BAB V BATUAN SEDIMEN KARBONAT
V.7. FAMILI BATUGAMPING
Ada tiga tipe famili batugamping, yaitu: 1. Sparry allochemical rocks/mud-free allochems
Batugamping tipe ini merupakan batugamping yang tersaring dan identik dengan konglomerat dan batupasir yang well rounded dan pada umumnya terbentuk pada kondisi pengendapan yang dipengaruhi oleh arus yang mempunyai tenaga yang penuh. Daerah pengendapanseperti itu misalnya daerah pantai, bar ataupun daerah submarin yang dangkal.
Tapi biarpun demikian dapat juga sparry allochemical rocks terbentuk pada lingkungan dengan arus yang lebih lemah.
2. Microcrystalline allochemical rocks
Batugamping tipe ini identik dengan batupasir lempungan ataupun konglomerat dan terbentuk pada lingkungan pengendapan yang dipengaruhi oleh arus yang tidak begitu kuat dan begitu cepat.
Batugamping tipe ini identik dengan batulempung dan terbentuk pada lingkungan yang tidak dipengaruhi oleh arus yang kuat.
Daerah pengendapannya pada laut amat dangkal, dengan laguna yang terlindunglereng yang landai dan terendam serta mempunyai tingkat kedalaman yang sedang. Disamping pada daerah-daerah tersebut diatas
Microcrystalline rocks dapat juga terbentuk di dalam daerah lepas pantai yang lebih dalam dari daerah-daerah diatas.
Dari semua partikel alkimia, intraklast adalah paling penting karena terbentuk di air dangkal, dibawah garis gelombang, atau mencirikan kemungkinan adanya pengangkatan tektonik.
Akan tetapi tidaklah dapat dipungkiri bahwa satuhal dapat terjadi diantara banyak kemungkinan yang merupakan suatu kelainan. Kelainan-kelainan tersebut misalnya, mikrit dapat terbentuk di dalam zone energi yang tinggi jika lumpur karbonat tersebut terperangkap oleh algae yang kotor (penuh lumpur) dan diangkut dengan keras oleh gelombang.
Sedangkan sparit mungkin saja terjadi pada suatu lingkungan air yang tenang apabila disitu terjadi suatu akumulasi fragmen-fragmen fossil, dan zat kimia yang terdapat pada lingkungan tersebut tidak bercampur dengan lumpur karbonat. Sparit tersebut dapat terbentuk oleh pretipitasi kimiawi ataupun oleh peristiwa abrasi dalam lingkungan yang tenang tersebut.
Mikrit atau diamikrit adalah analog dengan lempung/serpih yang terbentuk di tengah-tengah dari sebagian besar laguna ataupun terentuk di dalam air laut lepas pantai.
Batuan yang tersaring dari lumpur karbonat ataupun tersaring dari alokimia merupakan transisi biomikrit ke biosparit dan identik dengan immature
Batuan tersebut dapat terbentuk apabila gelombang atau arus tidak begitu kuat. Bila kegiatan arus tersebut berlangsung dengan sporadis maka semua mikrit tidak akan dapat dikikis ataupun diangkut.
Biosparite, intrasparite dan sebagainya adalah identik dengan super mature
sandstone.
Satu hal yang dipandang penting di dalam pembagian lingkungan pengendapan batugamping adalah adanya matriks lumpur gampingan dan semen sparry calsite yang diakibatkan oleh adanya pembagian antara kegiatan gelombang dan arus. Arus turbulen akan mempercepat proses pencucian lumpur gampingan dan lumpur gampingan tersebut kemudian bercampur satu sama lain hingga menjadi suatu suspensi lumpur karbonat. Suspensi lumpur karbonat tersebut kemudian diangkut ke dalam zone energi rendah. Proses tersebut merupakan garis pemisah antara tingkat mature dan sub mature dalam batupasir dan antara mikrit dan sparit dalam klasifikasi pertama Folk (1959).
Derajat sortasi/pemilahan
Derajat sortasi untuk pertama kalinya ditulis oleh Dunham, R.J. dan seperti halnya dalam batupasir derajat sortasi dalam batugamping merupakan fungsi dari mean grain size.
Sebagai contoh, bila semua material alokimia terdiri dari fossil, sehingga hanya mempuyai satu sifat saja, maka sortasinya akan bagus. Derajat sortasi tersebut tetap bagus walaupun pengaruh arus kuat, karena ukuran dari binatang-binatang tersebut tidak dapat dipisah-pisahkan satu sama lain dalam arti kata lain mempunyai ukuran yang mendekati seragam.
