L O S S O F P R O V E N A N C E I N F O R M A T I O N T H R O U G H S U B S U R F A C E D I A G E N E S I S I N P L I O - P L E I S T O C E N E S A N D S T O N E S , N O R T H E R N G U L F O F M E X I C O I
KITTY LOU MILLIKEN Department of Geological Sciences
University of Texas Austin, Texas 78713
ABSTRACT: Subsurface alteration of plagioclase, potassium feldspar, and a variety of nonopaque heavy minerals has substantially modified the detrital composition of Plio-Pleistocene sediments in the northern Gulf of Mexico. Albitization of plagioclase and dissolution of unstable grains are the two principal mechanisms of grain alteration. Grain alteration increases progressively with depth and has taken place without the influence of significant amounts of meteoric fluids. The degree to which strictly subsurface grain alteration has affected these young sediments suggests that grain assemblages in older rocks should be viewed most skeptically with regard to their state of preservation.
INTRODUCTION
Fundamental to petrographic inference of provenance is the assumption that between deposition and meta- morphism, detrital assemblages largely retain their pri- mary texture and composition. Whether or not this is so has been argued before, notably by Pettijohn and Kry- nine, who, in the 1940s, espoused divergent views on the importance of intrastratal solution versus tectonic con- trols on heavy mineral composition. Convincing evidence was put forth on both sides of this question, and the debate ended without substantial resolution.
In recent years, subsurface alteration of detrital feld- spars, principally through leaching and albitization, has been widely recognized (e.g., McBride 1977; Schmidt and MacDonald 1979; Land and Milliken 1981; Boles 1982; Fisher and Land 1986). Subsurface destruction of heavy minerals has likewise been clearly documented (Gazzi 1965; Morton 1984). The obvious significance of these grain alterations to provenance interpretations has been addressed several times (Blatt 1985; Morton 1985; Shan- mugam 1985; McBride 1985; Helmold 1985; Kugler and Milliken 1985), and McBride (1987) even coined the term diagenetic quartzarenite to emphasize the magnitude of the alteration which can occur.
In response to this growing body of evidence, much work has been done on varietal characteristics of the more stable minerals such as quartz, zircon, and tourmaline. But unstable heavy minerals, feldspars, and various QFL ratios have been, and continue to be, the foundation of many provenance studies.
How severe is the problem of subsurface grain altera- tion for interpretations based on the nature of detrital assemblages? Rapid burial rates, a low geothermal gra- dient, and shallow overpressuring make the Plio-Pleis- tocene sands in the Gulf of Mexico sedimentary basin a good test case to document the rate and extent of sub- surface grain alteration. It is difficult to envision burial conditions more amenable to preservation of detrital grains. These sediments indicate, however, that grain al- teration is indeed a major obstacle to traditional uses of sandstone compositional data. In less than 5 million years, without major recharge by meteoric water, and at tern-
Manuscript received 21 May 1987; revised 28 March 1988.
peratures less than 100°C, Plio-Pleistocene sands have undergone major alterations of the primary detfital as- semblage. Alterations include substantial loss of K-feld- spar, partial albitization of plagioclase, and reduction of a complex heavy mineral assemblage to zircon, tour- maline, and a minor amount of garnet (Milliken 1985). A more advanced stage of information loss is seen in the deeper parts of older Tertiary units along the Texas coast where albitization of plagioclase and K-feldspar removal are complete (Land and Milliken 1981; Boles 1982; Land 1984; Fisher and Land 1986). The comparatively incip- ient nature of feldspar alteration in the Plio-Pleistocene is especially significant because the remaining feldspars comprise a plausible detrital assemblage despite the sub- stantial degree of their alteration. Clearly, detrital assem- blages in older or hotter or hydrostatically pressured sand- stones are even more likely to have undergone alteration and should be viewed with great skepticism in terms of their degree of preservation.
CONDITIONS OF DIAGENESIS
Plio-Pleistocene sediments in the Gulf of Mexico rep- resent the youngest o f a series ofterrigenous elastic prisms that prograded the basin margin during the Cenozoic. Extremely high sedimentation rates in the principal de- pocenters have resulted in extensive growth faulting of Cenozoic units. As a result, it is possible to observe sed- iments of similar age and provenance over a wide range of depths, from the surface to depths in excess of 6 km. Deposition in the basin is ongoing, and it is reasonable to assume that present depths (and temperatures) are maximum ones, especially in the younger units. Rapid deposition has also resulted in extensive overpressufing, thus severely limiting the possible extent of freshwater influx. For these reasons, the Gulf of Mexico in general, and the Gul f s Plio-Pleistocene units in particular, pro- vide an excellent natural laboratory for the study of sub- surface grain alteration. For a summary of the Gulf's depositional, structural, hydrologic, and diagenetic fea- tures, see Sharp et at. (in press).
