AsSrRACT: The El Imperial Formation (mid-Carboniferous-Lower Per- mian) constitutes a progradational sandstone-rich succession deposited in the San Rafael ...
S O U R C E V E R S U S D E P O S I T I O N A L C O N T R O L S O N S A N D S T O N E C O M P O S I T I O N IN A F O R E L A N D B A S I N : T H E EL I M P E R I A L F O R M A T I O N ( M I D C A R B O N I F E R O U S - L O W E R P E R M I A N ) , S A N R A F A E L BASIN, W E S T E R N A R G E N T I N A IRENE S. ESPEJO ~ANDOSCAR R. LOPEZ-GAMUND[ 2 Amoco Production Company, P.O. Box 3092, Houston, Texas 77253.3092 USA 2 Texaco Inc., FrontierExploration Department, P.O. Box 430, Bellaire, Texas 77401-2324 USA AsSrRACT: The El Imperial Formation (mid-Carboniferous-Lower Permian) constitutes a progradational sandstone-rich succession deposited in the San Rafael foreland basin of western Argentina. Four facies associations have been identified: a basal glacial marine association, a shallow marine association, a deltaic association, and an uppermost fluvial association. Sand-prone deposits in the deltaic association are represented by prodelta and delta-front shales and subordinatefine sandstones (Facies A), deltaic platform, wave-reworkedchannel mouth-barsandstones (Facies B), and fluvial-dominateddistributary channel sandstones (Facies C). Analysis of framework grains of sandstone samples from Facies B and C shows two distinct mineral assemblages or petrofacies. The quartzose petrofacies is characterized by high contents of quartz and low percentages of feldspar and lithic grains. The quartzolithic petrofacles shows an increase in labile components, in particular lithic fragments, and a concomitant decrease in quartz. The quartzolithic petrofacies shows a source signature. Average detrital modes of sandstones from this petrofacies are similar to those from overlying fluvial sandstones. All wave-reworked,channel mouth-barsandstones (Facies B) correspond eompositionally to the quartzose petrofacies, whereas detrital modes from the distributary-channel sandstones (Facies C) fall into the quartzolithic petrofacies. This correspondence between depositional environment and potrofacies suggests a strong depositional influence on composition (deposifional signature). Abrasion (mechanical breakdown) by wave action in shallow marine environments accounts for the quartzrich nature and paucity of labile grains in the quartzose petrofaeies.
INTRODUCTION
Analysis of sandstone composition is a useful approach to unravel sourcearea lithology and, indirectly, to characterize the paleotectonic setting of siliciclastic deposits. Since the seminal work by Krynine (1943), and particularly after the work by Dickinson and Suczek (1979), an increasing number of papers have shown the detrital modes representative of different provenance types in a variety of plate-tectonic settings (Ingersoll and Suczek 1979; Dickinson and Valloni 1980; Valloni and Maynard 1981; Dickinson et al. 1983; Valloni and Mezzadri 1984; Valloni 1985; Schwab 1986; Dickinson 1988; DeCelles and Hertel 1989). Other contributions call attention to factors other than provenance that might modify, and in a few extreme cases drastically change, the final composition of sandstones. Among these other factors, climate and relief of source areas, abrasion and reworking in some specific environments, and diagenesis are the most significant (Krynine 1935; Folk 1980; Suttner et al. 1981; Basu 1985; Pettijohn et al. 1987). Most notably, Johnsson et al. (1988, 1991) and Savage and Potter (1991) have recently documented the effect of chemical weathering during fluvial transport under tropical conditions on sand-size material. Abrasion during transport, resulting in elimination of less stable mineral types, has been reported from fluvial and shallow marine environments (e.g., Mack 1978; Cotter 1983). In shallow marine environments prolonged residence periods improve the chances of wave reworking and consequent elimination of labile grains shifting the resultant sand-size sediment composition towards the mature quartz-rich end of the spectrum. Our objective is to document the complex interplay between source lithology and depositional environment in controlling sandstone comJOURNALOFSEDIMENTARYRESEARCH,VOL.A64, No. 1, JANUARL1994, P. 8--16 Copynghl © 1994, SEPM(Societyfor SedimentaryGeology) 1073.130X/94/0A64-8/$03.00