Penyaringan, pemilahan dan pembundaran dalam karbonat
Penyaringan dari matriks lumpur karbonat terjadi pada tingkat energi yang rendah karena lumpur karbonat mempunyai diameter yang begitu sangat halusnya dan mempunyai sifat mudah diangkut atau dipindahkan ke tempat lain. Batuan yang yang di dalam proses pembentukkannya tidak mengalami penyaringan (winnowing) akan tercirikan oleh melimpahnya kandungan lumpur karbonat (seperti biomikrit), pada umumnya mempunyai indikasi diendapkan pada lingkungan dengan energi yang rendah.
A B C Gambar V. 1. Allochemical Limestones
a. Foraminiferal biomicrite (Eocene), Italy. Diam. 3 mm. Abundant foraminifers in a matrix of microcrystalline calcite (stippled). Orbitoids predominate, but a variety of other forms is included.
b. Gastropod biomicrite (Miocene), Ulm, Germany. Diam. 3 mm. Fresh-water limestone containing abundant whole and broken Planorbis shells. Matrixes turbid microcrystalline calcite (dark stippling) containing patches of clear coarser calcite. Larger shells were partly filled with carbonate mud at the time of deposition. Voids remaining within shells, and also cavities under shell fragments, were later filled with coarser spar as a result of authigenic precipitation. The filling within several shells is an example of geopetal structure; contact between microcrystalline calcite and sparry calcite within shells is the bedding surface and is shown right side up.
c. Trilobite sparite (Silurian), Asker, Norway. Diam. 3 mm. Very abundant carapaces of the trilobite Olenus enclosed in sparry calcite cement in which crudely columnar crystals stand approximately normal to the shell surfaces.
A B C
Gambar V. 2. Allochemical Limestones
A. Biomicrite, Twin Creek Limestone (Jurassic), near Jackson, Wyoming. Diarn. 2.7 mm. Poorly sorted, ragged organic fragments enclosed in a matrix of calcite mud (stippled). Most larger fragments are fibrous calcite and may be bits of brachiopod or of certain molluscan shells; two coarse calcite fragments are bits of echinoids. Ragged, disoriented character of the organic fragments suggests bioturbation.
B. Crinoidal limestone, Trenton Limestone (Ordovician). Trenton Falls, New York. Diam. 3 mm. Medium-grained limestone composed of tightly interlocking crinoid fragments. Pressure solution along grain boundaries has produced microstylolites between the grains. One phosphate shell fragment in lower part of diagram. '
C. Cephalopod biomicrite (Silurian), Chuohle, Bohemia. Diam. 4 mm. Casts of the nautiloid cephalopod Orthoceras (circular cross-sections) composed of medium-grained sparry calcite are embedded in a matrix of microcrystalline calcite and small shell fragments. Absence of any trace of shell in the large casts suggests that the original shells were removed by solution and the resulting molds later filled with calcite spar,
A B C
Gambar V. 3. Oolitic Limestones
A. Pleistocene ooids. Great Salt Lake, Utah. Diam. 3 mm. Ooids consist of sub-angular detrital quartz grains enclosed by aragonite having both concentric and radial fibrous structure. Incipient cement.
B. Oomicrite, Volksen, Deister Mountains, Germany. Diam. 3 mrp. Loosely packed ooids consist of nuclei encased by microcrystalline calcite (dark stippling); nuclei are shell fragments, some of which have been recrystallized to calcite mosaics. Ooids occur in a micrite matrix that has been partially recrystallized; note patches of neomorphic microspar and fine-grained spar. The allochems are called ooids, because nuclei are visible and also because vague relics of concentric structure are visible in some (not illustrated); they have probably been micritized.
C. Composite ooids (Pleistocene), Pyramid Lake, Nevada. Diam. 6 mm. Large ooids consisting of microcrystalline (stippled) and radial fibrous (clear) concentric layers. Nuclei are fragments of broken ooids, clusters of tiny ooids (right and center), and bits of granular carbonate (lower right). Incipient cementation as in A.
A B C
Gambar V. 4. Oolitic Limestones
A. Oolitic biosparite (Jurassic), Bath, England. Diam. 2.5 mm. Radial fibrous calcite ooids (upper right), microgranular calcite pellets (heavily stippled, at bottom), and abraded shell fragments, all cemented with fine-grained calcite. Cement fabric consists of bladed calcite crystals rimming each carbonate fragment, with coarse calcite crystals (lightly stippled, near bottom) occupying the centers of original pores. Some shell fragments are original fibrous calcite; some are abraded single crystals, probably from echinoids (right and left); some are recrystallized granular calcite and were probably aragonite originally. Micrite envelopes on most allochems.
B. Recent ooids, coast of southern Florida. Diam. 2.5 mm. Dark microcrystalline ooids having distinct concentric structure. Nuclei are microcrystalline pellets; concentric carbonate is aragonite. Partly cemented with fine-grained calcite, which probably formed in the vadose environment. Remaining pores are blank.
