Lithofacies of the Nashville Ordovician
Pertidal environments consist of those settings exposed by the daily or twice-daily rise and fall of the tides. Some of the peritidal facies in the Nashville Ordovician suggest arid climates (dolomitic and laminated facies), whereas others suggest humid climates (bioturbated and fenestral lime mud facies).
Fining-upward beds that grade from ostracod-bearing lime mudstone and wackestone into pure lime mudstone. Laminated to very thin-bedded. Contains laminoid fenestrae, planar lamination, and small-scale cross-lamination. Occurs in the M1 sequence (Murfreesboro).
Fine-grained dolomite and greenish dolomitic shale. Very thin-bedded to laminated. Contains vortex ripples and abundant desiccation cracks. Occurs in the M4 sequence (Upper Carters).
Ranges from entirely dolomitic mudstone to even proportions of dolomitic mudstone and calcisiltite. Mudstones are light green to greenish-gray; calcisiltites are light to medium gray. Mudstones are silty, glauconitic, and usually unfossiliferous. Calcisiltites are locally coarse enough to be very fine-grained skeletal grainstones. Thin- to medium-bedded mudstones with very thin to thin beds of calcisiltite. Mudstones are structureless, nonfissile, blocky; presumably thoroughly bioturbated. Beds of calcisiltite are commonly discontinuous and may display erosional bases, fining-upward grain size, planar lamination, wave-ripple lamination, short vertical burrows, and nonlaminar bubble fenestrae. Occurs in the M6 sequence (Catheys).
Light-Colored Fine Grainstone
Light to medium gray silty skeletal to peloidal packstones and poorly washed grainstones; so fine-grained as to almost be calcisiltites. Packstone and grainstone beds commonly fine upwards. Generally unfossiliferous, but may contain ostracod valves and echinoderm ossicles. Dolomitic mudstones are green, argillaceous, and silty. Thin- to medium-bedded packstones and grainstones separated by very thin beds to partings of dolomitic mudstone. Moderately to heavily bioturbated (ii2-ii4). Nonlaminar bubble fenestra present. Occurs in the M6 sequence (Catheys).
Lime mudstone, rarely dolomitized; typically unfossiliferous, but locally contains rare ostracods. Thin- to medium-bedded. Common to abundant nonlaminar fenestral vugs. Less common vertical branching tubular pseudo-fenestrae. Occurs in the M2, M3, M4, M5, and M6 sequences (Ridley, Lebanon, Carters, Bigby-Cannon, Catheys).
Burrowed Lime Mudstone
Lime mudstones. Contains sparse fossil fragments, usually of ostracods and brachiopods, rarely rich enough to form a wackestone. Interbeds of very thin beds of dolomitic shale. Thin- to very thin-bedded. Abundant dolomite-filled burrows in lime mudstones, most commonly of Chondrites. Occurs in the M2, M4, and M5 sequences (Ridley, Carters, Bigby-Cannon).
Shallow subtidal environments consist of relatively calm and protected shallow-water (less than 20 m water depths) settings characterized by pervasive bioturbation and relatively minor influence of waves and tidal currents.
Dark-Colored Fine Grainstone
Skeletal grainstones, dark gray to dark bluish-black, fine-grained, phosphatic. Locally contains whole brachiopod valves. Mudstones are fissile, dark gray to black. Grainstones are medium-bedded and separated by partings or very thin beds of shale. Slightly bioturbated (ii2). Lacks obvious sedimentary structures such as cross-bedding commonly seen in other grainstone facies. Occurs in the M6 and C1 sequences (Catheys).
Peloidal grainstone, locally skeletal in part, locally packstone to poorly washed grainstone. Peloids large, up to 1 mm long. Common Girvanella and uncommon Solenopora. Some dolomitic mottling. Oolitic in some places. Medium- to thick-bedded. Burrow mottling (ii3–4) increases upwards within parasequence. Uncommon planar laminae. Occurs in the M2, M3, and M4 sequences (Pierce, Ridley, Lebanon, Carters).
Bioturbated Wackestone to Packstone
Skeletal to peloidal packstone and wackestone. Most fossils finely fragmented, but some layers with abundant coarsely broken to whole brachiopods, bryozoans, gastropods, and trilobites. Grainstones are intraclastic and skeletal. Thick-bedded. Grainstones are thin but laterally continuous. Heavily bioturbated (ii4-ii5), with abundant dolomitic burrow mottling. Occurs in the M1, M2, M3, and M4 sequences (Murfreesboro, Pierce, Ridley, Lebanon, Carters).
Silty Bioturbated Packstone
Skeletal packstones with abundant quartz silt and clay. Many whole or coarsely broken fossils of brachiopods, bryozoans, gastropods, nautiloids. Grades into Nodular Packstone-Shale facies. Thick- to very thick-bedded; individual depositional units obscured by intense bioturbation (ii5 to ii6). Sedimentary structures and discrete trace fossils rare to absent. Occurs in the C3 and C4 sequences (Leipers, Arnheim).
Skeletal packstones with coarsely broken and whole fossils of brachiopods, bryozoans, gastropods, and nautiloids. Shales and packstones may contain abundant quartz silt. Grades into Silty Bioturbated Packstone facies. Packstones are nodular-bedded to very thin-bedded and discontinuous; shales are very thin-bedded. Some beds of skeletal grainstone and packstone are very thin- to thin-bedded and laterally continuous. Occurs in the M5, M6, C1, C2, C3, and C4 sequences (Bigby-Cannon, Catheys, Leipers, Arnheim).
