PETROLOGY AND GEOCHEMISTRY OF MIDDLE ORDOVICIAN ...

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The petrological, mineralogical and geochemical investigation of two coeval Lower Middle Ordovician (upper Chazyan to Blackriveran) carbonate sections ...
PETROLOGY AND GEOCHEMISTRY OF MIDDLE ORDOVICIAN FOREDEEP AND PLATFORM CARBONATES AT TWO LOCATIONS IN THE APPALACHIAN BASIN

by Heikki Bauert

A Thesis submitted to the faculty ofthe University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Master of Science in the Department of Geology

Chapel Hill

1994

Approved by:

HE I K K I B AU E R T . Petrology and Geochemistry of Middle Ordovician Foredeep and Platform Carbonates at Two Locations in the Appalachian Basin (Under the direction of John M. Dennison).

ABSTRACT

The petrological, mineralogical and geochemical investigation of two coeval Lower Middle Ordovician (upper Chazyan to Blackriveran) carbonate sections, deposited in different depositional environments, record substantial differences in the studied carbonates. The core section of well ACK-2, central Kentucky, represents supratidal to shallow subtidal carbonate deposition (Wells Creek to Tyrone Formations) throughout the studied time span in the peripheral bulge area of the Cincinnati arch. These calcilutites are dolomitized to various degrees and contain little siliciclastic (quartz, feldspars) admixture. The textural characteristics permit distinction of early- and late-diagenetic dolomites. Both dolomites contain near-stoichiometric to calcian dolomites (up to 6 mole% CaC03). Very low Sr (mostly "·/! 1.5 to 2 2 to 3

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Fig. 4. Disrupted white, late-diagenetic, calcite veins which cut the Lincolnshire Limestone, indicate the influence of tectonic stresses. Tumbling Run roadcut, Virginia.

signatures of carbonates, no effort was made to geochemically analyze samples from the Tumbling Run roadcut section.

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METHODS Sampling The Tumbling Run roadcut section is located about 2.5 km southwest of Strasburg (at the junction ofUS Highway 11 and State Road 601), Shenandoah County, northwest Virginia. It was chosen as a study section because of its continuous exposure. No other such quality exposure of Middle Ordovician rocks is known in northern Virginia. Also, this exposure site has been mentioned in several papers and guidebooks (Cooper and Cooper, 1946; Cooper, 1955; Fichter and Diecchio, 1986; Mussman and others, 1992). The lithostratigraphical nomenclature used herein is from the Strasburg quadrangle geologic map (scale 1:24,000) and report by Rader and Biggs (1976). No mineralogical or geochemical investigations on Middle Ordovician carbonates have been reported from this site. Bulk samples were taken when different succeeding lithologies were recognized in the outcrop. Within thicker lithostratigraphic units a roughly uniform sampling interval was used. Field description of sedimentary structures in the Tumbling Run roadcut was much obscured by dark gray to black color oflimestones. Altogether, 28 samples were collected, with the sample spacings shown along the stratigraphic column (see Fig. 26). The core from the well ACK-2, drilled by the American Smelting and Refining Co. (ASARCO) on the Jessamine Dome in central Kentucky (Fayette County, Coletown quadrangle), was obtained from the Kentucky Geological Survey. The core is stored in the Lexington core repository under call number 190. Because of the small core diameter(- 5 em), only 5 to 8 em long and about 1 em thick core slices were permitted for further studies. The sample spacing depended on thicknesses ofindividuallithologic units and ranged from 0.4 m to 4 m. Altogether, 70 samples were collected from the core, with the sample spacings shown along the stratigraphic column (see Figs. 23 through 32).

Thin Sections Thin sections were made from all collected samples. In order to facilitate dolomite and ferroan calcite distinction in thin sections, about 2/3 of each thin section was stained with both Alizarin-RedS and potassium ferricyanide using 9

the method of Lindholm and Finkelman (1972). For mineralogical and geochemical analyses, a strip of rock was trimmed off the unweathered part of the lithologically most representative and uniform part of the slab using a diamond trim-saw. Then the trimmed strips were cleaned and crushed in a porcelain-bladed jaw crusher and reduced to powder in a tungsten carbide ball mill.

