TIN MOUNTAIN LIMESTONE
The Tin Mountain Limestone is exposed in the Funeral Mountains only at the southeast end. Here the main exposure extends continuously along the crest of the Bat Mountain block northward from hill 2997 (fig. 1). The Tin Mountain Limestone from a distance is clearly distinguished by its broad dark-gray and light-gray bands (fig. 7) above the brownish uppermost part of the Lost Burro Formation. In a section measured from the underlying Lost Burro to the overlying Perdido Formation at hill 2997, the Tin Mountain Limestone is 315 feet thick.
The limestone of the Tin Mountain contains some nodular chert and thinly interstratified shale or argillaceous limestone that is a distinctive feature of some units. The texture of the limestone is conspicuously diverse within the range of very fine grained to bioclastically coarse grained. The formation is exceptionally fossiliferous in this area as well as throughout the region. The association and stratigraphic distribution of fossils collected from the measured section are shown graphically on plate 3. The most useful fossils for dating the rocks are referred to in comments on the ages of units in the Tin Mountain Limestone and Perdido Formation.
The lowest part of the Tin Mountain Limestone, a commonly concealed weak unit (t1) about 25 feet thick, consists of thinly interstratified limestone and shale or argillaceous limestone. The limestone, which is medium dark gray and weathers medium gray, is sparsely cherty and very fine grained to bioclastically coarse grained. The argillaceous thin beds and partings are tinged red or yellow. The basal bed is gradational within 1 foot upward from calcareous sandstone of the underlying Lost Burro to slightly sandy limestone in the Tin Mountain. The sandy limestone contains conspicuous Syringopora, which is regionally characteristic of the Tin Mountain Limestone.
Fossils in collections (21950PC to 21953PC) from the basal bed and higher in unit t1 (collns. 21947PC, 21948PC, 19772PC, 19773PC, pl. 3), according to reports by W. J. Sando (written communs., 1964, 1965) on the corals and by Mackenzie Gordon, Jr. (written communs., 1964, 1967) on the brachiopods, include taxa that are widely known from the Tin Mountain and are Early Mississippian in age. Conodonts in collection 24652PC (pl. 3) from about 5 feet above the base of the formation, according to J. W. Huddle (written commun., 1965), indicate more restrictively a Kinderhookian age. Foraminifers in collection 21948PC (pl. 3) from a few feet higher above the base, according to Betty Skipp (written commun., 1965, amended 1971), are late Kinderhookian and are common to zone 1 of the Redwall Limestone, Arizona (Skipp, 1969, p. 184-185), and to foraminiferal Zone 6 at the base of the lower Carboniferous in the type region in Belgium and France (Mamet and Skipp, 1970, fig. 7, p. 1134).
Unit t2 of the Tin Mountain Limestone, about 75 feet thick, is prominent in a dark cliff above the rubbly slope that generally conceals unit t1 and below a light-gray band in unit t3 (fig. 7). Limestone that constitutes unit t2 is medium dark gray, weathers medium gray, and is varied in texture. Near the lower boundary, which is gradational and poorly defined, beds are thin. Elongate chert nodules, 1 inch or less thick, are sparse in a 12-foot interval 30 feet above the base of the unit.
Brachiopods are common at the base of unit t2, corals are common near the top of the unit, and microfossils are associated with both. The brachiopods, none of which were collected, occur in the lowest 10 feet of the unit and extend the underlying range represented by collections 19772PC and 19773PC (pl. 3) at the top of unit t1. The corals near the top of unit t2 are laterally persistent in a zone 5 feet thick. Six collections (21941PC to 21946PC) spaced irregularly about 1,000 feet along the strike of the zone (see section on "Register of Fossil Collections") contain silicified corals identified by W. J. Sando (written commun., 1965) and given on plate 3. Their relative abundance, not shown by the chart, is suggested by the occurence of Rylstonia in all six collections, Homalophyllites in four, Syringopora in three, Vesiculophyllum in two, and Caninia? in one collection.
