A study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of ...
Tectonophysics,
633
94 (1983) 633-658
Elsevier Science Publishers
ACCUMULATION
B.V., Amsterdam
- Printed
in The Netherlands
OF FOSSIL FUELS AND METALLIC
MINERALS IN
ACTIVE AND ANCIENT RIFT LAKES
ELEANORA
IBERALL
ROBBINS
U.S. Geological Survey, Reston, (Revised
VA 22092 (U.S.A.)
version received August
23, 1982)
ABSTRACT
Robbins,
E.I., 1983. Accumulation
P. Morgan
A study of active and ancient and metallic minerals place
in and
around
Cu-Pb-Zn
consequent
input of abundant
volcanism
add other nutrients and anoxic
nutrients
river,
Postdepositional and metallic organicpotentially
talus,
and metal-rich
begins
fan deposits.
94: 633-658. of fossil fuels
high biological lake deposits
gas, oil shale,
of uplifted
and tectonic
coal,
and
the
creates oxidized/re-
metal-bearing
are in contact
Earthquake-induced
areas,
lakes. Hot springs and
productivity
waters which preserves
phases, the fine-grained alluvial
of petroleum,
with erosion
and solute loads into swamps bottom
that accumulations
rift lakes. In:
that form rift valleys with those that take
of the precursors
uranium
Tectonophysics,
organic
tissues
with coarse-grained
turbidites
also are common
of rift lakes.
processes
components economic
and
in active and ancient
Rifting.
the world suggests of processes
and solutes. The resulting
beach,
deposits
and
and HaSrich
In the depositional
delta,
The deposition sulfides,
and horizons. coarse-grained
of Continental
rift systems around
rift lakes.
barite,
interfaces,
Processes
are related to the interactions
phosphate,
duced
of fossil fuels and metallic minerals
and B.H. Baker (Editors),
in rifts include
that were dispersed sourcebeds
deposits
in contact
is therefore
high heat flow and a resulting throughout
the lakebeds.
with coarse-grained
a characteristic
concentration
Postdepositional
host and reservoir
of the organic faulting
brings
rocks. A suite of
of rift valleys.
INTRODUCTION
Organic fuels and metallic minerals are found in the sediments of active rifts and in the rocks of ancient rifts. For example, in active rifts, petroleum, oil shale, lignite, and bituminous coal, are exploited in the Rhine rift in Germany (Teichmtiller, 1970; Ltittig, 1980). In the Dead Sea rift in Israel, widespread deposits of peat and lignite underlie Lake Hula (Huleh) (Brenner et al., 1978), and evaporite minerals such as potash are extracted from brines in the Dead Sea (Horowitz, 1979). Degens and Kulbicki (1973b) have calculated that 60,000 metric tons of copper, 270,000 tons of lead, and 60,000 tons of zinc accumulated in Lake Kivu in the East African rift during the last 5,000 years. In ancient rifts, petroleum is being generated today from Early Cretaceous 0040-1951/83/%03.00
0 1983 Elsevier Science Publishers
B.V.
634
lakebeds in rift basins along the South Atlantic margin in Angola, Brazil, and Gabon (Ghignone and Andrade, 1970; Brink, 1974; Brice and Pardo, 1981) (Fig. 1). (The location
of rift basins
discussed
in this paper
barite deposits
of Cretaceous
Africa (Fitton,
1980). Coal, phosphate,
mined
in North
Carolina
1955) of the Newark
is shown
age have been identified
in Triassic
rift of eastern
in Fig. 1.) Lead,
and nitrogen-rich
lakebeds North
zinc.
and
in the Benue trough of West black-shale
fertilizer
were
in the Deep River basin (Reinemund. America
(Burke,
1976: Wiegand
and
Ragland. 1970; Manspeizer, 1981: Robbins, 1981). Copper and barite were mined in Connecticut in the Hartford basin of the Newark rift (Fritts, 1962; Schnabel and Eric, 1964). Much of the current prospecting in the Newark rift is for economic deposits of uranium (Washington Post, 1982) and petroleum. In northern Australia a Proterozoic rift (Batten trough-Paradise along the Gulf of Carpenteria, graben-Leichhardt River rift) is the site of mining of lead and zinc from the Urquhart shale lakebeds in the Mt. Isa deposit; lead, zinc and barite in the Lady Loretta deposit (Large. 1980); and uranium in the Westmoreland River deposits (Hills and Richards, 1972). Furthermore, phosphate
and Alligator is mined along
the western edge of the Batten trough (Cook and Shergold, 1979). Copper is mined in Michigan and Wisconsin in the lakebeds of the Nonesuch Shale of Proterozoic age in the Midcontinent rift (Leone et al., 1971: Cannon, in press; Klasner et al.. 1982). But also, petroleum seeps into abandoned mine drifts at the White Pine copper mine in the Nonesuch The usual
Shale (G. Scott, written
explanation
commun.,
for ore deposits
1981).