Carbonate Carbon and Organic Carbon Analysis The carbonate and organic carbon contents were determined on a UIC Inc. model5011 C02 coulometer fitted with total carbon analysis system. Total carbon is measured through combustion, and carbonate (inorganic) carbon is measured by coulometric titration of acid-evolved C02 (Jackson and Roof, 1992). Organic carbon is calculated as the difference between the total and carbonate carbon contents. Knowing carbonate carbon and organic carbon contents allows one to calculate the contents of carbonates and siliciclastics in studied samples. Reagent grade calcium carbonate (FLUKA) was used as a standard. X-ray Diffraction Analysis X-ray diffraction analyses (XRD) using a Phillips Cu-a X-ray Diffraction Unit were carried out in two steps. At first a back-filled powder mount was scanned for major minerals at speed of 1° 28/min. The second step involved scanning of samples for d(104) values of dolomite and calcite. The exact d(104) value was determined relative to that of internal sodium chloride standard (Blatt and others, 1972). A homogenous, very fine-grained powder was mixed with 15 wt% NaCl and scanned in a 28 range from 29.00 to 320 at 0.50/min. The relative proportion of calcite to dolomite in individual samples was derived by taking the ratio of the heights of d(104) calcite peak and d(104) dolomite peak, where dolomite % = (dolomite/calcite+ dolomite) x 100 (Sperber et al., 1984). This approach has an estimated error of± 5% dolomite. Dolomite nonstoichiometry, expressed as the mole% of CaC03 in the dolomite lattice was calculated following the formula given by Lumsden and Chimahusky (1980): NCaC03 = Md + B where NCaC03 is the mole percent CaC03 in the dolomite lattice, dis the observed d-spacing in Angstroms, M and B are constants (M = 333.33; B = -911.99). 10

Chave (1952) reported a linear relationship between the position of the d104 diffraction peak of skeletal calcites and their Mg concentrations (as determined by wet chemical analysis) over the range 0-18 mole% MgC03. He concluded "that Mg was replacing Ca in the calcite lattice, shrinking it, and forming a solid solution between calcite and dolomite". Therefore, by measuring the shift of the calcite peak (d104 = 3.035 A; Joint Committee on Powder Diffraction Standards, 1970) toward that of dolomite (d104

= 2.886 A; Howie

and Broadhurst, 1958), one can determine the amount of Mg in calcite (for review, see Milliman 1974). It is known that up to 3 mole% FeC03 is necessary to produce a measurable change in dolomite cell size (Goldsmith and Graf, 1958). Because all studied samples contain less than 1% FeO, as they did not react with potassium ferricyanide (Lindholm and Finkelman, 1972) and which was confirmed by inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis, Fe can not be considered responsible for dolomite nonstoichiometry. Therefore, the degree of nonstoichiometry can be given in terms of mole % CaC03.

ICP-AES Analysis In most cases, aliquots of powdered samples were used for inductively coupled plasma atomic emissiqn spectrometry analyses ofFe, Mn, Mg, Sr, Na. However, in case of dolomite-mottled limestone, a dental drill was used to obtain separate samples from both calcite matrix and .dolomite lenses. ICPAES analyses were done at Svensk GrundamnesAnalys AB laboratory in Sweden. For analyses, 0.1 to 0.5 g of sample was leached in 1M HCl. Oxygen and Carbon Isotopic Analyses For isotopic analyses, samples were milled using a dental mill. Care was taken to sample only homogenous matrix and to avoid any areas containing birdseye-like occlusions as well as later-diagenetic calcitic fracture fills. Numerous analyses of recent dolomites indicate enrichment of at least 2 to 3 %o in 18o relative to calcite (for review, see Land, 1980). Because most of Middle Ordovician carbonates in the ACK-2 core are dolomitized limestones with very variable dolomite content, the results of direct oxygen and carbon isotopic analyses are hard to interpret. Therefore, samples containing between 10-90% of dolomite were leached with Di Na EDTA, which selectively removed calcite (Fig. 5; for review of method, see Glover, 1961). This leaching has been 11

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Fig. 5. X-ray diffractograms of sample ACK-2 - 98.2 m. A- original powdered sample; B - after treating with EDTA which selectively removed most calcite. shown not to affect the original isotopic composition of dolomite (Videtich, 1981). Approximately 20 mg of each sample was dissolved at 100° C in 100% H3P04. The C02 gas was collected for 15 minutes. Isotopic analyses were done on a Finnigan MAT DELTA-E stable isotope ratio mass spectrometer at the Institute of Geology, Estonia. All results are reported relative to PDB standard in %o. Reproducibility of results is± 0.1 %o for 818o and± 0.1 %o for 813c, based on replicate analyses of an internal standard TLN- C1(Institute of Geology) which is calibrated relative to NBS-18 and NBS-19 standards. The specific isotope compositions for oxygen and carbon are expressed in 8-values (given in per mil), which are stated as: 8 = [(RsamplefRstandard) - 1]

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where R is the ratio of the heavy to light isotopes (Veizer, 1992; p. 4).