A microfossil assemblage (colln. JFM65517C, pl. 3) from beds near the base of unit t2 contains Kinderhookian foraminifers. According to Betty Skipp (written commun., 1965, amended 1971), "The fauna is that of zone 1 of the Redwall Limestone, Arizona, and is typical of Zone 6 of the foraminiferal zonation of Mamet." She reports further that collections (21941PC and 21946PC, pl. 3) from the coral zone, 60-65 feet higher in unit t2, significantly include Latiendothyra of the group L. parakosvensis Lipina, 1955, (Plectogyra tumesepta Zeller, 1957, [part]), a designation that is abbreviated in the chart Palaeospiroplectammina aff. P. parva (Chernysheva), 1940, Septaglomospiranella primaeva (Chernysheva), 1940, and S. primaeva subsp. noda Skipp, 1966. She concludes, "This is a late Kinderhookian fauna similar to that of the lowest part of zone 2A of the Redwall and the lower part of Zone 7 of the foraminiferal zonation of Mamet." An inferred relationship of global foraminiferal Zone 7 with Cordilleran megafossil zones, principal European and American time-stratigraphic divisions, and Cordilleran and type Mississippian formations is show diagrammatically by Sando, Mamet, and Dutro (1969, fig. 7).
Conodonts are abundant and varied in one of the collections (21946PC) from the coral zone and in a collection (21940PC) from 5 feet higher, at the top of unit t2. The fauna listed (pl. 3) by J. W. Huddle (written commun., 1965, amended 1971) is similar to that from the base of the formation; both assemblages contain predominantly Siphonodella cooperi Hass, but the assemblage from unit t2 contains in addition S. obsoleta Hass, Polygnathus symmetricus E. R. Branson, and P. inornatus E. R. Branson. The conodont fauna is Early Mississippian in age and is probably equivalent in age to late but not the latest Kinderhookian of southwestern Missouri and adjacent areas as reported by Thompson and Fellows (1969, table 1).
Coarsely bioclastic crinoidal limestone characterizes unit t3, which is 75 feet thick in the section at hill 2997. Much of the unit makes a light-gray band on the mountain spur (fig. 7), in contrast to darker gray beds in the base of the unit and in underlying unit t2. The darker gray basal beds contain sparse, large chert nodules in a 15-foot interval where the section was measured. A thousand feet northward, the cherty interval is 25 feet thick, and the chert is abundant in its lower part. The nodules that occur together abundantly are commonly 8-10 inches long and 3-4 inches thick. They consist of dark-gray to olive-black chert that weathers brownish black. The light-gray main part of the unit consists of broad crinoid columnals and some long segments of crinoid stems.
A few corals and brachiopods were collected near the base and near the top of unit t3 (collns. 21938PC, 21939PC, pl. 3). The higher collection (21938PC), 160-165 feet above the base of the Tin Mountain Limestone contains Dimegelasma sp., identified by Mackenzie Gordon, Jr. (written commun., 1967). He states that Dimegelasma is considered to be a post-Kinderhookian brachiopod genus. Judging from this and from the distribution of corals listed by Sando (pl. 3), he believes that this and the overlying part of the Tin Mountain are Osagean in age.
Unit t4, which is 50 feet thick, forms a rubbly bench below a steeper slope or cliff. In the less resistant, lower part of the unit, thin limestone beds are interstratified with pale-red or pinkish-gray argillaceous beds. Reddish argillaceous material forms partings in the upper part. The limestone, which is medium gray or darker and weathers somewhat lighter gray is fine- to coarse-grained biocalcarenite that consists of fragmented mixed fossils. Unbroken fossils are conspicuous and varied. Elongate chert nodules, which almost coalesce in some beds, are abundant in the upper part of the unit.
Specialists who identified the corals, brachiopods, conodonts, and ostracodes in collections from unit t4 (tabulated on pl. 3) agree that the faunas are Early Mississippian but they do not agree whether they indicate an Osagean or a Kinderhookian age. The corals that were identified by W. J. Sando (written commun., 196S) are not diagnostic as to which of these ages. He states that the coral zones established for the Madison Group in the northern Cordillera region (Sando and others, 1969) and the coral zones established for the Redwall Limestone in Arizona (Sando, 1969, p. 268-271) are not clearly evident in the Funeral Mountains section. The seven genera of brachiopods that Mackenzie Gordon, Jr. (written commun., 1967), identified from unit t4 (pl. 3) do not by themselves indicate Osagean age, but they occur above the Dimegelasma-bearing bed that he believes is Osagean in age.