position in a wave-influenced delta in the Upper Paleozoic El Imperial Formation of the San Rafael basin, western Argentina (Fig. 1). Our intention is to illustrate through the study of sandstone petrography the compositional modifications introduced by marine reworking. We use the term lithofacies, or simply facies, as "a distinctive rock that forms under certain conditions of sedimentation and that reflects a particular process or environment" (Reading 1986). In contrast, we use the term petrofacies to describe mineral assemblages that form the framework grains of sandstones; petrofacies boundaries commonly do not follow the boundaries of lithofacies or lithostratigraphic units (Mansfield 1971). Different petrofacies are recognized on the basis of relative abundances of detrital modes. GEOLOGICAND STRATIGRAPHICSE'ITING
The San Rafael Basin is part of a series of backarc-foreland basins developed along the South American part of the Gondwanan paleo-Pacific margin (L6pez-Gamundi et al. 1989). Basement along the eastern margins of the basins is made up of Precambrian igneous (mostly plutonic) and high-grade metamorphic rocks and Cambrian to Devonian marine sedimentary successions. This suite of source-area lithologies accounts for the mixed, sedimentary-igneous composition of the upper Paleozoic sandstones in the San Rafael and adjacent Calingasta-Uspallata basins. The El Imperial Formation is a siliciclastic, sandstone-rich unit of Middie Carboniferous to Early Permian age (Espejo and Cesari 1987; Espejo 1990; Espejo et al. 1991; Garcia 1992). It is well exposed in the northern pan of the San Rafael Basin (Fig. 1), and consists of basal shallow marine deposits that grade upward into fluvial sediments (Fig. 2). In spite of some changes in the paleocurrent pattern towards the top of the formation, the petrology of the source areas and paleotectonic setting remained constant throughout deposition of the entire unit (Espejo 1990). DEPOSmON~ ENW~ONMENTSOF THE EL IMPEmXLFORMATION The type locality of the El Imperial Formation is exposed in Rio Diamante Canyon (Fig. 1B). There the El Imperial Formation reaches 2500 m in thickness. Four facies associations were distinguished (Fig. 2). The El Imperial Formation starts with basal transgressive marine shales and fine sandstones that pass upwards into ice-rafted mudstones and pebbly mudstones of the glacial marine facies association. These glacial marine deposits grade upwards into cross-bedded fine to medium sandstones of the shallow marine facies association. These latter sandstones are grouped into tabular beds and represent nearshore sand bars that formed parallel to the shoreline. The shallow marine sandstones pass into mudstones and fine and medium sandstones of the deltaic facies association. The El Imperial Formation culminates with trough-cross-bedded sandstones and scarce fine conglomerates and massive mudstones of the fluvial facies association. The deltaic facies association can in turn be subdivided into three facies. At the base are prodelta laminated shales and rippled fine sandstones (Facies A), followed by deltaic-platform, wave-reworked mouth-bar sandstones (Facies B), and fluvial-dominated distributary channel sandstones and interdistributary bay mudstones and sandstones (Facies C). Deposits
S. tNDSTO.rYh" ('O.~lf"OSITION l.\ ~A FORELAND BASIN
9
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El.. IW~RIAL FORMATION
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of the two latter facies are interbedded throughout a thickness of 530 m and are object of our petrologic study (Fig. 2).
0I
25 I Krn
Fm(~.1.-A) Study area. B) location of outcrops of the El Imperial Formation in western Argentina. Adapted from Espejo(1990).