12

Strontium Isotope Analysis Strontium isotope analysis was done on the subsamples used for oxygen and carbon isotope analysis. To minimize possible contamination from siliciclastic material, samples containing less than 15% of siliciclastics were selected (XRD analysis and thin section inspection revealed that clastic material was overwhelmingly dominated by quartz grains ). About 100 mg of sample, spiked with 84sr, was dissolved in 2x distilled 1M HCl. Mter complete dissolution of carbonate, Sr was separated in 2x distilled 5M HN03, using ionexchange columns filled with strontium-specific EIChrom Sr•Spec® resin. Total procedural blanks ranged from 100 to 150 pg Sr. Isotopic composition of Sr was measured on a VG Sector 54 mass spectrometer at the University of North Carolina at Chapel Hill in dynamic multicollector mode. Each analysis for Sr consisted of ten cycles of data per block, with ten blocks measured for each sample. All 87sr;86sr values were normalized to 86sr;88sr = 0.1194. The NBS SRM 987 standard had a 87Srf86sr value of0.710263 (n=2).

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STRATIGRAPHY and LITHOLOGY Middle Ordovician Carbonate Succession, Core ACK-2, Kentucky The examined core interval ranges from depth 55.2 m to 245.7 m and includes dolomites of the Wells Creek Formation, overlain by carbonates of the Camp Nelson, Oregon and Tyrone Formations of the High Bridge Group (Fig. 2). The lithostratigraphic subdivison of the studied interval was done according to suggestions by P.J. Gooding (Kentucky Geological Survey) and the stratigraphic nomenclature used herein follows that of Gooding (1992). Descriptions of dolomite rock textures are after Sibley and Gregg (1987). My core description of the studied interval is given in Appendix A.

Wells Creek Formation The 11.2 m thick Wells Creek Formation is composed mainly oflight yellowish gray to pale olive soft, friable dolomicrite. The upper and middle parts of this formation display rather distinctive grayish mottling. The unconformable contact between the Middle Ordovician Wells Creek Formation dolostone and the Lower Ordovician Mascot Formation dolostone of the Knox Group (Fig. 6) is placed at the base of an intradolorudite bed. This bed is 0. 7 m thick and comprises pebble-size subrounded dolomite intraclasts and chert nodules reworked from the Knox Group and embedded into a fine-grained planar-s dolomite matrix (Fig. 7). Siliciclastic material consists of subrounded to well-rounded silt to medium sand grains of quartz and feldspar (Fig. 8). Fine to medium silt-size quartz grains are common throughout the whole unit. A quartz sandstone bed (up to 10m thick) is reported to occur on the Knox erosion surface less than 15 km east of the well ACK-2 (Wolcott and others, 1972). Depos itional environment : The lowermost intradolorudite represents a

basal conglomerate, derived from the emerged seafloor during the Knox hiatus and subsequently buried by early Middle Ordovician marine transgression sediments. The remainder of Wells Creek dolostones reflects deposition under diminishing influence of clastics influx. Although dolomitized, the intermediate position relative to the subaerial Knox unconformity and lower Camp Nelson 14

Fig. 6(L). Massive dolomicrite (sample ACK-2: 244.8 m; Mascot Formation). Fig. 7(R). Dolomicrite with dolomite (gray) and chert (white) intraclasts (sample ACK-2: 242.7 m; Wells Creek Formation). Both samples are 3.5 em wide.

Fig. 8. Dolomicrite intraclast surrounded by silt- and sand-sized quartz grains in a dolomicrite matrix (sample ACK-2: 242.7 m; Wells Creek Formation). Horizontal field of view is 2.5 mm. Alizarine-RedS, XN.

15

intertidal dolomites, suggests similar peritidal depositional environment for Well Creek dolomicrites.