A goniatite that is associated with brachiopods in collection 21936 PC is a Protocanites or young Merocanites sp. indet., according to Mackenzie Gordon, Jr. (written commun., 1967), who comments, "The young goniatite***is neither large enough nor well enough preserved to enable us to distinguish between Kinderhook and Osage in this bed. It is, however, certainly a Lower Mississippian form."
A conodont fauna in this collection (21936PC) from the middle of unit t4 has predominantly Gnathodus punctatus (Cooper) and Polygnathus communis Branson and Mehl (pl. 3), according to J. W. Huddle (written commun., 1965). He notes, "Gnathodus punctatus has been found in the upper part of the Joana Limestone of Nevada, in the Chappel Limestone of central Texas, and the Welden Limestone of Oklahoma. The age of this part of the Tin Mountain is Early Mississippian, probably late Kinderhookian." G. punctatus characterizes but is not restricted to the uppermost part of the Kinderhookian Series in the southern midcontinent where its name has been used for an upper Kinderhookianlower Osagean zone in Texas (Hass, 1959, p. 369; Thompson and Fellows, 1969, p. 60) and for a subzone limited to the uppermost Kinderhookian of southwestern Missouri (Thompson and Fellows, 1969, p. 5657). In southwestern Missouri and adjacent areas G. punctatus occurs also in overlying beds of Osagean conodont zones (Thompson and Fellows, 1969, for example, see distribution charts 2, 13, 19).
An abundant silicified ostracode assemblage, which contains more than 40 species, mostly new, was recovered by I. G. Sohn from a collection (12858PC) within the basal 15 feet of unit t4. The following is from his report on the assemblage (I. G. Sohn, written commun., 1966, reviewed 1971):
The etched residue consists of thousands of specimens in various stages of preservation. Some of the specimens, though intact, show bending and lines of breakage, indicating that postdepositional and presilicification movement took place. Many of the taxa are represented by growth stages, an indication the ostracodes were not transported. Although most of the specimens are single valves and only rare complete carapaces are recovered, I consider this to be the result of the laboratory process because an acetate peel of an etched surface has a few cross-sections of carapaces.
* * * This assemblage consists of practically all new species and several genera that I do not recognize. * * * One of the problems is that few silicified faunas have been described. * * *
The ostracodes have an aspect that is similar to etched forms from the middle and upper members of the type Banff Formation (Green, 1963) and from the Gattendorfia Beds of Thuringia (Grundel, 1961; 1962). As indicated above, however, this may be due more to the fact that the two assemblages listed are also etched from limestone than to age similarity.
Ranges in age of 14 ostracode species that are close relatives of those in collection 12858PC are shown in table 2 by Sohn (written commun., 1966, reviewed 1971). Seven of the related species are Osagean in age, such as Kirkbyella (Berdanella) reticulata Green, 1963, and Rectobairdia confragosa Green, 1963; two are Kinderhookian, Psilokirkbyella ozarkensis (Morey, 1936) and Monoceratina? elongata Benson and Collinson, 1958; and the other five species are both Osagean and Kinderhookian. Sohn comments that the comparison made in the table suggests an Osagean age for the assemblage.
Later Sohn collected in sequence through the same stratigraphic interval in the lowest part of unit t4. He recovered abundant silicified ostracodes from samples taken from 11-15.9 feet above the base. In reporting on his collections (written commun., 1971), he states that in addition to Shivaella macallisteri Sohn, 1972 (p. 5, pl. 3, figs. 1-38), 14 species are either identical or closely related, on the species level, to ostracodes from the Narrow Canyon Limestone and lower part of the Mercury Limestone in the Nevada Test Site, about 45 miles east northeast of the Funeral Mountains.