The geothermal gradient of the Louisiana-Texas shelf portion of the northern Gulf is typically in the range of 20°C/kin (A.A.P.G. Geothermal Gradient Map, Region 7, 1976). Bottom hole temperatures for wells used in this
GRAIN L O S S T H R O U G H S U B S U R F A C E D I A G E N E S I S 993
T ~ * ~ ~¢) r'7 Sru6y
0 50mi -F2Sa
80 km
• Locotlons Of well= tam~ld
FiG. 1.--Location map of wells used in this study. The major federal lease block areas of the shelf are indicated. The southern boundary is the approximate shelf edge.
study show linear variation with depth (C = 0.0204 x depth + 23.2; depth in meters, ~C _+ 7*; temperatures uncorrected). This equation yields a gradient of 20.4°C per kilometer.
SAMPLING AND METHODS OF STUDY
Samples for this study were obtained from 43 oil and gas wells on the Louisiana shelf (Fig. 1). Samples are mostly whole core, but they also include sidewall cores and cuttings. Subsurface depths for these samples range from 1,500 ft to 16,500 ft (450 m to 5,000 m). In general, sediments in the upper part of this depth range are un- lithified. In the range of l 1,000-13,000 ft (3,350-3,960 m), however, both sands and muds begin a transition to lithified materials; by 15,000 ft (4, 570 m), sediments have become lithified rocks.
Detrital grain assemblages were studied using a com- bination of several techniques. Seventy-seven thin sec- tions made with blue-dyed epoxy were point-counted with a polarizing microscope, accumulating a minimum of 200 grain counts per sample. Feldspars were stained using the technique of Houghton (1980). Individual grains and their alterations were further examined, in both whole rocks and in disaggregated >62-~m fractions, with the SEM.
Detailed information on feldspar compositions was ob- tained by point-counting 42 polished grain mounts of disaggregated >62-gm fractions with the electron micro- probe. A random selection of grains was obtained by traversing the sample in a spiral and making a quick inspection of the count rates for potassium, sodium, and calcium for each grain intersected by the cross-hair. Feld- spars were thus readily identified and then analyzed for 30 sec using combined wave-length and energy-dispersive analysis. Approximately 40 feldspar grains were analyzed in each sample.
Heavy minerals were separated from the > 125-um fraction of ten samples using standard heavy liquid sep- aration techniques. One hundred heavy mineral grains per sample were then point-counted using the SEM, ob- taining a random grain selection by a technique similar to that used for feldspars on the microprobe. Heavy min-
QUARTZ
Plio-Pleistocene ovo o 65 19 16 / " MISSISSIPPI RIVER SAND \
/ This studyo 65 17 18 \
FELDSPAR 50 ROCK FRAGMENTS FIG. Z--Quartz-feldspar rock-fragment proportions of Plio-Pleisto-
cene sands and Mississippi River sand. There is no systematic variation in QFR with depth; the wide scatter is attributable in part to the variable presence of syn-sedimentary mud clasts which were tabulated as rock fragments.
erals were identified by combining clues from grain mor- phology with elemental data from the energy dispersive analyzer.
COMPOSITION OF THE UNALTERED ASSEMBLAGE
The best approximation of primary detrital composi- tion for Plio-Pleistocene sand is modem sand from the Mississippi River. The position and orientation of the Neogene depocenters suggest that the pffncipal source of sediments in the northern Gulf has been essentially coin- cident with the present Mississippi drainage since the early Miocene (Winker 1981). Quartz-feldspar rock-frag- ment proportions for Plio-Pleistocene samples that were point-counted, and for a modern 'sand sample collected at South Pass on the Mississippi River delta, are shown in Figure 2. The composition of the river sand closely approximates the average for all samples and also match- es closely the composition reported by Russell (1937) in his classic study of Mississippi River sediments. Samples are primarily lithic arkoses and feldspathic litharenites.
