Heterostegina Zone Research Articles

Structure, Stratigraphy, and Depositional Environment of the Heterostegina Limestone and Overlying Sands in the Lake Pontchartrain Area of Southeast Louisiana: ABSTRACT

ABSTRACT The Heterostegina zone of the Oligocene Anahuac Formation in southeastern Louisiana occurs in the subsurface as an extensive shelf reef complex. The Heterostegina limestone is overlain by strata associated with the Oligocene Discorbis and lower Miocene Robulus (43) biostratigraphic zones. Examination of electric logs and drill cuttings from wells in the Lake Pontchartrain area of southeastern Louisiana reveals the importance of the Heterostegina reef as a paleoenvironmental punctuation marking a significant shift in regional depositional patterns that occurred between the generally transgressive Oligocene seas and the generally regressive Miocene seas. The Discorbis and Robulus (43) zone sedimentary rocks in the study area are interpreted to represent a transition of environmental conditions following the transgressive reefal carbonate environment in place during Heterostegina reef deposition. Fauna identified in thin section from the Heterostegina reef interval suggest deposition in a warm, shallow-marine environment relatively free of significant clastic influx. An eastward migration of late Oligocene-early Miocene stream systems introduced an influx of classic sediments onto the ancient shelf of the Lake Pontchartrain area, which influenced the termination of favorable conditions for Heterostegina reef growth. The Robulus (43) zone strata are characterized by a cyclic sequence of limestone, shale, and sandstone. Within this interval, two general lithofacies are identified. Litho-facies I is characterized by thick, shore-parallel sandstone deposits and is interpreted to have been deposited in association with a barrier-beach/tidal-inlet channel environment. Lithofacies II is characterized by shale-prone sandstone intervals, which are immediately overlain by calcareous mudstones and limestones deposited in the offshore inner-middle neritic environment. Five oil and gas fields in the study area have produced hydrocarbons from the interval of interest. The occurrence of hydrocarbons at these locations with respect to mechanisms of entrapment and areal extent of the reservoirs was characterized through detailed subsurface mapping. End_of_Record - Last_Page 773-------

Heterostegina Zone Carbonates, Southeastern Louisiana-Offshore Mississippi: Petrography, Seismic Stratigraphy, Hydrocarbon Potential

A biostratigraphically calibrated electric log section approximately 106 mi long through 11 exploratory wells drilled in nearshore waters of southeastern Louisiana and western Mississippi reveals a thick (387 - 1000 ft) section of carbonates both above and below the Heterostegina zone. Lithologic logs of four cores (total 229 ft) from the Heterostegina zone, and petrographic analysis of 45 thin sections cut from these cores, indicates the presence of hermatypic framework and binding organisms that constructed reefal or algal mound accumulations. These buildups accumulated along a late Oligocene-early Miocene rimmed accretionary carbonate shelf. About 160 mi of seismic line, short roughly perpendicular to this shelf edge, confirms the presence of these buildups in the subsurface. Oil shows in the relatively thick porous and permeable carbonates both above and below the Heterostegina zone interval, regional unconformities at the top and bottom of these carbonates, and the listric faults at the Heterostegina zone hinge line indicate that additional Frio-Anahuac hydrocarbon reservoirs await discovery in this neglected [open quotes]frontier[close quotes] area of the Gulf Coastal Province.

Oligocene carbonate platform evolution and reef development in the eastern Gulf Coastal Plain