TABLE 2.Ostracodes in collection 12858PC, Tin Mountain
Limestone, listed by I. G. Sohn, showing range in age of close
Unit t5 at the top of the Tin Mountain Limestone is more uniform in composition and bedding than the underlying units but is variable in thickness. Limestone in the unit is commonly very fine grained and medium dark gray that becomes medium light gray through weathering. A few chert nodules mark a zone 5-10 feet thick about 50 feet from the base of the unit. Unit t5 is at least 90 feet thick to alluvial cover at the top of the Tin Mountain in the continuous measured section across hill 2997, and 90 feet thick to the exposed contact with the Perdido Formation in a supplementary section 250 feet northward. It is only 55 feet thick about 250 feet farther north and 65 feet thick in the next ridge about 1,000 feet eastward, where the cherty zone is also about 50 feet above the base of the unit. The change in thickness occurring between the cherty zone and distinctive mudstone at the base of the Perdido implies that the upper contact of the Tin Mountain in the vicinity of hill 2997 is a disconformity if it is not an obscure bedding fault of local extent.
Fossils are scarce in unit t5. The only fossils found, except for a poorly preserved brachiopod (colln. 21934PC, pl. 3) at the base, are a few brachiopods and poorly preserved horn corals (colln. 21933PC) from the interval between 70 and 75 feet above the base of the unit. Betty Skipp examined two samples taken for possible foraminifers from lower beds in the unit and only found calcareous sponge spicules and fragments of bryozoans and algae. The brachiopod assemblage from the uppermost part of the formation (colln. 21933PC) contains Brachythyris cf. B. suborbicalaris (Hall) and Imbrexia sp., according to Mackenzie Gordon, Jr. (written commun., 1967). He comments "The Brachythyris indicates an Osagean, rather than Kinderhookian, age for this bed. This species, however, ranges into rocks of early Meramecian age."
In summary, the age of the Tin Mountain Limestone here is Early Mississippian. The lowest part of the formation is Kinderhookian in age on the basis of conodonts and is restricted to late Kinderhookian on the basis of foraminifers. The upper part above unit t3, on the basis of brachiopods and tentative support of ostracodes, is Osagean.
In the Funeral Mountains the Perdido Formation is exposed only on the Bat Mountain block (fig. 1), where it is the youngest Paleozoic formation under Cenozoic continental rocks. The Perdido here is incomplete, for it lacks the distinctive uppermost limestone and shale beds that contain Chesterian fossils in the type area, northern Panamint Range. It is distinguished at a distance by a moderate-brown upper part overlying a generally more resistant, brownish-gray lower part, which together contrast with light-gray limestone at the top of the Tin Mountain Limestone. The boundary between the formations is drawn at the base of a thin mudstone unit. The lithology of the Perdido Formation which is characteristically diverse, is predominantly medium-dark-gray limestone containing interbedded chert in the lower part, and it is mostly siltstone in the upper part. Much of the limestone is bioclastic, of varied grain size, and it contains irregularly interbedded calcareous quartz siltstone and much less sandstone. Uncomminuted fossils are rare. Abundant dark-gray chert, thinly interbedded in limestone, and brown-weathering calcareous siltstone are diagnostic features of the formation. At the top of the mountain 1.5 miles north of hill 2997, the thickness of the Perdido Formation is about 500 feet where the lower and upper parts of the formation are nearly equal. The lower 215 feet of the Perdido Formation, which is described below and shown on plate 3, was measured on the low ridge that is about 1,000 feet east of hill 2997.
Basal unit p1 of the Perdido is 18 feet thick in the measured section where it is concealed by slope rubble on a bench. In a good exposure a few hundred feet north of hill 2997, unit p1 consists of light-gray partly shaly mudstone that has some thinly interbedded limestone, especially in the upper part. The unit, although thin, extends to the farthest exposure of the base of the Perdido in the Bat Mountain block. Other units in the measured section (pl. 3) may not be as extensive, but they are characteristic of the general sequence in the lower part of the Perdido Formation.
Unit p2, which is 87 feet thick, contains evenly bedded medium-dark-gray limestone that weathers medium gray, medium light gray, and light olive gray. Some of the limestone is bioclastic. Abundant dark-gray chert forms beds that are less than an inch to 3 inches thick and commonly about 2 inches thick. Exposed surfaces of the chert are brownish gray, contributing to the color of the unit.
Unit p3, which is 20 feet thick, is the lower of two units that are browner than others in the lower part of the Perdido. Chert, siltstone, silty limestone, and subordinate limestone are generally in discontinuous beds less than 1 foot thick. Fine-grained sandstone, gray where fresh, occurs in irregular pods and in beds that pinch and swell. The quartzrose rocks weather brown.