Compositions of feldspars in Mississippi River sand and in Plio-Pleistocene sediments huffed less than 2 km are shown in Figure 3. The assemblage is slightly domi- nated by plagioclase with an average anorthite content of 24 mole percent. Most plagioclase is untwinned. Inherited feldspar alterations include minor vacuolization, sericit- ization, and minor secondary porosity such as that de- scribed by Passaretti and Eslinger (1987). Based on the presence or absence of twinning, K-feldspars are sub- equally divided between microcline and orthoclase. Elec- tron microprobe analysis confirms the presence of a mi- nor amount of Na-rich alkali feldspar.
994 K I T T Y L O U M I L L / K E N
AN
50 / i 50 ~ ....
o~ • 50
AB 50 OR
FIG. 3.--Compositional triangle for feldspar grains in Plio-Pleistocene samples buried less than 2 km and in Mississippi River sand. Values plotted represent mole percent of the three feldspar end members. Lines are drawn at 5 and 10 percent OR.
Nonopaque heavy minerals of the Mississippi River and in modern surface sediments of the Gulf have been well documented (e.g., Goldstein 1942; van Andel 1960; Davies and Moore 1970). The assemblage is dominated by green hornblende and pyroxenes. Other abundant heavy minerals are staurolite, kyanite, epidote, sphene, garnet, tourmaline, and zircon.
FIG. 4.--Percent of potassium feldspar in total feldspar (> 62-tAm fraction) versus depth, determined from point counts on the electron microprobe.
It.
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EXPLANATION o Above 3 3 0 0 m • Below 3 3 0 0 m
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GRAIN SIZE (¢1 FIG. L--Percent of potassium feldspar in total feldspar versus grain
size (4~ units) for samples above and below 3,300 m. The correlation coefficient refers only to the shallow samples. Dashed lines represent one standard deviation.
ALTERATION OF FELDSPARS
Potassium Feldspar
Potassium feldspar shows little petrographic evidence of alteration. Most K-feldspar grains appear quite fresh or only slightly vacuolized over the entire range of depths examined. Despite its slight alteration, below about 3,600 m K-feldspar is lost from the sands (Figs. 4, 5). Appar- ently the K-feldspar grains do not alter slowly over a wide depth range, but instead show little or no evidence of instability up until a time when they disappear "rapidly" and completely.
For samples shallower than 3,300 m, K-feldspar con- tent decreases with decreasing grain size (Fig. 5). Average grain size decreases with depth (Fig. 6) and thus, some portion of the depth-related decline in K-feldspar content is a function of grain size. Figure 5 also shows, however, that samples deeper than 3,300 m have less K-feldspar than shallower samples of equivalent grain size. Thus, loss of K-feldspar through some diagenetic process is con- vincing.
Evidence from both SEM and thin sections substan- tiates the observation that K-feldspar is subject to dis- solution and does not disappear through albitization or provenance effects (Fig. 7A, B).
The mechanism by which K-feldspar is removed from the sands appears to be primarily a "surface-controlled" process as described by Berner and Holdren (1979). This is evidenced by the strong tendency for dissolution to be localized along cleavage and/or other crystallographically
GRAIN LOSS THROUGH SUBSURFACE DIAGENESIS 995
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Flo. 6.--Grain size of Plio-Pleistocene samples versus depth. Tick marks on dots indicate multiple points.
controlled features (Fig. 7B). In a few of the deepest sam- pies, however, there is evidence that another mechanism, perhaps best described as "'transport-controlled," may also play a role (Fig. 7C). In these grains, K-feldspar dis- solution seems to be more random. In the SEM, grain surfaces appear karst-like; in thin section, grains are tra- versed by irregular canals. The appearance of this type of dissolution texture near the depth of complete K-feld- spar removal is most likely a reflection of the increasing instability of the detrital grains in this zone. Apparently, the tendency of K-feldspar to dissolve becomes sufficient- ly great that relative energy differences between different areas of the crystal lattice become negligible and the crys- tal dissolves randomly, at a rate controlled principally by transport of dissolved material away from the grain.