Throughout the Early Oligocene, the eastern Gulf Coast was characterized by three paleogeographic/carbonate facies provinces: (1) Shelf Province (Alabama/NW Florida), with foramol/bryomol limestones and local red algal pavements; (2) Gulf Trough/Apalachicola Embayment Province (N Florida/SW Georgia), a deep, current-swept structure with shallow, flanking coralgal reefs; and (3) Florida Platform Province (peninsular Florida/SE Georgia), a miliolid, peloidal, chlorozoan limestone with local patch reefs and coral thickets.During the mid-Oligocene highstand (TO1.2 depositional sequence), the Shelf Province became a drowned ramp with a shelf margin condensed section (Glendon LS). This carbonate system was unable to keep-up with sealevel rise because of the slow-growing, temperate water character of its fauna (30 °N paleolatitude). Around the Gulf Trough, however, coralgal reefs (Bridgeboro LS) kept pace with sealevel rise and formed a rimmed platform. Despite their comparatively high paleolatitude (29–32°N), the tropical fauna of these carbonates thrived because of the influence of warm Gulf Trough waters originating in the Caribbean. The Florida Platform also kept pace with sealevel rise, and was partially emergent (so-called “Orange Island”).This mosaic of adjacent tropical and temperate water carbonates, developed on rimmed platform and ramp settings, is comparable to the modern shelf margin from south Florida (Florida Reef Tract), westward to the west Florida carbonate ramp slope. A good modern analog for the Gulf Trough-Florida Platform Facies Provinces is the Florida Reef Tract-Florida Straits-Bahama Bank system.During the 30 Ma eustatic sealevel fall, coralgal reefs moved from the flanks of the Gulf Trough (Bridgeboro LS) into the Trough (Okapilco Member, Suwannee Limestone). This time-transgressive shift in reef development continued to step down the bathymetric gradient (tracking sealevel fall) until by the Late Oligocene, reefs were growing along the northern Gulf shelf margin in southern Mississippi and Alabama (Heterostegina Zone) as a rimmed shelf. Isolated, time-equivalent reefs grew on the shoals produced by emerging salt domes in Texas and Louisiana (e.g., Damon Mound).

Hydrocarbon Seepage and Salt Dome-Related Carbonate Reservoir Rocks of the U.S. Gulf Coast

ABSTRACT Although most salt dome-related oil production in the Gulf Coast is from clastic reservoirs, oil has been produced from Tertiary carbonate reservoir facies within (1) carbonate cap rocks over salt domes, and (2) the Heterostegina zone coral reef facies of the Anahuac Formation. The only known surface exposure of both carbonates occurs over the Damon Mound salt dome, near Houston, where oil and thermogenic gas have been produced from early Oligocene to early Miocene reservoirs flanking the dome crest. The origin of carbonate cap rock with extremely light 13C values at Damon Mound is related to long-term microbial oxidation of crude oil and thermogenic gas that commenced during or prior to the early Oligocene. Exposure of the salt-dome cap rock by erosion offered a shallow-water carbonate hardground that favored localized development of the late Oligocene-early Miocene Heterostegina coral reef facies. Carbonate cements with extremely light 13C values in the reef facies suggest that hydrocarbon migration continued during reef development, simultaneous with downward development of the carbonate cap rock by replacement of anhydrite. Processes similar to those that operated at Damon Mound appear to explain the origin of carbonates in cold hydrocarbon seeps of the Gulf continental slope. Generalization of these observations suggests that intense hydrocarbon seepage could result in development of carbonate reservoir facies in other oil basins characterized by siliciclastic sediments and shallow salt.

Heterostegina Zone--A Shallow Anahuac (Late Oligocene-Early Miocene) Oil Frontier in Southern Louisiana and Mississippi: ABSTRACT

ABSTRACT The Heterostegina (Het) zone (23.6-23.0 ma, N4a -- Globorotalia kugleri zone) is recognized throughout the Gulf Coast as an excellent correlative datum and an erratic, but prolific producer of hydrocarbons. Its late Oligocene-early Miocene? chronozone represents the maximum development of Neogene reefs in the Caribbean/West Atlantic biogeographic province. Although many of the western Gulf Coast Heterostegina reservoirs consist of reefal limestones that grew atop piercement salt domes, eastern Heterostegina equivalents in Louisiana (St. Martin Parish), southern Mississippi, and Alabama (Mobile Bay area) comprise a rimmed accretionary carbonate shelf. The existence of this shelf is confirmed by seismic data as well as by a core from the Sapphire Petroleum Company #1 State well (MS-85-4-OS) located south of Ship Island, Mississippi. Petrographic study of seventeen feet of Anahuac-age core (5660-5677 ft) recovered from the Sapphire well has delineated three carbonate paleoenvironments-in ascending order: 1) shallow-water, windward algal thickets (5671.0-5677.0 ft); 2) bryozoan-mollusc-foraminiferal-algal limestones deposited below fairweather wave base (5664.2-5671.0 ft); and 3) foraminiferal-algal-rhodolitic limestone laid down above fairweather wave base (5660.0-5664.2 ft).