Unit p4, which is 25 feet thick, consists mostly of medium-gray limestone in beds that are commonly about 1 foot thick but range in thickness from a few inches to 1-1/2 feet. Coarse crinoidal debris forms beds that pinch and swell, and individual lenses that are as much as 4 feet thick. Quartzose rocks ranging from silty fine-grained limestone to calcareous sandstone are interbedded with the limestone.
Unit 5, which is 15 feet thick, is the upper brown-weathering unit. It is lithologically similar to unit p3, the lower brown unit.
Unit p6 measures 50 feet thick to a fault of undetermined displacement. Beyond the fault similar beds are probably less than a hundred feet thick below alluvial cover. Fine- to coarse-grained limestone, which constitutes about half the unit, is medium dark gray, weathers the same gray or somewhat lighter, and occurs in beds about 1 foot thick or less. Chert and siltstone occur in beds that are generally a fraction of an inch to several inches thick and of local continuity.
Very few fossils were obtained from the Perdido Formation, except for microfossils of Late Mississippian (Meramecian) age in the collection at the top of the section (24651PC, pl. 3). Fossils in lower collections (21931PC, 21932PC, pl. 3), as evaluated in the following comments, are inadequate to distinguish an Osagean from a Meramecian age. The corals in collection 21932PC (pl. 3) only indicate a Mississippian age (W. J. Sando, written commun., 1965) and the associated conodonts are not diagnostic (J. W. Huddle, written commun., 1965). Mackenzie Gordon, Jr. (written commun., 1967), comments on a moderately large form of productoid in collection 21931PC (pl. 3), as follows: "This productoid suggests by its fine ribbing either of two genera: Setigerites and Striatifera. These have very distinctive shapes but this one specimen is not sufficiently complete to allow us to distinguish which genus is represented. Both occur in rocks of Meramecian (Late Mississippian) age. Setigerites also occurs in rocks of Osagean (Early Mississippian) age." Concerning one of the conodonts from the same collection, J. W. Huddle (written commun., 1965) points out, "Gnathodus texanus ranges from Early to Late Mississippian. It has been found in the Mississippi Valley in rocks as old as the Burlington Limestone [in the middle part of the Osagean Series] and as young as the Bethel sandstone in the lower part of the Chesterian Series. It is abundant in the Keokuk, Warsaw, Salem, and lower St. Louis Limestones."
The sample at the top of the section (colln. 24651PC, pl. 3), which was collected only for possible microfossils 205 feet above the base of the Perdido Formation, yielded diagnostic foraminifers and conodonts among those listed on the chart (pl. 3). The following designations of foraminifers as reported by Betty Skipp (written commun., 1965, amended 1971) are abbreviated on the chart:
Endothyra of the group E. prisca
Rauser-Chernousova and Reitlinger, 1936
Although Calcisphaera packysphaerica Pronina, 1963, and Stacheia Brady, 1876, are listed as foraminifers, Skipp adds the following comment:
Calcisphaera and Stacheia are considered algae by both Mamet and me. Calcisphaera suggests a very shallow water environment. This assemblage is definitely Late Mississippian (middle Visean to early Namurian) in age. All of the forms are common in rocks of Meramecian age. But the age is not earliest Meramecian because both G. tomiliensis and E. bowmani appear in early St. Louis time. Also, G. tomiliensis is very scarce in late Chesterian faunas. Therefore, a middle Meramecian to early Chesterian age range fits best.
Concerning the conodonts recovered from the sample, J. W. Huddle (written commun., 1965) reports, "This fauna is early Late Mississippian in age. Taphrognathus varians is abundant in the Keokuk, Warsaw, Salem, and lower St. Louis Limestones in the Mississippi Valley."
The age of this bed in the Perdido, based on the foraminifers and conodont occurring together, is Late Mississippian. Furthermore, the lowest occurence of G. tomiliensis and E. bowmani and the highest occurence of abundant T. varians in the provincial series of the Mississippi Valley indicate that its age is comparable with that of the lower part of the St. Louis Limestone, about middle Meramecian. Systematic sampling for microfossils is necessary in order to determine whether beds of Osagean age are limited to the upper part of the Tin Mountain Limestone or whether they continue into the lower part of the Perdido Formation.
Last Updated: 23-Jun-2009