show abraded overgrowths of silica and/or coating of iron, indicating a second-cycle sedimentary origin. Trains of fluid inclusions, of probable plutonic origin, are common. Where abundant, fluid inclusions give a turbid aspect to crystal fragments ("bubbly quartz"; Fig. 3A). A hydroMETHODS thermal origin is also indicated by the presence of vermicular chlorite (Fig. Thin sections were made of sandstone samples collected approximately 3B) and less clustered epidote inclusions (cf. Scholle 1979', Folk 1980). 30-50 m apart in a composite section. Due to abundant faults, this in- Qm grains of probable volcanic origin are represented by euhedral forms tegrated section is made up of five partial sections exposed in the Diamante and grains with embayments and nonundulatory extinction (Fig. 3C). Canyon (Fig. 1, B); a total of 32 thin sections (15 from the deltaic facies Polyerystalline quartz (Qp) is present in minor amounts. Although the association and 17 from the fluvial facies association) were analyzed in number of internal crystals varies depending on clast size, different varithis study• Severely altered samples were disregarded in order to avoid eties can be distinguished: (1) a population where internal crystals are misidentification and consequent underestimation of the percentage of randomly arranged, (2) a population with polygonal-shaped internal cryslabile components. tals, (3) a population with strongly sutured intercrystalline boundaries, Approximately 800 points (400 by each author) were counted in all the and (4) crystals that show moderate to strong elongation and preferential thin sections. Point-counting parameters (Tables l, 2) are mostly those crystallographic orientation (strained quartz or recrystallized metamorphic defined by Dickinson (1970, 1985}. Metamorphic lithic fragments (Lm) quartz; Fig. 3D). Population 1 is probably derived from plutonic or highwere distinguished from sedimentary lithic types (Ls) by the alignment of grade metamorphic rocks (Scbolle 1979; Folk 1980). Chert of sedimentary crystals (tectonite or metamorphic fabric) in the former grain type. Cor- origin or other equally durable microcrystalline particles are subordinate relation coefficientsbetween the calculated percentages of the various point- (Fig. 4A). counted grain types measured by both operators are above +0.84. Due Both plagioctase (P) and K-feldspar (K) are present and included as to these high correlation coefficients, the final values shown in Table 1 are feldspars (F). Neither is clearly predominant, and the total percentage of the result of averaging the point-count results from both operators. feldspar is also extremely variable with respect to quartz. The point-counting technique used was that proposed by Ingersoll el al. The K-feldspar population is composed of unlwinned and, uncommon(1984). Counting error was estimated using the chart of Van der Plas and ly, twinned orthoclase and subordinate microcline. A few grains show Tobi (1965). Long axes of two hundred sand-size grains per thin section secondary overgrowths. Commonly observed etched and corroded grain were measured and converted to equivalent sieve sizes using the method margins are due to partial replacement by calcite. Seficite and chlorte are proposed by Friedman (1958). Mean size of sands in each thin section common alteration products. Clay pseudomorphs replacing whole K-feldwas thus computed. spar grains were observed in some specimens. K-feldspar grains are mostly inhomogeneous in composition and include cryptoperthites to maculose SANDSTONE PETROGRAPHY perthites (Fig. 4B). Some K-feldspar grains may have been derived from cataclastic rocks, because they are associated with foliated quartzofeldDescription of Graln Types spathic rock fragments with cataclastic texture and feldspar "eyes" with Texturally, sandstones from both petrofacies are moderately to well similar characteristics and crushed mineral "fails". A few well-developed sorted (Folk 1980) with subrounded to rounded clasts. elongate orthoclase crystals of volcanic origin are present. Monoerystalline quartz (Qm) shows nonundulatory to weak undulatory Plagiodase (P) is generally twinned; repeated (sometimes chessboard) extinction in fine sandstones. Clasts are subangular to subrounded; a few and Carlsbad twinnings are found (Fig. 4C). Twin lamellae are offset along
10
IRENE S. ESPEJO AND OSCAR R, LOPEZ-GAMUND[
1 u P P E R
FLUVIAL FACIES ASSOCIATION
M E M B E R
FACIES C FACIES B
L
o w
FACIES C
M
FACIES B FACIES C
E R
E M B E R
DELTAIC FACIES ASSOCIATION
FACIES B
'N
FACIES B -~
500" m
250'
FACIES C
FACIES A
SHALLOW MARINE FACIES ASSOCIATION
GLACIAL MARINE FACIES ASSOCIATION
WE-CARBONIFEROUS METASEDIMENTARY BASEMENT
fractures. Myrmekites are also present. As in K-feldspar grains, seficite and chlorite are the principal alteration components. Grains are partly replaced by calcite. Rock fragments (L) consist of sedimentary (Ls), metamorphic (Lm), and volcanic (Lv) lithics and a few fine aplitic clasts. Sedimentary rock fragments are dominated by argillaceous, silty matrix-rich (chlorite-sericite) quartzose wackes (Fig. 4D); matrix-poor quartzose siltstones cemented by iron oxides constitute a subordinate subpopulation (Fig. 5A). Limestone clasts are rare. Metamorphic rock fragments, the most abundant lithic suhpopulation, consist of subangular to subrounded clasts of phyllites, schists, and slates (in decreasing frequency). Low-grade metamorphic fragments vary from quanz-chlorite-sericite to quartz-muscovite in composition; microporphyroclastic quartz is common in these grains (Fig. 5B, C). In some cases, severe mashing of these ductile grains (pseudomatrix, Dickinson 1970) makes it difficult to differentiate them from intrabasinal rip-up clasts or detrital matrix. Few clasts with pseudo-hornfelsic texture are present, suggesting derivation from contact-metamorphic sources. Aplites are clasts with microgranular hypidiomorphic texture and granitic (acidic) composition of hypabyssal origin. Since quartz and feldspar grains show strong similarities with the monocrystalline quartz and K-feldspar grains described above, aplites were grouped with the plutonic and highgrade metamorphic lithics. Volcanic and pyroclastic grains were included within the volcanic rock
Fi6. L-Generalized stratigraphiccolumn of the El Imperial Formation. Facies associations and subdivision of the deltaic associationarea adapted from Espejo(1990).