Subsurface Geology of St. Helena, Tangipahoa, Washington and St. Tammany Parishes, Louisiana: ABSTRACT

ABSTRACT St. Helena, Tangipahoa, Washington, and St. Tammany Parishes comprise the eastern half of the Florida Parishes, an area strategically located between two major producing provinces, the Cretaceous and Wilcox production of south Mississippi to the north and the prolific Miocene production of south Louisiana to the south. Strata seen to date range in age from surface Pleistocene terraces and recent alluvium to the Lower Cretaceous. This paper summarizes the stratigraphy, structure, and oil and gas production of the area, including a brief discussion of pertinent oil and gas production in the immediately adjacent Mississippi counties. Subsurface structure is shown on a series of maps contoured on datums of the top of the Heterostegina zone, the Wilcox group, the Clayton-Selma chalk, the Eutaw shale, and the Lower Tuscaloosa formation. Four geological crow-sections illustrate stratigraphic, structural, and facies relationships. Structural contours demonstrate downflexing of Tertiary strata, show a shifting of strike to a more northerly trend with progressively deeper strata, and are strongly deflected southward by the Hancock County High. Correlation of electric logs show pronounced structural thinning on the Hancock structure in the Comanchean, Gulfian, Paleocene, Eocene, and Oligocene Series. Truncation of the upper Wilcox, Claiborne, Jackson, and possibly Vicksburg beneath Miocene strata indicates a major period of growth occurred sometime prior to the early Miocene. History of development, production, and structure of Angie-East Angie-Sandy Hook field is summarized. A map showing the structure of the top of the producing Lower Tuscaloosa is included.

Miocene-Oligocene Boundary Problems in Gulf Coast: ABSTRACT

The Oligocene Vicksburg marine beds, the Frio massive sands and shales, and the Anahuac shales and limestones constitute one of the best known, yet controversial petroliferous sequences in the coastal Louisiana-Texas subsurface. The position of the Miocene-Oligocene boundary within this sequence has been the subject of many debates among Gulf Coast geologists. The problem originated when the middle Anahuac Heterostegina species were erroneously identified as Heterostegina antillea of the middle Oligocene on the Island of Antigua, British West Indies. The problem resulting from the assignment of the Heterostegina zone to the middle Oligocene on the basis of this species determination was further complicated by the application of the Texas surface term Frio to the sands and shales that occur between the Anahuac and Vicksburg. Later it was determined that the surface Frio of Texas was actually the equivalent of the subsurface Vicksburg; and that the so-called subsurface Frio was younger than previously believed. Many proposals have been made for the placement of the Miocene-Oligocene contact in the Louisiana and Texas subsurface, each at a different stratigraphic level. The purpose of this paper is to establish the relation of the subsurface Cibicides hazzardi zone of the upper Frio with the fossiliferous surface units on the east in Mississippi, Alabama, and Florida by means of Ostracoda; not to establish an indisputable Miocene-Oligocene boundary. The Tampa limestone (basal Miocene, Florida Geological Survey) has been assigned to the Aquitanian stage of Europe and correlated with the Paynes Hammock sand of Alabama and Mississippi by the United States Geological Survey. A detailed study of the Ostracoda of the basal Tampa and Paynes Hammock formations shows that they contain the same ostracode fauna. A similar study of the Ostracoda of the Cibicides hazzardi zone in the Superior Oil Company's Duplantier well No. 1, University field, East Baton Rouge Parish, Louisiana, points to a correlation of this unit with the basal Tampa and Paynes Hammock formations. Whether the correlation of the Tampa with the Aquitanian is correct is beyond the scope of this study; however, if correct, this does not necessarily establish a definite Mio ene age for the upper Frio since the Miocene-Oligocene boundary in Europe is still in dispute. End_of_Article - Last_Page 1600------------

Heterostegina Reef on Piercement Salt Domes, with Special Reference to Nash and Others in Northwestern Brazoria County, Texas: ABSTRACT

