous clays, montmorillonite can alter to a mixed layer kaolinite-montmorillonite. In noncalcareous sediments it alters to amorphous Si-Al which then crystallized into kaolinite. In some areas the kaolinite, in turn, alters to an Fe-chlorite. Chemical analyses indicate the montmorillonites are the Wyoming-type. Soil montmorillonites have a relatively high Fe content. Approximately half the octahedral positions of the palygorskite are occupied by Al. Calculations suggest the smectite in the palygorskite-rich clay beds is stevensite. The palygorskites contain an average of 24 ppm Li, suggesting they were formed in waters of less than normal salinity. Chemical calculations confirm that most of the palygorskite formed by the direct alteration of montmorillonite. The Al and Fe remained constant and additional Si, Mg and H were obtained from solution. When the Si and Mg content of the solution is sufficiently high and the pH is in the range of 8–9 montmorillonite will convert to palygorskite. When montmorillonite is not present sepiolite will precipitate. Dolomite is commonly formed contemporaneously with both sepiolite and palygorskite. Calcite is commonly deposited out of phase with the Mg minerals. Much of the palygorskite in limestones is detrital. Thermodynamic calculations indicate that there is a strong temperature effect on the stability of sepiolite. With increasing temperature, the stability field of sepiolite increases relative to the dolomite field. Thermodynamic calculations (25°C) have been made for three reactions of direct concern: montmorillonite-palygorskite, palygorskite-aqueous solution, and sepiolite aqueous solution. The stability-field boundaries for these reactions are defined by: log[ Mg 2+]+2 pH+2 log[ H 4 SiO 4 0]=5.75 0.69 log[ Mg 2+]+0.76 pH+2.6 log[ H 4 SiO 4 0]+6.2 log[ Al(OH) 4 −]=10.70 and: log [ Mg 2+]+2 pH+1.5 log[ H 4 SiO 4 0]=7.95 respectively. In all cases the chain silicates are favored by an increase in one or more of [Mg 2+], pH, and [H 4SiO 4 0]. Palygorskite also requires an appropriate input of Al (and Fe), either inherited directly from the precursor clay or taken from solution. Sepiolite requires log [H 4SiO 4 0] = 4.25 (around 3.0 ppm SiO 2 in sea water, assuming γ(H 4SiO 4 0) = 1.13) for stability with respect to aqueous solution. Palygorskite should form from montmorillonite at log [H 4SiO 4 0] ⩾ −4.29 (around 2.7 ppm SiO 2). Thus, from the point of view of thermodynamic calculations, only slight modifications of normal sea water conditions are required to form sepiolite and palygorskite. However, if this were true these minerals should be more common. The calculations indicate the chain silicates are found by an increase in [Mg 2+], pH, and [H 4SiO 4 0]. Field observations indicate they are also favored by less than normal salinity and by high temperature. Calculated stability field boundaries between palygorskite, montmorillonite, and a series of different corrensites, show that regardless of the choice of corrensite composition, it is favored over montmorillonite by higher [Mg 2+] and pH. The [H 4SiO 4 0] effect is minor. For the corrensite-palygorskite reaction the importance of [Mg 2+] and pH is variable depending on the choice of corrensite composition. However, in all cases high [H 4SiO 4 0] favors palygorskite. These calculations tend to confirm the idea that corrensite is more abundant in the Paleozoic and Early Mesozoic because evaporatic environments with high pH were abundant. Small evolutionary or periodic fluctuations in the composition of the ocean may have increased the possibility of Mg-silicate versus Mg-carbonate formation in the younger Phanerozoic. The first appearance of palygorskite in the Early Mesozoic may be related to the initiating of sea-floor spreading and attendant introduction of silica (and perhaps Mg) into the oceans. Development was further enhanced in the Late Creteous by the proliferation of diatoms. In the Georgia-Florida area palygorskite and limpid dolomite developed in shallow, coastal brackish to schizohaline waters. Warm temperature caused a high pH. Both increased the solubility of silica (largely from diatoms). Cooler conditions during the Middle Miocene made conditions unfavorable for the development of palygorskite. Magnesium was obtained from sea water. There is a mutual antipathy between palygorskite and clinoptilolite, with palygorskite being the fresher-water mineral and clinoptilolite the more saline equivalent. The results of thermodynamic calculations are compatible with this distribution. The phosphate deposits are largely restricted to the Atlantic facies. Much of the phosphate was derived from diatoms in shallow coastal waters and concentrated by replacing clay pebbles and clay-rich fecal pellets. A review of the literature on ‘marine’ palygorskites indicates there is little, if any, data, to indicate they formed in a normal-marine environment. We believe that peri-marine palygorskite deposits form only in brackish water and montmorillonite (and glauconite) is usually the stable clay in the normal-marine waters. Chloritic clays (mixed-layer chlorite-montmorillonite) are the stable phase under hypersaline conditions. The global distribution of the major palygorskite deposits indicates they are restricted to the belt of tropical-subtropical temperatures. To a large extent the distribution was controlled by the pattern of the warm Tethys currents. During the Early Cenozoic the westward-flowing Tethys currents supplied warm waters to the Caribbean region. The convergence of the African and Eurasian plates in the Late Oligocene and Early Miocene allowed these currents to swing to the north and increase temperatures in the coastal waters of the southeastern United States, allowing palygorskite and phosphate to form. The collision of Europe and North Africa at Gibraltar at the beginning of the Middle Miocene modified the Atlantic circulation pattern, allowing cold Arctic waters to enter the western North Atlantic. With the decrease in temperature the growth of palygorskite ceased. Palygorskite is relatively abundant in deep-sea cores from the southern Gulf of Mexico, the Bahama area and off the northwest and northeast coasts of Africa. An evaluation of the data, other than hydrothermal deposits, indicates it is probably detrital in all instances. In lacustrine environments the needed silica and magnesium is supplied by weathering; however, the association of phosphate and palygorskite in the peri-marine deposits suggests the general source of ions is the sea. An estimate of the temporal distribution of authigenic Mg minerals shows that dolomite and corrensite are relatively abundant in the Paleozoic and Early Mesozoic and decrease in the younger sediments. Palygorskite-sepiolite and kaolinite (and perhaps limpid dolomite) are relatively abundant in the Late Mesozoic and Cenezoic. A comparison of the temporal distribution of palygorskite, kaolinite, and evaporites in terms of paleolatitude suggests humidity may be more important than temperature in producing conditions favorable for the formation of palygorskite.