fragments. Volcanic grains display a felsitic matrix with few quartz and K-feldspar microphenocrysts, suggesting a rhyolitic composition. Pyroclastic rock fragments comprise fine-grained tufts with vitric textures where ghosts of shards are still recognizable (Fig. 5D) and recrystallized ignimbrite clasts. Silicification is common. In these latter types, mineralogical identification was precluded by extensive sericitization and replacement by various silica-rich minerals. Petrofacles Discrimination
Two distinct pctrofacies can be defined on the basis of relative abundances of detrital modes (Tables 1, 2; Fig. 6). The mineralogically more mature qaartzose petrofacies (mean QmFLt %: 82.5-3.0-14.5; mean QFL %: 85.8-3.1-11.1) is clearly segregated from the quartzolithic petrofacies (mean QmFLt %, 50.8-11.2-38.0; mean QFL %, 55.8-11.2-33.0), characterized by abundant labile fragments. In the latter petrofacies, the principaI mode falls invariably in quartz grains, and the secondary mode fails in either lithics or feldspar grains. Variations in the modes are slightly more than in the quartzosc petrofacies. Labile components (Lb = F + L) and phyllosilicates (chlorite, muscovite, and biotite) are more common than in the quartzose petrofacies. More durable accessory minerals, such as futile and zircon, are present in small percentages in the quartzose pctrofacies.
SANDSTONE COMPOSITION IN A FORELAb~T)BASIN
TAntE1.-- Detrital modes of the samplesfiom quartzosepetrofacies Quartzose petrofacies
Sample Qm
Qp
Q1
K
P
F
Ls
Lv
Lm
L
Lt
Y 27 % a 20
9128 0.66 1.32
6.51 0.58 1.16
97.79 0.34 1.16
0.11 0.08 0.69
0.00 0.00 0.00
0.11 0.08 0.15
1.14 0.25 0.50
0.00 0.00 0.00
0,83 0,21 0,43
1.97 0.32 0.65
8.48 0.65 1.31
Y 33 % ~r 2a
78.94 1.01 2.02
4.43 0.51 1.02
83.37 0.92 1.84
2.59 0.39 0.779
1.23 0.27 0.55
3.82 0.47 0.95
9.24 0.72 1.44
0.12 0.08 0.17
3.45 0,45 0,90
12.81 0.83 1.65
17.24 0.94 1.87
Y 34 % ~r 2a
77.54 1.04 2.08
2.23 0.37 0.73
79.77 1.00 2,00
2.36 0.38 0.76
0.50 0.17 0.35
2.86 0.41 0.82
7.07 0.64 1.28
6.95 0.63 1.27
3.35 0.45 0,90
17.37 0.94 1.89
19.60 0.99 1.98
Y 36 % # 2~
82.53 0.91 1.82
2,87 0.40 0.80
85.40 0,85 1.69
0.92 0.23 0.46
0.92 0.23 0.46
1.84 0.32 0.64
11.61 0.77 1.53
0.46 0.16 0.32
0.69 1.28 2.56
12.76 0.80 1.60
15.63 0.87 1.74
20
80,22 0.98 1.96
3.28 0.44 0.88
83.50 0.91 1.83
2.30 0.37 0.74
0.24 0.12 0.22
2.54 0.39 0.77
8.86 0.70 1.40
0.61 0.19 0.38
4,49 0,51 1.02
13.96 0.85 1.71
17.24 0.93 1.86
122 % a 2a
76.48 1.02 2.05
2.33 0.36 0.73
78.81 0.98 1,97
6.52 0.59 1.19
5.70 0.56 1.13
12.22 0,79 1.58
0.23 0.11 0.22
0.00 0.00 0.00
8.73 0.68 1,36
8.96 0.69 1.38
11,29 0.76 1.53
125 % a 20
93.40 0.58 1.15
3.78 0.44 0.89
97.18 038 0.77
0,97 0.23 0.45
0.00 0.00 0.00
0.97 0.23 0.45
1.08 0.24 0.48
0.11 0.08 0.15
0.65 0.19 0.37
1.84 0.31 0,62
5.62 0.53 1.07
I 33 % ~r 20
78.43 0.97 1.94
2.01 0.33 0.66
80.44 0,94 1.87
0.45 0.16 0.32
0.00 0.00 0.00
0.45 0.16 0.32
5.25 0.53 1.05
2.79 0.39 0.78
11.06 0,74 1,48
19.10 0.93 1.86
21.11 0.96 1.93
Y 47 %