The volume of secondary porosity that might be ex- pected from wholesale removal of K-feldspar is not ob- served in these sediments. This is not surprising in light of the unconsolidated nature of the sediments. It should
FIG. 7.--Evidence of potassium feldspar dissolution. A) Secondary pore (p) with skeletal remnants of K-feldspar (indicated by arrows). Depth, 3,626 m. Plane-polarized light. B) Evidence of dissolution seen with the SEM. Depth, 569 m. C) Karst-like dissolution texture which characterizes some potassium feldspar grains in very deeply buried Plio- Pleistocene samples. Depth, 4,376 m.
also be noted that the amount of authigenic clay, prin- cipally kaolinite, observed within Plio-Pleistocene sands (< 1% by volume) does not nearly approach the amount of K-feldspar that has apparently dissolved (around 5%
996 K I T T Y LO U MILLIKEN
V
• 47
MISSISSIPPI RIVER SAND , \
32% K-SPAR in TOTAL FELDSPAR
%
PLEISTOCENE 1.5 km k OR
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PLIOCENE 4.Skin k
13% K-SPAR in TOTAL FELDSPAR k " = 1 4 6 '~ -
V 4,.,, V V V V %1111 • . . . .
FIG. 8. --Variation in potassium feldspar composition with depth. The n values represent the total number of analyses (K-feldspar + plagioclase) used to calculate the percent K-feldspar in total feldspar. Data are in mole percent.
of the rock volume). This implies that grain destruction in these sediments is accomplished by a rather open- system process, as similarly noted for older Gulf Coast units (e.g., Land 1984, p. 60) and in contrast to the limited aluminum mobility suggested by Stoessell and Thomson (1985).
Variation in K-feldspar composition also diminishes with depth (Fig. 8). The more sodic K-feldspar grains are not observed in the deeper samples. Land and Milliken (1981) observed a similar contraction in the range of K-feldspar compositions in Oligocene sandstones in Bra- zoria County, Texas. Maynard (1984) suggests that sodic K-feldspars are more readily lost from sediments during
P
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%ANo-AN 2 %AN30-ANio 0 FiG. 9.--Plagioclase composition versus depth. Each point represents
an average for a sample point-counted on the electron microprobe. Open circle is Mississippi River sand. The quantities given on the fight are averages for samples above and below 3,300 m. The statistical signif- icance of differences (Student's t-test) between shallow and deep samples is shown at left.
weathering and transport than other varieties, and this enhanced solubility holds for subsurface dissolution as well.
Albitization of at least a portion of the K-feldspar is possible but difficult to confirm in the absence of partially albitized K-feldspar grains.
Plagioclase
Microprobe analyses of plagioclase (Figs. 9, 10) also reveal depth-related changes. Calcium contents, averag- ing around An 25 in samples shallower than 3,300 m, are greater than in unalbitized feldspar in Eocene, Oligocene, and Miocene units of the Texas Gulf Coast (Land 1984; Fisher and Land 1986; Gold 1987). Below 3,300 m the lower An content reflects both loss of the more calcic plagioclase and the addition of substantial amounts of new albite. The distribution of points shown in Figure 10A could arise from many combinations of plagioclase dissolution and albitization of either plagioclase, K-feld- spar, or both. But loss of the more calcic plagioclase in samples below 3,300 m is certain. In Figure 10A, unal- tered plagioclase combined with albitized K-feldspar would show decreasing average anorthite content with no change in the anorthite content calculated excluding al- bite. Figure 10B shows that samples with less calcic pla- gioclase also contain more albite.
I f the loss of all plagioelase more calcic than An 30 is ascribed to dissolution, then the proportion of albite in the remaining plagioclase would increase by default, but only to about 12 percent. Thus, a substantial portion of the increase observed in the amount of albite below a depth of 3,300 m can be reasonably attributed to albiti- zation:
21.4% (albite, > 3,300 m) - 12.2% (original detrital al- bite, < 3,300 m) = 9.2% (albitized).
Table 1 summarizes the depth-related changes in the plagioclase population.
GRAIN L O S S T H R O U G H S U B S U R F A C E D I A G E N E S I S 997
(A) 4o I-- Z LU 3f i
Z ~0 o
z ~ 2 s
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• Above 3300 m o Below 3300 m
o
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AVERAGE AN CONTENT (mole %)
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,8 zb -" 3o AVERAGE AN CONTENT (mole %)
FIG. 10.--A) Average anorthite content (mole percent) in plagioclase versus the average anorthite content excluding grains of albite. B) Average anorthite content (mole percent) versus percent albite in pla- gioclase.
In thin section and with the SEM, plagioclase shows evidence of dissolution at all depths (Fig. 11). As with K-feldspar, there is no evidence that formation of authi- genic clay minerals accompanies plagioclase dissolution.