Reef limestone as much as 300 feet thick occurs in the Heterostegina zone in part of northwestern Brazoria County about 40 miles south-southwest of Houston, between Damon Mound, Nash, and West Columbia piercement salt domes. This locally developed limestone is completely surrounded by normal Heterostegina calcareous shale or shale with one or more very thin limestone beds. At Nash dome the upper part of the limestone is porous, consisting chiefly of honeycomb corals, and the lower part is more dense. Indications are that the Heterostegina reef developed in a near-shore, shallow-water environment, and that its growth did not stop at the end of Anahuac time but continued even into the early Miocene. Large-diameter conventional cores are recommended for evaluating the production potential of the Heterostegina limestone. Although the best porosity is found in the top of the limestone, it may develop also in other intervals. The limestone production at Nash field does not have a common oil-water contact. Substantial oil production has been obtained from the Heterostegina limestone and more may be expected at Nash, Damon Mound, and West Columbia fields, at depths ranging from approximately 2,000 feet at Damon to 4,350 feet at Nash. End_of_Article - Last_Page 2515------------

Petroleum Geology of Anahuac and Frio Formations of Northeastern Mexico: ABSTRACT

During the past 14 years, Petroleos Mexicanos has carried on an extensive drilling program. This has been principally concentrated along the Frio-Vicksburg trend. The Frio has been the most prolific producing formation in northeastern Mexico. Of its three facies, non-marine, brackish, and marine, the first named is the most productive. Locally overlying the Frio, both on the surface and in the subsurface, is the Norma End_Page 1775------------------------------ conglomerate. It is a large fan deposit, which may be correlative to the Anahuac formation, or the Soledad formation. Unconformably above the Norma, or the Frio, is the Anahuac formation. The three foramineferal zones, Discorbis, Heterostegina, and Marginulina, are recognized in northeastern Mexico. All are productive. The most important fields discovered in the Frio-Vicksburg trend are Brasil, Reynosa, Cano, and Trevino, where development drilling is still going on. The 18 de Marzo field is important as a gas producer from the Marginulina zone of the Anahuac, and the Trevino field produces from the Heterostegina zone. End_of_Article - Last_Page 1776------------

Tertiary Reef and Associated Limestone Facies from Louisiana and Guam

Similarities in fossil content and lithology of reef and associated limestone facies from southern Louisiana and Guam indicate that biologically similar reefs existed in these areas during middle Tertiary time. The reefs of southern Louisiana are found in the subsurface Heterostegina zone of late Oligocene or early Miocene age; the outcropping Guam reefs are considered to be of early Miocene age. The geologic settings of the reefs differ greatly. The Louisiana reefs are found localized on piercement salt-dome structures and as an extensive reef complex developed on a shallow-water shelf area in southeastern Louisiana. On Guam the flanks of an intermittently active volcano and a ridge of folded volcanics were the sites of reef growth. Six limestone facies are common to these two reef complexes: (1) reef-wall, (2) reef-breccia, (3) off-reef, (4) fore-reef de-trital, (5) fore-reef transitional, and (6) basinal. Material from Middle East reefs of Eocene and Oligocene age is presented for comparison. Reconstructions of depositional environments of the southern Louisiana and Guam reefs are based on stratigraphic, structural, and facies analyses. Present-day counterparts of the Louisiana fossil reefs are submarine topographic prominences on the Continental Shelf off Louisiana and Texas; the Guam fossil reefs are considered analogous to the reef around Borabora in the Society Islands. The authors conclude that Tertiary reefs contain diagnostic limestone facies in predictable spatial relationships and that the nature of the substratum and the local geologic settings do not appear to be controlling factors, provided that ecological conditions are favorable.

Miogypsinids, Planktonic Foraminifera, and Gulf Coast Oligocene-Miocene Correlations