C. Oosparite, St. Louis Limestone (Mississippian), Bowling Green, Kentucky. Diam. 2.5 mm. Ooids consisting of radial fibrous calcite, but with distinct concentric banding, tightly packed and firmly cemented by fine-grained clear calcite. Nuclei in ooids are mostly microcrystalline calcite pellets, but a few appear organic (right edge and lower right). Compare the looser packing in B.
A B C
Gambar V. 5..Dolomitized Limestones
A. Dolomitized Devonian coral. Bear River Range, northern Utah. Diam. 8 mm. Limestone matrix and septa of coral replaced by very fine-grained dolomite; coarser dolomite has filled in between septa in coral; dolomite euhedra near the center are enclosed in a single large calcite crystal.
B. Dolomitized crinoidal limestone (Silurian), Niagara River, \New York. Diam. 6 mm. Coarse calcite crystals (stippled) are remnants of crinoid plates and stem segments enclosed and marginally replaced by a fine-grained mosaic of subhedral dolomite crystals.
C. Dolomitized Devonian coral {Cyathophyllum}, Eifel, Germany. Diam. 3 mm. Coral structure cut longitudinally. Septa consist of cross-oriented prismatic dolomite; dolomite mosaic between septa is composed of interlocking larger anhedral grains, generally elongated parallel to septa.
A B C
Gambar V. 6. Dolomites
A. Lone Mountain Dolomite (Silurian), 3000 m below surface, near Eureka, Nevada. Diam. 2.5 mm. Mosaic of dolomite anhedra, not visibly different from some recrystallized calcite mosaics.
B. Glauconitic Bonneterre Dolomite (Cambrian), near St. Louis, Missouri. Diam. 2.5 mm. Inequigranular dolomite mosaic, with patches of microcrystalline glauconite between dolomite grains. Local ferric oxide (black), Compare pellet form of glauconite (stippled) in C. Relict ovoid in large dolomite grain at right may be organic. The rock contains some detrital quartz grains (not shown in this field) and is perhaps a dolomitized glauconitic calcarenite.
C. Sandy glauconitic dolomite (Cambrian, Sawatch Formation), Ute Pass, El 1'aso County, Colorado. Subrounded quartz grains and glauconite pellets Healing in a dolomite mosaic; probably a dolomitized calcarenite. Compare the non-porous mosaic of anhedral dolomite grains at the bottom with porous aggregate of dolomite rhombs in upper part of figure. Local ferric oxide stain (black).
A B C
Gambar V. 7. Cherts
A. Cherty portion of Madison Limestone (Mississippian), Bear River Range, northern Utah. Diam. 2.5 mm. Dolomite rhombohedra and detrital quartz sporadic grains (blank and irregular) set in a matrix of microcrystalline quartz. Chert bands like that in center parallel the bedding and alternate with others, like that at bottom, composed almost entirely of dolomite. Opaque lamina in dolomite is probably organic material. Secondary veinlet of chalcedony.
B. Foraminiferal chert (Upper Miocene, McLure Formation), Reef Ridge, California. Diam. 2 mm. In lower half, well-preserved calcite tests, infilled partly with coarse calcite (two cleavages) and partly with chalcedony (blank), are set in a matrix of opal (stippled). In upper half, matrix is clear chalcedony (blank), and calcite tests (without distinct outlines) have been largely replaced by chalcedony.
C. Chert in Helderberg Limestone (Devonian), Genesee County, New York. Diam. 2.5 mm. An irregular patch of uniformly oriented calcite (dark stippling plus cleavage) is enclosed and seemingly replaced by microcrystalline quartz (light stippling). Dolomite euhedra, some of which are zoned, are scattered through both chert and calcite.
A B C
Gambar V. 8. Ironstones
A. Frodingham Ironstone (Lias), Scunthrope, Lincolnshire, England. Diam. 2 mm. Ovoid limonite ooids in a shelly limestone. Ooids are brown, concentrically banded, and translucent in thin section. The matrix is finely granular calcite, containing a variety of abraded shell fragments, some of which are granular and some fibrous. Cavities in three shell fragments (center and lower part) are filled with green chamosite (stippled).
B. Northampton Sand Ironstone (Lias), Corby, Northamptonshire, England. Diam. 2 mm. Sideritic limestone containing numerous chamosite ooids (stippled lightly) and also shell fragments and grains of detrital quartz (blank). One ooid has quartz nucleus. An abraded phosphate shell fragment (stippled) in lower center, two fibrous shell fragments marginally replaced by siderite.
C. Northampton Sand Ironstone (Lias), Irthlingborough, Northamptonshire, England. Diam. 2 mm. Chamosite ooids in a matrix of chamosite mud. Both matrix and ooids partly replaced by patches of granular siderite.