Textural evidence of albitization is also observed (Fig. 12). Albitized grains are typically composed of aggregates of l -5-pm euhedral albite crystals. Similar textures have been recognized in albitized grains in older Tertiary sand- stones in the subsurface elsewhere in the Gulf(Boles 1982; Gold 1987).
A L T E R A T I O N O F H E A V Y M I N E R A L S
Nonopaque heavy minerals also show a depth-related variation (Fig. 13). Below a depth of approximately 1,500 m the heavy mineral assemblage changes as the relatively less stable minerals become less abundant and ultimately disappear. In the deepest samples, only tourmaline, zir- con, and a minor amount of garnet remain. The extent of heavy mineral loss is demonstrated in Figure 14, a plot of zircon + tourmaline relative to the total nonopaque assemblage.
Fio. 11.--Plagioclase shows evidence of dissolution over the entire depth range examined. Secondary pores (P) are surrounded by plagio- clase (I). Depth, 3,514 m.
Petrographic evidence confirms that, indeed, non- opaque heavy minerals have undergone dissolution (Fig. 15). Again, authigenic phases such as chlorite or kaolinite are not observed around leached heavy minerals. Thus, dissolution was a rather open-system process, at least on the scale of a thin section, and most probably over greater distances.
Preferential removal of relatively unstable minerals is the trend expected if dissolution is responsible for the changing mix of heavy minerals. If the change resulted instead from a changing balance of metamorphic versus volcanic sources, the order of disappearance would not necessarily correlate so closely with relative chemical sta- bilities. Morton (1984) reports similar diagenetic trends in the distribution of heavy minerals in Tertiary sands of the North Sea basin. In fact, similar depth-related trends in heavy minerals have been noted previously in post- Miocene rocks of the northern Gulf (Bornhauser 1940; Cogen 1940) but have been interpreted in the context of possible stratigraphic significance rather than diagenesis.
ALTERATION OF THE BULK ASSEMBLAGE
As has been demonstrated, deeply buried samples from Plio-Pleistocene units have undergone substantial mod- ification of their detrital assemblage, including substantial loss of feldspar. That there are no significant changes in the QFR ratio with depth in no way refutes this fact. Shallow and deep samples cannot be strictly compared in terms of QFR because there is a systematic decrease of grain size with depth (Fig. 6). Loss of feldspar through diagenesis with increasing depth is offset by the tendency of finer grain sizes to contain more feldspar.
T A ~ l.--Depth-related changes in detrital plagioclase
Depth ~ of Feld. Average An Content Average An Content (-Ab) % <An2 % >An30 No.
< 3,300 m 56.0 24.1 26.6 10.6 35.9 26 > 3,300 m 70.6 16.7 20.8 21.4 22.8 16
FtG. 12.--Aggregates of euhedral albite crystals in the place of former plagioclase grains show that depth-related changes in average plagioclase compositions likely arise through albitization. Depth, 4,518 m.
DISCUSSION
Similar, but more extensive, feldspar alterations are documented in the Eocene (Boles 1982; Fisher and Land 1986), Oligocene (Land and Milliken 1981; Land 1984), and Miocene (Gold 1987) units of the Gulf. Other ex- amples of complete K-feldspar removal or complete al- bit ization should be easy to recognize, even in older rocks of more uncertain source. The importance of alteration in these Plio-Pleistocene sediments is that it shows the rapidity and subtlety with which these processes begin. In less than 5 m.y. plagioclase more calcic than An 30 has been d iminished by one-third, albite has doubled in abundance, and half the K-feldspar has dissolved (speak- ing in general about the sediments of burial depth below 3 km; Table 1), all without the rigors of meteoric-water diagenesis.
If found in a much older sandstone, the modified con- di t ion of a similar detrital assemblage could easily go unrecognized. Unfortunately, secondary porosity and au- thigenic clay minerals are not inevitable consequences of grain dissolution. Interpretations of source area litholo- gies and tectonics are very much compromised by sub- surface alteration of detrital mineral assemblages.