CONTROLSON SANDSTONECOMFO~rrloN Sandstone composition can be envisioned as an open system whose final product is the result of provenance (lithoiogy, relief, and climate of the source areas) and grain size (Basu 1976; Folk 1980; Suttner et al. 1981). The resultant composition can be modified greatly by mechanisms operating during sediment transport (i.e., abrasion) and during and after deposition (Davies and Ethridge 1975; Odom et al. 1976; Mack 1978; Suttner 1974; McBride 1985, 1986). Only after a full accounting of these factors can sandstone composition be considered to be a confined system in which the only variable involved is the source-area lithology. Consequently, a careful analysis must be earned out to evaluate fully the relative importance of the factors controlling sandstone composition. Compositional Variations Due to Climate and Relief in the Source Areas
Climate can be a primary control on sand mineralogy (Suttner et al. 1981; Franzinelli and Potter 1983; Mack 1984; Basu 1985; Stewart 1991). Climatically controlled weathering in the source area, collectively known as paleoweathering, modifies the primary composition prior to introduction of the weathered assemblage into the dispersal system (Velbel and Saad 1991). It has been suggested that paleoweathering in humid climates may significantly reduce the content of labile sand-sized components (Suttner et al. 1981), leading to a "paleoweathering" signature in the framework composition recognized in the resulting quartz-rich sandstones. In the case of the San Rafael Basin regional evidence gathered from studies on the late Paleozoic palcoclimatic evolution of west-central Argentina (L6pez-
Gamundi et al. 1992) suggest that the El Imperial deltaic sandstones were deposited under cool to temperate and semiarid conditions with incipient development of seasonality after a glacial period and previous to widespread arid conditions that prevailed during the latest Early Permian (Limarino and Spalletti 1986; Espejo 1992a). Under such conditions, elimination of labile framework-grain types by chemical weathering was mostly inhibited due to low to moderate temperatures and humidity. The overlying fluvial sandstones of the Upper Member of the El Imperial Formation provide additional evidence of the influence of climate (weathering) and relief. The thickness and facies association of these fluvial sandstones suggest that abundant sediment supply, subsidence, and/or slope gradient were sufficient to preclude significant chemical weathering. The fluvial sandstones reach a maximum thickness of 1700 m and were deposited by gravel-deficient sandy braided systems (Espejo 1986, 1990), suggesting short residence times (Basu 1985) and moderate slopes in the surrounding areas. Graln-Size Control
Although some sandstone samples show considerable variations in sand grain size ranging between 2.71 ~ (fine sand) and 0.05 0 (coarse sand), compositional changes related to grain size are considered insignificant. Variations greater than 10% in each parameter are not observed when samples of different average grain size are compared (Tables 1, 2). On the contrary, greater deviations are found, particularly in feldspars, if samples of similar grain size are compared. This relationship suggests that, although changes in sandstone composition might be expected when fractions of