Camp Nelson Formation This formation is the oldest part of the High Bridge Group. The contact between the Wells Creek and Camp Nelson is poorly defined and has been placed at widely different levels by different people (Wolcott and others, 1972). In my investigation, the contact was drawn at the level where friable and silty dolomicrites of the Wells Creek Formation grade up into dense, thinly laminated dolostones (Figs. 9-10). The thickness ofthe Camp Nelson Formation in the studied section is 133 meters. It consists largely of dense, yellowish gray to dusky yellow, variably dolomitized calcilutites with light olive brown to dark yellowish brown dolostone stringers as well as greenish gray shaly stringers. Several of these beds are peloidal. Some beds (98 to 101m and 189 to 191m) contain higher concentrations of silt-sized quartz and feldspar grams. The uppermost and lower middle part contain abundant dolomite patches which weather preferentially to give a "honeycomb" appearance (Fig. 11) on exposed sections (Fisher, 1970). This kind of distinctive dolomite mottling has been usually interpreted as dolomite-filled burrows. It has been suggested that these burrows originally contained more permeable sediment, produced by the digging activity of organisms, than the surrounding sediment and thus acted as conduits for dolomitizing fluids leading to preferential dolomitization of burrow fillings (Cressman and Noger, 1976; Morrow, 1978). However, no central tubular cavities like those that are common in dolomitemottled Ordovician Yeo man and Red River Formations in southern Saskatchewan and Manitoba (Kendall, 1977), were seen in the studied sequence. Another type of mottling is characterized by the presence of dense gray mottles with very irregular shape in dolomitized calcilutites. These mottles can occur as individual blotches (Fig. 12), but they usually are interconnected forming a maze-like pattern. The origin of this kind mottling is unknown. It can be the result of bioturbation. However, it should be noted that no clearly outlined burrows are observed. Thin section studies indicate that fine-grained dolomite crystals are both planar-e and planar-s types. Euhedral dolomite rhombs mainly occur in a 16

Fig. 9(L). Laminated dolostone (sample ACK-2: 218.1 m). Fig. 10(R). Laminated dolostone (sample ACK-2: 226.6 m). Both samples are from the Camp Nelson Fm.

Fig. 11(L). "Honeycomb"-pattern due to tan dolomite in slightly dolomitized pale dusky yellow calcilutite (sample ACK-2: 175.8 m). Fig. 12(R). Mottled dolostone (sample ACK-2: 223.5 m). Both samples are from the Camp Nelson Formation. All samples are 3.5 em wide, except in Fig. 11, which is 2.9 em wide.

17

micrite matrix (Fig. 13), while tightly packed planar-s dolomite forms burrow fillings. Millimeter-scale dolomite stringers are rather common and contain well-developed planar-e dolomite rhombs. These dolomite stringers are often associated with fenestral fabric and birdseyes (Fig. 14). Partly silicified faunal remains are observed, but they are not common. The rock is often cut by short fractures (length - less than 5 em; width a:>a:>a:>OO

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containing less than 2.5 mole% MgC03. The dismicrites of the New Market Limestone (3.0349 to 3.0369 A) have a MgC03 content close to zero and are the purest limestones studied. Almost all studied dolostones in the Middle Ordovician carbonates in ACK-2 section consist ofnonstoichiometric dolomite. The dolomite dl04 values range from 2.8856 to 2.9048 A(49.9 to 56.3 mole% CaC03 in dolomite; Fig. 28). All dolostone units have near-stoichiometric values (Oregon: x= 51.8 mole % CaC03; Wells Creek: x= 51.6 mole% CaC03; Mascot: x= 50.9 mole% CaC03). Dolomicrites of the Wells Creek Formation have a similar CaC03 content to that of Oregon medium-grained dolomite, indicating that there is no apparent correlation between the dolomite crystal size and the stoichiometry. The dolomitized Tyrone and Camp Nelson limestones contained less ideal dolomite (Tyrone: x= 53.9 mole% CaC03; Camp Nelson: x= 52.7 mole% CaC03). A very low to negligible dolomite content in the Middle Ordovician carbonates at the Tumbling Run section did not permit determination of the CaC03 molar percentage of these dolomites.