The foraminiferal family, Miogypsinidae, contains evolutionary series, the successive units (species) of which are common to the Louisiana Gulf Coast, Florida, the Caribbean region, and the Mediterranean region. It is suggested that the miogypsinids had a monophyletic origin and that dispersal of some of the evolving groups during the Oligocene and the Miocene was a rapid process. Stratigraphic relationships between miogypsinid species and planktonic Foraminifera in the Gulf Coast offer the key to world-wide correlations and proper use of some of the European terminology. Oligocene sediments on the western side of the Atlantic contain the probable Rotalia ancestors of the Miogypsinidae, which show only slight differences from widely distributed primitive Miogypsinidae. Chattian beds in both the Old and the New World contain the primitive Miogypsina complanata Schlumberger, while M. bermudezi Drooger, M. thalmanni Drooger and M. panamensis (Cushman) are late Oligocene species restricted to the western hemisphere. M. gunteri Cole and M. tani Drooger are found in the Aquitanian on both sides of the Atlantic. Burdigalian beds are marked by M. irregularis (Michelotti) and M. intermedia Drooger both in America and in the Western Mediterranean area. The American species, M. bronnimanni Drogger, is thought to be yet another Burdigalian-Helvetian representative of the family. M. cushmani Vaughan occurs in Helvetian sediments on both sides of the Atlantic, but M. mexicana Nuttall seems to have been confined to the Helvetian of the western side. M. mexicana was the most advanced miogypsinid in the Gulf Coast and the last member of the family to become extinct here. Both nepionic acceleration and centripetal trends are repeatedly confirmed by the Gulf Coast data. Further study is needed on planktonic Foraminifera, but at present a late Helvetian age is logical for the ranges of Globigerinatella insueta Cushman and Stainforth, Globigerinoides bispherica Todd, and Globorotalia praemenardii Cushman and Stainforth. Orbulina and Globorotalia menardii (d'Orbigny) seem to have appeared contemporaneously in the Gulf Coast records; the beginning of Orbulina in Europe coincides with the Helvetian-Tortonian boundary, and the range of Globorotalia fohsi Cushman and Ellisor is mainly Tortonian. It is apparent that this new faunal evidence affords a better understanding of time relationships within the Gulf Coast, which can be extended to areas of tropical America. These fossils indicate that Suwannee limestone which is exposed at numerous Florida localities and fossiliferous carbonate rocks to which the name has also been applied in various Florida wells should be correlated in part with the Chickasawhay formation of Mississippi and in part with the subsurface Anahuac formation and the lower Fleming of Texas and Louisiana. The Tampa limestone of Florida is correlated with the lower middle Fleming. Some of the faunal zones of the Gulf Coast subsurface, such as the well known Heterostegina zone, are considered to be time-transgressive units.

Geology of West Tepetate Oil Field Jefferson Davis Parish, Louisiana

The West Tepetate oil field, located in the Louisiana Gulf Coast area, was discovered by the Barnsdall Oil Company and Vincent and Welch in 1944. The presence of the structure was first suggested in 1930 by torsion-balance exploration conducted by Vincent and Welch and was subsequently disclosed by various seismograph surveys during 1941-1944. The producing section lies below the Heterostegina zone in the middle Miocene, and consists of twelve oil- and gas-bearing sands ranging from 7,600 to 9,900 feet in depth. The structure is a gentle, irregularly elongate dome, believed to have been produced by the deep-seated intrusion of salt, though none has been encountered to date. There is nearly 100 feet of effective closure at the producing levels. Only one fault has been enco ntered in drilling, though a major regional fault, downthrown on the south, is indicated by seismograph on the north side of the field, probably being a westward extension of the similarly bounding fault in the Tepetate field. The productive limits thus far established enclose 1,400 acres, offering a reserve of 30-40 million barrels of liquid hydrocarbons, and 60-70 billion cubic feet of gas.

Marine Sedimentary Cycles of Tertiary in Mississippi Embayment and Central Gulf Coast Area

The principles of marine cyclic deposition are discussed briefly and a classification of marine sedimentary cycles is presented. The principles are applied to the Tertiary section of the Mississippi embayment. Five major marine cycles are differentiated, each representing a major transgression of the sea into this embayment. Facies and thickness distribution of the older Tertiary cycles indicate the presence of a positive area (submarine plateau) in southeast Mississippi, which is bounded on the north and northwest by negative areas (synclines). Isopach studies also show a progressive expansion of this positive area north and northwest during Eocene time, with a corresponding north and northwestward shifting of the bordering synclinal areas. The theory is advanced that the southeast Mississippi plateau constitutes a part of the northern front of the Gulf of Mexico neutral plate, and that its progressive northward expansion is caused by a gradual rise and possible northwestward drift of this plate, in connection with the Tertiary orogenic movements in the Greater Antilles. Epeirogenic movements in the Miss ssippi embayment and its borderlands are considered to be responsible for the different marine transgressions during the Tertiary. These movements reached a climax at the end of Jackson and during early Oligocene time, after which the sea retreated from the Mississippi embayment. Marine deposition during Miocene time is restricted to the Gulf Coast areas with one minor transgression occurring during the deposition of the Heterostegina zone.