12
IRENE S. ESPEJO AND OSCAR R. LOPEZ GAMUND[ I'~BLE2.-- Detrital modes of the samplesfrom quartzolithicpetrofacies
Qtlartzolithic
Sample
petro-
facies
Qm
Qp
Qt
K
P
F
Ls
Lv
Lm
L
Lt
2a
48.51 1.27 2.54
5.71 0.59 1.18
54,22 1.27 2,54
7.52 0.67 1.34
1.30 0.29 0.58
8,82 0.72 1,44
20.23 1.02 2.04
2,98 0,43 0,87
13.75 0.88 1.75
36.96 1.23 2.46
42.67 1.26 2.52
Y 38 % o 2#
44.58 1.25 2.49
t.76 0,33 0.66
46.34 1.25 2.50
9.95 0.75 1.50
12.72 0.84 1.67
22.67 1.05 2.10
11.21 0.79 1.58
4.28 0,51 1,01
15.49 0.91 1.81
30.98 1.16 2.32
32.74 1,18 2.35
Y40 % a 2#
41.35 1.23 2.46
5.14 0.55 1.10
46.49 1.25 2.50
3,00 0,43 0.85
9.02 0.72 1.43
12.02 0,81 1.63
7,02 0,64 1,28
0,50 0.18 0,35
33.96 1.18 2.37
41.48 1.23 2.47
46,42 1.25 2.50
Y 42 % a 2#
57.99 1.28 2,57
3.52 0.48 0.96
61.51 1.27 2.53
5.01 0.57 1,13
12.87 0.87 1.74
17.88 1,00 1.99
3.12 0.45 0.90
0.54 0.19 0.38
16.94 0.98 1.95
20.60 1.05 2.10
24,12 1.11 2.23
127 % o 2a
57,49 1.19 2.37
6,22 0.58 1.16
63.71 1.15 2.30
t.27 0.27 0.54
0.00 0.00 0,00
1.27 0.27 0.54
23.04 1.01 2.02
1.96 0.33 0.66
10.02 0.72 1.44
35.02 1.14 2.29
41.24 1.18 2.36
1 27' % ~r 2a
52.04 I.t6 2.32
6,45 0,57 I. 14
58.49 1.14 2.28
0.86 0.21 0.42
0.00 0.00 0.00
0.86 0.21 0.42
24.52 1.00 1.99
0,43 0,15 0,30
15.70 0.84 1.69
40.65 1.14 2.28
47,10 1.16 2.31
131 % # 2~
53,78 1.23 2.46
5.73 0.57 1.15
59.51 1.21 2.42
3,54 0.46 0.91
11.22 0.78 1.56
14.76 0.87 1.75
5.00 0.54 1.08
2.32 0.37 0.74
18.41 0.96 1.91
25.73 1.08 2.16
31.46 1.15 2.29
Y31' % a
different grain sizes are studied (Odom 1975; Odom et al. 1976; Basu, 1976; Johansen 1988), larger fluctuations may be attributed in this case to other factors. We further consider that feldspar and lithic fragments may have been lost from the coarse grain sizes and subsequently concentrated in the fine grain sizes (fine sand) during transport in littoral/eolian systems (e.g., Johansen 1988). To that end, the distribution of monocrystalline quartz (Qm), feldspar (F), and labile lithics (L) in both the quartzose and quartzolithic petrofacies with respect to the mean grain size of each sample was plotted (Figs. 7, 8). A very weak negative correlation between quartz content and grain size was observed in samples of the quartzolithic pelrofacies (Fig. 7), indicating a possible concentration of monocrystalline quartz grains in the finer sandstones. This trend is also clear in samples of the quartzose petrofacies. Enrichment in Qm is clearly independent of grain size, because both straight lines derived from regression equations are nearly parallel (Fig. 7). No overlap is observed between quartz contents from quartzose and quartzolithic petrofacies; this confirms the validity of the two petrofacies independent of grain-size variations. Additionally, the results show virtually no correlation between feldspar content and grain size (Fig. 7) and a moderate increase in labile lithics (L) toward the coarser grained end (Fig. 8).