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GEOCHEMISTRY Minor and Trace Elements Theoretical Concepts Veizer in his comprehensive reviews (1983a, b) has envisioned the following understanding about the chemical stabilization of carbonates. Aragonite and high-Mg calcite (> 4 mole% MgC03) are the major carbonate phases deposited in shallow water subtropic and tropic zones at present. LowMg calcite (0-4 mole% MgC03) is usually a minor component, and dolomites form only in specific environments. Both aragonite and high-Mg calcite are thermodynamically unstable and proceed through a stabilization process that ultimately produces sediments composed oflow-Mg calcite and dolomite. In the diagenetic environment, stabilization of an original metastable carbonate assemblage is achieved through complementary textural, mineralogical and chemical changes. The original carbonate phases (aragonite and high-Mg calcite), precipitated in equilibrium with seawater, incorporate minor and trace elements as well as stable isotopes from the ambient seawater. These phases, upon exposure to meteoric water, will dissolve partially or fully, exchange and mix its trace elements and stable isotopes with those in the interstitial water, and reprecipitate as diagenetic low-Mg calcite. The low-Mg calcite will, therefore, have a trace element and isotopic composition shifted in the direction of equilibrium with the interstitial meteoric water. Since such water, in general, contains less Sr, Na and Mg (and lighter o18o and o13c) and more Mn and Fe than seawater (Drever, 1982), this process should lead to a decrease ofSr and Na and an increase in Mn, Fe (Brand and Veizer, 1980) in a transformed carbonate. The concentration of Mg will either be lowered or increased, depending on what was the original carbonate phase. A high water/rock ratio signifies an open diagenetic system and /or numerous dissolution-reprecipitation events. Consequently, the precipitated low magnesium calcite is in chemical equilibrium with the ambient diagenetic meteoric water. In contrast, a low water/rock ratio indicates a partly closed diagenetic system with a few dissolution-reprecipitation events. In a partly closed system, interstitial water is buffered by the host rock. Therefore, the precipitated low-Mg calcite derives much of its identity from the host 38

carbonate. The present consensus is that the mineralogical stabilization of the original carbonate phase into low-Mg calcite is usually accomplished under low to intermediate water/rock ratios, whereas the precipitation of pore cements requires an intermediate to high water/rock ratio (Veizer, 1983a).

Stratigraphic Variations Stratigraphic variations of Fe, Mn, Sr and Na in the ACK-2 core are given on Figs. 29-32. The raw ICP-AES data are included in Table 3. Manganese. The Mn content in the Mascot to Tyrone carbonate succession ranges from 16.9 to 270 ppm. Element versus sample depth plots (Fig. 33) show that Mn concentrations are lowest in Camp Nelson limestones (x= 43.4 ppm; Table 4), followed by slightly higher values in Mascot and Oregon dolomites ( x= 55.8 and 68.5 ppm, respectively). The uppermost section, starting in Tyrone, shows a gradual increase in Mn concentrations. The highest Mn value (1140 ppm) was obtained in the biointrasparrudite sample of Lexington age (55.3 m; Figs. 29; 33). Published average Mn values from lower Ordovician Upper Knox intertidal to subtidal dolomites in Virginia (Table 4) are close to those of studied Wells Creek dolomites, but are slightly elevated relative to Oregon and Mascot dolomites in the ACK-2 core. Shanmugam and Benedict III (1983) observed a marked increase in Mn content from shallow to deep marine lithofacies. The tidal flat carbonates from the Lower Middle Ordovician Lenoir Formation in Tennessee, yielded Mn values (76-141 ppm) similar to those measured in the ACK-2 core, while skeletal packstones and grainstones of the Whitesburg Formation that deposited in slope settings, had much higher Mn values (346-1321 ppm). Iron. Fe content in the studied sequence ranges from 913 ppm to 9000 ppm (Figs. 30; 33. Some Tyrone Limestone samples that are enriched in siliciclastics (at depth 62.3 and 73.9 m; Fig. 30), have very high Fe contents (9000 ppm and 5190 ppm, respectively). This implies that an additional Fe leaching from clay particles may have occurred during 1M HCl treatment of samples for ICP-AES analysis. The same may be partly the reason for elevated values in Wells Creek dolomites (x= 2870 ppm) compared to Mascot (x= 1150 ppm), Camp Nelson (x= 1019 ppm) and Oregon carbonates (x= 1361 ppm). Another possible explanation for higher Fe concentrations in Wells Creek and lowermost Camp Nelson dolomicrites includes meteoric water influence during diagenesis. Except for elevated Fe values in the Wells Creek 39

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Fig. 47. Histogram ofMn distributions in brachiopods and host limestones of Upper Ordovician age, Anticosti Island (Al-Aasm and Veizer, 1982) compared with Mn histogram of Middle Ordovician Wells Creek Fm. and High Bridge Group carbonates, core ACK-2, central Kentucky. Csw stands for calcite in equilibrium with seawater; CMW stands for calcite(= diagenetic low-Mg calcite) in equilibrium with average meteoric water. Samples bearing evidence of textural alteration and late-diagenetic dolomitization along stylolite sutures, are omitted.