Skeletal packstones to poorly washed grainstones. Skeletal; fossils usually coarsely broken or whole. Girvanella locally common. Thin- to medium-bedded packstones separated by laminae or very thin beds of silty shale. Tabular bedding; sedimentary structures uncommon. Occurs in the M6, C1, and C5 sequences (Catheys, Inman, Fernvale).
Undulatory Wackestone to Packstone
Wackestones and packstones are skeletal. Commonly finely dolomitized. Color ranges from medium gray to dark black; tan where dolomitized. Fossil fragments finely broken. Stromatoporoids common. Thin- to medium-bedded wackestones separated by very thin shale beds to partings. Upper and lower surfaces of wackestones are commonly undulatory, but may be planar. Intense bioturbation within wackestones (ii3-ii5). Vugs (2–10 cm) filled with sparry calcite locally common. Occurs in the M5 and M6 sequences (Bigby-Cannon, Catheys).
Sand-shoal environments in the Nashville Dome consist of shoreface environments and tidal bar belts. Shoreface environments flank beaches and are developed where waves break on a daily basis. Tidal bar belts are found along the edges of embayments (such as the western side of the Nashville Dome in the M5-M6 sequences), where tidal currents are vigorous. Some occurrences of sand-shoal facies in the Nashville Ordovician may represent other environments.
Crinoidal grainstone. Variably phosphatic; some laminae may be composed entirely of phosphatic peloids and phosphatic internal molds of mollusks, bryozoan zooecia, and echinoderm stereom. Locally with lags of ramose to massive bryozoans, or with discontinuous horizons of large colonial rugosans, colonial tabulates, or stromatoporoids. At some localities, grainstone composed of numerous unbroken high-spired gastropods and echinoderm debris. Solenopora locally present. Uncommon to locally abundant shale intraclasts. Grain size varies widely within facies from fine to very coarse, but individual beds are well to very well sorted. Numerous microfacies types at any given outcrop; microfacies distinguished chiefly on cross-bedding, phosphate, grain size, and fossils. Medium- to thick-bedded. Abundant large-scale trough cross-bedding; large-scale troughs locally have clay drapes. Abundant thin pyritic hardgrounds. Rare small-scale ripple lamination. Occurs in the M3, M4, M5, M6, C2, C3, and C5 sequences (Lebanon, Carters, Hermitage, Bigby-Cannon, Catheys, Leipers, Fernvale).
Deep subtidal lithofacies
Deep subtidal environments are present in areas below normal wave base, yet above the wave-base of typical storms. As a result, deep subtidal lithofacies contain numerous storm beds that reflect the alternation of storm and background conditions.
Calcisiltites with varying concentrations of very fine quartz sand to silt. Mudstones also contain varying concentrations of quartz silt. Generally unfossiliferous, although some calcisiltites may rarely have lags of brachiopods and bivalves at their bases. Thin- to medium-bedded calcisiltites are interbedded with thin-bedded silty mudstones in varying but roughly equal proportions. Calcisiltites have erosional lower surfaces with tool marks; tops are burrowed; beds display abundant planar laminae, small-scale hummocky cross-stratification, and megaripples. Tool marks common, gutter casts also present. Ball and pillow locally common, but discontinuous across an outcrop. Diplocraterion locally present; strong bioturbation at some localities (ii4). Calcisiltites fine upwards into overlying silty mudstones. Occurs in the M5 sequence (Hermitage).
Packstones are skeletal, with abundant whole to fragmented brachiopods, bryozoans, gastropods, trilobites and echinoderms. Skeletal grainstones composed of both whole and broken fossils locally common. Calcisiltites generally quartz-poor. Shales dolomitic. Packstones, calcisiltites, and shales are all interbedded and very thin- to thin-bedded. All three lithologies are laterally discontinuous, but where packstones dominate, beds may be relatively continuous. Calcisiltites with common planar laminae, small-scale ripple lamination, small-scale hummocky cross-lamination, and common to abundant burrows (Chondrites) filled with dolomitic shale. Bioturbation variable (ii2 to ii4), but increases up-cycle and obscures bedding. Abundant hardgrounds. Girvanella locally. Occurs in the M2, M3, M4, C1 and C4 sequences (Pierce, Lebanon, Carters, Catheys, Arnheim).
Offshore environments are the deepest-water facies present on the Nashville Dome and reflect settings only rarely disturbed by the strongest storms. As a result, these facies are fine-grained and contain only rare and thin sandy beds deposited during storms.
Mudstone, dolomitic and commonly silt-poor. Calcisiltites are rare and contain quartz silt. Mudstones are medium-bedded; calcisiltites to skeletal packstones are thin to very thin-bedded. Calcisiltites may display small-scale hummocky cross-stratification. Occurs in the M6 and C1 sequences (Catheys).
Greenish to gray dolomitic mudstone. Medium- to thick-bedded; often interbedded with cross-bedded crinoidal grainstone facies containing intraclasts of green to gray dolomitic mudstone. Occurs in the M6 and C5 sequences (Catheys, Sequatchie).