Compositional Variationsduring Transportand Deposition Compositional variations are related to the elimination of labile components by either physical abrasion of the detrital constituents during transport or chemical weathering during temporary storage in various depositional sites during transport. The degree of compositional modification is a function of both intensity and time. Loss of rock fragments and feldspar grains during fluvial transport has been documented extensively in the literature (Cameron and Blair 1971; Ethridge 1977; Winn et
al. 1984; Savage and Potter 1991, among others). Also, abrasion produces significant changes in composition on first-cycle sands (Valloni 1985). Compositional variations caused by current and/or wave reworking in certain depositional environments (i.e., littoral and inner-shelf areas) are considered because of the similarity of mechanical processes operative during transport to the depositional basin and in depositional sites before final burial. In shallow marine environments, especially in wave-dominated settings, tractional processes precede final deposition. If so, abrasion can cause a strong reduction of labile components: sand-sized feldspar grains are reduced due to fracturing along cleavage and twinning planes, making them more susceptible to chemical weathering, and a significant proportion of the rock fragments is destroyed by wave abrasion (cf. Savage et al. 1988). It can be predicted that, all other variables remaining equal, deltaic mouth-bar sandstones should be mineralogically less mature (richer in labile components) than their offshore wave-reworked counterparts. Compositional Variations Due to Diagenesis
Postdepositional changes that modify detfital composition must be taken into account in order to reconstruct the original detrital modes. Strong modifications of detfital composition can be produced by dissolution of nonquartz grains under severe weatherng, and replacement of detfital grains by authigenic carbonates, clays, zeolites, and other minerals (McBride 1985). Cementation by carbonate and alteration to clay are considered the main diagenetic processes that affected the El Imperial sandstones. However, the authigenic clay seems to be related not only to feldspar alteration but also to biotite decomposition. In this case, flakes of detfital biotite were altered to secondary muscovite and a mixture of fine-grained carbonates, iron oxides, and hydrous clay minerals, which in turn were squeezed
SANDSTONE COMPOSITION IN A FORELAND BASIN
FIG.3.-Photomicrographs of principal grain types. A) Monocrystallinequartz ofhydrothermal origin with abundant fluid inclusions (bubbly quartz); note secondary quartz overgrowths indicated by arrows. Crossed nicols. Scale bar: 0.3 mm. B) Monocrystallinequartz ofhydrothermal origin with vermicular inclusions of chlorite. Scale bar: 0.3 ram. C) Monocrystallinequartz of volcanic origin. Note partial euhedral shape and embayments with attached aphanitic and glassy blebs. Crossed nicols. Scale bar: 0.3 ram. D) Well rounded polycrystalline quartz of metamorphic origin with preferentialorientation of elongatedcrystalswith sutured boundaries. Crossed nicols. Scale bar: 0.3 ram.
between rigid grains (Hayes 1979). Thus, clay filling &large primary pores during early diagenesis (accompanied by silica and uncommon feldspathic overgrowths and carbonate cementation) cannot be attributed entirely to complete feldspar alteration. Feldspar kaolinization and replacements by other clay minerals are mostly incomplete in the E1 Imperial sandstones. Illite-smectite and kaolinite aggregates identified by clay analysis in the quartzose petrofacies might be attributed to complete feldspar alteration. However, pervasive alteration of feldspar to clay minerals has not been observed. Moreover, no aggregates that might represent replacement of feldspar have been observed in the more feldspar-rich quartzolithic petrofacies or in the overlying feldspar-rich sandstones of the fluvial facies association. For this reason, the idea of a low feldspar content due to diagenetic processes within the quartzose petrofacies was disregarded. Another common diagenetic process that leads to feldspar replacement is carbonatization. This process is, like clay alteration, mostly partial. Where it is severe, pseudomorphous grains can be identified. In order to avoid misinterpretations of the feldspar percentages in any given sample, specimens with high contents of carbonate cement were not considered (McBride 1986). In summary, no significant modifications of the percentages of detrital modes due to diagenetic processes were observed.
13
F~, 4.-Photomicrographs of principal grain types. A) Polycrystalline quartz (sedimentary chert). Crossed nieols. Scale bar: 0,3 mm. B) Microcline with stringtype pcrthites, Crossed nicols. Scale bar: 0.3 mm, C) Plagioclasewith Carlsbad and repeated twinning partially altered to serieite. Crossed nicols. Scale bar: 0,3 ram. D) Sedimentary rock fragment (fine-grainedwacke) with quartz and feldspar elasts and chlorite-rich matrix. Plane-i~larized light. Scale bar: 0.3 mm
The overlying fluvial sandstones of the upper member of the El Imperial Formation share a similar framework composition with the deltaic sandstones (quartzolithic petrofacies), suggesting a possible common source area (Fig. 9). It is thus clear that only those sandstones with a "provenance (source)" signature (quartzolithic petrofacies) can be used for analysis of source-area lithology, disregarding those sandstones with an unequivocal "depositionar' signature (quartzose petrofacies). The contrasting compositions of the quartzose and quartzolithic petrofacies defined for the deltaic sandstones becomes evident when the principal detrital modes are compared (Tables 1, 2). The variance in detfital modes between the petrofacies partly obscures the relationship of sandstone composition to provenance. This relationship is clarified only when lithofacies and petrofacies are compared with one other (Fig. 10).