Geology of West Tepetate Oil Field, Jefferson Davis Parish, Louisiana: ABSTRACT

The West Tepetate oil field, located in the Louisiana Gulf Coast area, was discovered by the Barnsdall Oil Company and Vincent and Welch in 1944. The presence of the structure was first suggested in 1930 by torsion balance exploration conducted by Vincent and Welch and was subsequently disclosed by various seismograph surveys during 1941 to 1944. The producing section lies below the Heterostegina zone in the Middle Miocene, and consists of nine oil- and gas-bearing sands ranging from 7,600 to 9,500 feet in depth. The structure is a gentle, irregularly elongate dome, believed to have been produced by the deep-seated intrusion of salt, though none has been encountered to date. There is about one hundred feet of effective closure at the producing levels. Only one fault has b en End_Page 1966------------------------------ proved, though a major regional fault, downthrown on the south, is indicated by seismograph north of the field, probably being a westward extension of the similarly bounding fault in the Tepetate field. The productive limits thus far established enclose 1,400 acres, offering a reserve of 30-40 million barrels of liquid hydrocarbons. End_of_Article - Last_Page 1967------------

Geology and Geophysics Showing Cap Rock and Salt Overhang of High Island Dome, Galveston County, Texas

The High Island dome was first drilled for oil in 1901; however, the first production was obtained from the porous cap rock in 1922, on the super-dome structure. The Yount-Lee Oil Company in 1931 proved the existence of an overhang at High Island and has since developed the field from the northwest flank to the southeast flank. The development at present is toward the east flank. The cap rock at High Island is divided into a false cap rock and a true cap rock. The false cap rock is a hard sand and lime rock ranging in thickness from 200 to 1,300 feet. The true cap rock lies underneath the false cap rock and consists of a series of caps. At the base of the series is anhydrite which grades upward into gypsum and a calcite cap above the gypsum. The overhang has so far been proved to exist on all flanks that have been drilled. There is a cap-rock overhang as well as a salt overhang; the cap-rock overhang extends outward and away from the salt. Production is just as prolific under the cap-rock overhang as under the salt overhang. The dome is somewhat lopsided in that the southeast flank is approximately 1,500 feet higher than the northwest flank. Faulting has been detected on the south and southeast flanks. A theoretical peripheral fault is mapped from the northwest to the southwest flank. Oil has been produced in commercial quantities from the cap rock, Pliocene formation, Miocene formation, M Dv zone, Discorbis zone, and Marginulina zone. The Heterostegina zone at High Island has not yet been penetrated or encountered. The P iocene and Miocene production is very small and practically all of the sands in these two formations contain water. The M Dv zone is a new paleontological zone mapped by the writer as lying immediately above the true Discorbis zone of the Middle Oligocene, and equivalent to Lower Miocene in age. This new zone is characterized by both Lower Miocene and true Discorbis fauna. Therefore, the name M Dv has been chosen to represent the zone because of the occurrence of Miocene fauna associated with Discorbis cf. D. vilardeboana and its fauna; thus, the name: M Dv. The M Dv zone has so far been proved to be the most prolific of any productive zone. Six different sand horizons have been found in this zone. The Discorbis zone production has been limited; however, some of the company's latest and best wells are producing from this zone. Only one sand has been found in the Marginulina formation. The oil at High Island is divided into three types, ranging from 24° to 44° A.P.I. gravity.

The Relationship of the Vicksburg Group to the Formations which overlie it in Mississippi: ABSTRACT

End_Page 138------------------------------A certain amount of confusion in the nomenclature and in the correlation of certain formations encountered in deep wells in the Gulf Coastal areas which are now being exploited by oil development has necessarily occurred, perhaps primarily due to the fact that the more detailed stratigraphic and paleontological features of a number of the standard Tertiary sections of Mississippi and Alabama have not been known in greater comparative detail. The exact demarcation of the upper and lower limits of the Oligocene is still a subject of controversy among paleontologists and geologists, particularly within the Gulf Coastal areas of Louisiana and Texas which are now undergoing exploitation by oil-field developments. It is particularly interesting that we may go to Mississippi for what appears to be the proper elucidation of the relationships of the standard Oligocene section of the Gulf Coastal Plains and the lowermost Miocene of Mississippi. A marine formation lying between the Catahoula and the Byram marl of the Upper Vicksburg will be described. The evidence for the age of this formation and its position with respect to the Heterostegina zone will be discussed. End_of_Article - Last_Page 139------------