In light of what is now known about grain alteration in general, and specifically, those studies of grain altera- tion in Tertiary basins, it is interesting to review some of the earlier literature on preservability of provenance in- formation. Petti john (1941) documented the occurrence of heavy minerals in sandstones as a function of age, finding that with increasing age rocks contain progres- sively fewer varieties of heavy minerals. At about the same time, Krynine (1941 a, b, c, d) outl ined a tectonically oriented view of provenance in which depth-related vari- ations in detrital mineralogy reflected progressive un- roofing of sedimentary, metamorphic, and plutonic rocks and the resulting " inver ted stratigraphy" of the sedimen- tary basin. Despite the convincing data on the age cor- relation of heavy mineral suites, this disagreement on the
0
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I I - 12. W 3 - C~
4 -
i I I 2
Q. E
J I I I I
O O
I LiJ I
, g
f N
I
5 - I FIG. 13.--The assemblage of nonopaque heavy minerals undergoes a
dramatic change with depth in Plio-Pleistocene sediments of the north- ern Gulf of Mexico. This diagram is based on the presence or absence of the minerals as observed in thin section. Lines are solid where the mineral can be found in most thin sections from that depth; lines are dashed in zones wher6 the mineral is only observed in a few samples. Note that the depth variation corresponds well with Krynine's model for progressive unroofing of"sedimentary," "metamorphic," and "plu- tonic" layers of the crust. But, clearly, the post-Miocene history of the Mississippi drainage basin does not allow for such an interpretation.
significance of detrital assemblages could not be resolved because of one powerful argument on the side of Kry- nine 's tectonic interpretation. Elucidated by van Andel (1959), the observation that the sequence from stable to unstable assemblages is found in basins of diverse ages,
~90
+~.~ Z ' 8 g
N W
,o[
o PLIO- PLEISTOCENE MIOCENE o o • •
0o
O O
FIG. 14.--Ultra-stable heavy minerals (zircon and tourmaline) in- crease in their relative proportion of the total nonopaque assemblage with increasing depth. Miocene samples from the Louisiana coastal region are included to emphasize that the trend is depth- and temper- ature-related and not a function of relative ages.
o 9 I I I I I
I 2 3 4 5
DEPTH (kin)
G R A I N L O S S T H R O U G H S U B S U R F A C E D 1 A G E N E S I S 999
FIG. 15.--SEM view of dissolution textures which characterize common heavy minerals. Dissolution features were observed on all heavy mineral species except zircon and tourmaline. A) Pyroxene. Depth, 807 m. B) Amphibole. Depth, 2,771 m. C) Epidote. Depth, 2,265 m. D) Garnet (Fe > Ca). Depth, 4,898 m.
including many with Tertiary sediments, argued against a temporal (i.e., diagenetic) control. A compromise was suggested (Pettijohn et al. 1987) to reconcile these points of view: both diagenetic and tectonic controls are im- portant determinants ofdetrital mineralogy, but they op- erate on different time scales--tectonic events leaving their mark over tens of millions of years, and diagenesis op- erating on the scale of hundreds of millions of years (Pet- tijohn et al. 1987, p. 437). The nature of grain alteration in the Plio-Pleistocene sediments examined in this study suggests that this compromise is unnecessary. Diagenesis can bring about dramatic changes in a detrital assemblage over time spans of 5 m.y. or less. The repeated occurrence of increasing mineralogical maturity with increasing age in various basins can be viewed as the expected result of the geothermal gradient. The older rocks in any particular sequence are buried deeper, are heated to higher tem- peratures, and hence, show the effects of more extensive alteration. Pettijohn was quite right: postdepositional al- teration of grains is a major control on the composition of sandstones. That this is as true for feldspars as for
heavy minerals is a finding that probably was not antic- ipated in the 1940s. Krynine (1950, p. 85) did point out that if heavy minerals were compromised as tectonic indicators, feldspars would be compromised as well He simply did not believe either situation to be true. The recognition that detrital components are not always inert, permanent constituents of sandstones has come about largely through developments (blue-dyed epoxy, electron microprobe techniques [Trevena and Nash 1981], and deep drilling in Tertiary basins) of the past twenty years.
It is probably unwise to suggest that detrital assem- blages are never preserved. It is reasonable to suggest that for sediments older than one million years, complete pres- ervation seems unlikely. It has been extensively dem- onstrated that meteoric water, in either weathering or subsurface environments, is a powerful agent for grain alteration (Friis 1974; Walker et al. 1978; Mathisen 1984) over a wide range of temperatures. Any evidence that a sandstone has been flushed by meteoric fluids demands that the detrital assemblage be viewed as potentially al- tered. In the Plio-Pleistocene sediments examined in this
1000 K I T T Y LO U M 1 L L I K E N
study, with little or no meteoric influx, subsurface tem- peratures above 50"C are accompanied by extensive loss of heavy minerals; above 80°C feldspars are also altered. An assessment of thermal history should be a part of every provenance study.