DETRITALMODES VS. SEDIMEN'IAR¥ENVIRONMENTS: RI,:.STRICtIONTO PROVENANCESTUDIES
Although two distinct petrofacies can be identified throughout the section analyzed, there is no evidence of a change in provenance types. Types oflithics (especially slates and schists, wackes, and quartz-rich sedimentary rock fragments) remain identical. Distinctive features in the monocrystalline population, such as feldspars (with perthites and myrmekites) and inclusions in quartz ("bubbly quartz" varieties, vermicular chlorite, and rutile needles), can be detected throughout the entire stratigraphic section, suggesting that rock types in the source area remained unchanged during the sedimentation of the El Imperial Formation (Espejo 1990, 1992a, 1992b).
FIG. 5.--A) Sedimentary rock fragment (quartzose sandy siltstone with ferruginous cement). Note plastic deformation by compaction. Plane-polarized light. Scale bar: 0.3 ram. B) Low-grade metamorphic fragment (slate); note cleavage and crenulations. Plane-polarized light. Scale bar: 0.3 ram. C) Low-grade metamorphic fragment (mica-quartz phyllite); note alignment of micas (mostly sericite and chlorite) and quartz porphyroclasts. Plane-polarized light. Scale bar: 0.3 ram. D) Vitric tuffwith euhedral quartz phenocryst (arrow). Plane-polarized light. Scale bar: 0.3 mm.
14
IRENE S. ESPEJO AND OSCAR R. LOPEZ-GAMUND[ Om
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petrofacies Fla. 6.-Q-F-L and Qm-F-Ltprovenance discriminatingdiagramsafter Dickinsonet al. (1983) for the quarlzose(8 samples)and quartzolithic petrofacies(7 samples).Means for both petrofaciesare plotted with one standarddeviation. Main provenancefieldsand subdivisions are as follows: 1, continentalblock; 1A, craton interior; IB, transitionalcontinental;IC, basement uplift;2, magmaticarc; 2A, undissected; 2B, transitional;2C, dissected; 3, recycledorogen; 3A, lithic;3B, transitional;3C, quartzose; 4, mixed.
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The compositional changes observed in the deltaic sandstones are recurrent throughout the section. Furthermore, these compositional variations show a striking COtTelationwith the lithofacies defined in the deltaic association. Sandstones from lithofacies B, of "marine" affinity, correspond to the quartzose petrofacies, whereas the sandstones oflithofacies C, with an evident "fluvial" affinity, are mineralogically characterized by the more immature quartzolithic petrofacies (Fig. 10). Abrasion by wave action in shallow marine environments, where grains can experience frequent movement parallel and/or transverse to the shoreline, can result in winnowing of labile components and enrichment in quartz. This seems to be the case for the sandstones of lithofacies B, in which labile (feldspar, F, and lithics, L) fragments were reduced by selective destruction due to wave action, resulting in a more mature end product. The quartz-rich, more mature composition of these sandstones can be considered to be the "depositionar' signature of the detfital modes. Contrastingly, sandstones of lithofacies C show a remarkable similarity in framework composition
with the overlying fluvial sandstones of the Upper Member (Fig. 9). This petrofacies, less mature and richer in labile components, is characterized by a "provenance" (more specifically "source") signature. CONCLUSIONS
Framework compositional analysis of the sandstones of the El Imperial Formation illustrates clearly the complex interaction between source areas and depositional environments. The quartzolithic petrofacies characterizes the fluvial and deltaic sandstones with insignificant wave reworking, whereas the quartzose petrofacies, dominant in wave-reworked channel mouth-bar sandstones, is indicative of the strong influence of the depositional dynamics of shallow marine areas on sand composition. Sandstones of the quartzolithic petrofacies are characterized by a "provenance" signature and are thus suitable for provenance evaluations. In contrast, the quartz-rich quartzose petrofacies displays a "depositional" signature
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