63

dissolved carbon in modern seawater is about +1 ± 0.5%o, with surficial waters generally heavier and deep waters lighter than this average (Tan, 1988). The organic carbon is strongly depleted in 13c (since most warm-water phytoplankton has o13c values between -17%o and -22%o; Anderson and Arthur, 1983). This organic matter is easily oxidized in shallow-water depositional environments, causing a negative shift in o13c values. The oxygen isotopic composition of modern equatorial and temperate surface waters is relatively uniform with o18o around + 0.5%o SMOW (Marshall, 1992; SMOW stands for Standard Mean Ocean Water). This value varies slightly with changes in salinity (a decrease of0.11%o per 1%o decrease in salinity). The oxygen isotopic composition of a carbonate mineral which is precipitated in equilibrium with its environment is determined by the oxygen isotopic composition of the fluid from which the mineral precipitated and by the temperature of precipitation. Holocene dolomites from Baffin Bay, Texas and the Persian Gulf show that isotopic fractionation occurs during coprecipitation of calcite and dolomite, so that dolomite is enriched in 18Q relative to calcite at least 2 to 4%o (Land, 1980). Sr isotopes can provide useful information about the nature of the fluids from which the carbonates were precipitated, thus helping to constrain the possible mechanisms of diagenesis and the times deposition or diagenesis occurred. Analyses of least-altered marine carbonates indicate that the 87sr;86sr ratio of ocean waters has varied systematically throughout Phanerozoic. Because of the long residence time of Sr in seawater (2-4 million years; Holland, 1984), that ratio has a constant value in the open ocean at any given time (Burke and others, 1982). In contrast to oxygen and carbon isotopes, 87 Sr and 86sr are incorporated into carbonate minerals with no measurable isotope fractionation (Veizer, 1992). For this reason, the Sr isotopic compositions of marine carbonates are assumed to be identical to those at the time of deposition. The Sr isotopic compositions of dolomites reflects that of fluids from which they were precipitated initially or subsequently altered (Mazzullo, 1992). In those cases where marine fluids are involved in these processes, the timing of single or multiple episodes of dolomitization often can be inferred from the 87sr and 86sr ratios in the dolomites upon comparison of data with 87 Sr/ 86Sr seawater curve, prepared by Burke and others (1982).

64

Many shallow-water limestones do not preserve a simple geochemical record of their depositional environment because the sediments are exposed to meteoric water before they have reached mineralogical stability. Carbon isotopic values are less prone to alteration during diagenesis than oxygen values, but shifts can be significant where organogenic carbon is incorporated. Because of the relative insolubility of C02, it is difficult to alter the carbon isotopic composition of carbonate sediment from its initial value (Land, 1980). Therefore, carbonates that did not contain much organic matter initially, have retained its carbon isotopic value even during early-diagenetic dolomitization. Meteoric calcite cements and replacive calcite have negative oxygen isotopic signatures which are dominantly controlled by the isotopic composition of the water. In areas of intense evaporation, however, the oxygen isotopic composition of pore waters and cements will be enriched in 18o in the local evaporation process (Marshall, 1992). The oxygen added to the water from unstable carbonates during stabilization, except at very low water-rock ratios, is very limited. Therefore, the isotopic composition of carbonates precipitated in meteoric water, reflects by and large the isotopic composition of the water and the temperature of precipitation. The trend of decreasing ()18o of marine carbonates with increasing age has been well documented (Veizer and Hoefs, 1976; Veizer and others, 1986; Wadleigh and Veizer, 1992). Similar trends have also been observed for cherts and phosphorites (Knauth and Epstein, 1976; Karhu and Epstein, 1986). However, the cause of this variation is still the subject of controversy. Secular variations in o13c (Veizer and others, 1980; Veizer and others, 1986) and 87sr;86sr (Veizer and Compston, 1974; Burke and others, 1982) in carbonates are also recognized in the Phanerozoic oceans. The 87sr;86sr secular changes in the Lower Paleozoic are better explained than o18o variations and these secular changes are interpreted as resulting from shifts in balance between oceanic crust-seawater interaction and surficial and groundwater runoff into oceans (Veizer, 1989).

Stratigraphic Variations in Isotopic Compositions Variations in o18o and o13c values along the log of well ACK-2 are shown on Figs. 48-49. o13c values range from -3.06 to +1.66%o (Table 5), and display a gradual increase in heavier 13c isotopic compositions upward in the section. No negative o13c values are recorded in the Oregon and Tyrone 65

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