One often-cited solution to the problem of intrastratal solution is the study of supposedly "insulated" detrital assemblages, as in early concretions (Bramlette 1941), early cemented zones (Walker 1984), oil-saturated inter- vals (Yurkova 1970), or mudstones (Blatt and Sutherland 1979). The role of fluid flow (i.e., the importance of highly open systems) in grain destruction observed in this and other Gulf Coast studies would suggest this solution is imperfect. For instance, the presence of an abundant ce- ment implies that tremendous fluid volumes have at one time passed through the rock (Land and Dutton 1979; Blatt 1979). That the detrital assemblage in a concretion might be different from that in the surrounding rock is not surprising because those two parts of the rock have had different diagenetic histories which may have in- cluded different types of grain alterations (e.g., replace- ment vs. dissolution). Similarly, while mudstones may have fluid flow that is less relative to flow in adjacent sandstones, the fluid "seen" by the mudstone may still be substantial. Depth-related loss of sand- and silt-sized K-feldspar has been documented in Tertiary mudstones of the Gulf Coast (Hower et al. 1976; Freed 1981, 1982); albitization and heavy mineral dissolution in mudstones are also quite possible and deserve further investigation.
There are three main categories of reliable petrographic evidence for provenance:
1 ) The presence of a particular mineral grain is certainly significant. The use of mineral assemblages or ratios, how- ever, imparts a significance to the absence of minerals. These measures should be avoided or applied with the utmost caution.
2) Varietal studies of quartz, tourmaline, and zircon have much potential for use in provenance determina- tion. Only a few applications (e.g., Siever and Potter 1956; Callender and Folk 1958), have taken advantage of work done in this field (e.g., Groves 1931; Krynine 1946; Basu et al. 1975). New advances in the use of luminescence (Matter and Ramseyer 1985; Sprunt et al. 1978; Zin- kernagel 1978) and application of trace element analysis to ultrastable minerals (Leckebusch 1978; Owen 1987) are improving the utility of ultrastable mineral varieties for provenance determination. Regrettably, the ultrasta- bility that assures preservation of these minerals also en- ables them to be recycled, limiting their usefulness some- what.
3) Nonquartz components in rock fragments are subject to alteration, the same as in monomineralic grains. How- ever, to the extent that information from rock fragments is textural rather than compositional, they are reliable indicators of provenance.
SUMMARY
1) Subsurface alteration of K-feldspar, plagioclase, and heavy minerals in Plio-Pleistocene sediments of the
northern Gulf of Mexico has caused substantial loss of provenance information in less than 5 m.y.
2) Compared to shallower samples, sediments with a burial depth below 3,300 m have lost half of the K-feld- spar, most likely through dissolution. Dissolution is also responsible for drastic modification of the heavy mineral assemblage and the loss of an unknown quantity of pla- gioclase.
3) Petrographic evidence of grain dissolution such as authigenic clays or secondary porosity is rare in these sediments. Lack of such petrographic indicators in an- cient sandstones is not reliable evidence for unaltered detfital assemblages.
4) Imbalance between the volume of authigenic clays and the volume of grains lost implies that grain loss has been an open-system process on a large, perhaps basinal, scale.
5) Albitization has caused modification of plagioclase grains. Compared to shallower samples, sediments below 3,300 m have lost one-third of the grains more calcic than An 30 and albite has doubled in abundance. Similar to older Tertiary units along the Texas Gulf Coast, albiti- zation of plagioclase is accompanied by distinctive tex- tural changes (Boles 1982; Gold 1987).
ACKNOWLEDGMENTS
I thank L. S. Land, R. L. Folk, and E. F. McBride for their wise guidance during the course of the dissertation study on which this paper is based and for their helpful suggestions on early drafts of the manuscript. Discussions with Ralph Kugler, Steve Johansen, and Paul Gold were instrumental to the development of many of the ideas presented here. Reviewer Ken Helmold's suggestions were most helpful. Samples for this study were provided by Amoco, Marathon, Mobil, Shell, and Texaco. Paul Gold kindly provided the Miocene samples included in Figure 13. Financial support in various forms was given by Ten- neco, Texaco, the Geology Foundation of The University of Texas at Austin (Owen-Coates Fund), and the Gulf Coast Association of Geological Societies.
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