associated
with rifting
involves
a fast
Fig. 1. Map of rift systems, rift valleys, and basins discussed in text. (Explanation of symbols: Newark rift - 1 = Fundy basin, 2 = Hartford basin, 3 = Newark basin, 4 = Gettysburg basin. 5 = Culpeper basin, 6 = Taylorsville Deep
basin, 7 = Richmond
River basin;
I1 = Midcontinent
basin, IO = basin, 8 = Farmville basin, 9 = Dan River-Danville rift, I2 = Reelfoot rift, 13 = Reconcavo basin, 14 = Rhine rift.
15 = Benue trough, 16 = Gabon basin, 17 = Cabinda basin, 18 = Mqamedes
basin. 19 = Dead Sea rift,
20 = East African rift, 21 = Karamay field, 22 = Baikal rift, 23 = Batten trough. 24 = Paradise graben, 25 = Leichhardt River rift.)
635
process, along
and that is injection
faults into the marine
of hot mineralizing environment
solutions
(Large,
vents occur in the fossil record (Russell
occurring
along
today
the East
operating
surface around
Pacific
Rise.
on a daily and annual
lakes, have been ignored,
the lower crust
1980). Certainly,
ated with marine accumulation,
through
The
deposits
and Smythe,
slower
associ-
198 l), and are
processes
resulting
in
basis in the upper crust and at the
and were chosen therefore
as the subject of
this paper. Although lake processes will be stressed, narrow arms of the ocean reaching into rift valleys share characteristics in common with lakes: the water column
can overturn,
bottom
can accumulate,
and numerous
ticulate
characteristic
elements
waters can become freshwater
anoxic,
large amounts
rivers can bring
of the catchment
in dissolved
of solutes and par-
area.
This paper is concerned with processes, and will be stressing biological limnological processes that enhance the accumulation that results in selected
and eco-
nomic deposits. Although it represents a synthesis, some new data are presented. Certain processes are glossed over because the data should be known by rift geophysicists. The usage of the words “tectonic lakes” is from the limnologist Hutchinson (1957). Tectonic lakes are those in grabens (Type 9) in tilted fault blocks (Type 8), and in a variety of other basins such as those between mountains rising (Type 4). Most large lakes in rift valleys are of these types. INTERACTION
OF RIFTING
that are actively
AND DEPOSITION
A study of active and ancient rift systems suggests that fossil fuels and metallic minerals accumulate as a result of the interactions of processes that form rift valleys with those that take place in rift lakes. The interacting processes include tectonic, thermal, climatic, hydrologic, sedimentological, limnological, chemical, and biological factors
(Fig. 2). The most important
ing, which causes
uplift
and shatters
surficial
interactions
rocks. Weathering
revolve around
and erosion
result
faultin the
release of inorganic nutrients required by organisms and metallic ions into the watersheds of tectonic lakes. Nutrients carried downstream may increase the productivity of organisms that are precursors of fossil fuels in tectonic lakes. Metallic ions are known to precipitate depending on oxidation/reduction states that are produced by living and dead organisms. Active and ancient rifts around the world contain similar sequences of rocks formed in response to similar tectonic processes. Rift systems are characterized by linear rupture in the crust caused by faulting and attendant earthquakes. Periodic faulting can result in uplifted highlands and horsts, down-dropped grabens, and tilted fault blocks. These structures are environments and act as geographic barriers which lead to biological isolation. Therefore, rift valleys are places where active speciation has been noted in crustaceans, gastropods, and fish (Brooks, 1950; Freyer, 1969). Earthquakes and accompanying tremors produce landslides and slumping of
Flowing under Hydraulic Head
Ground Water Heated into Convection Cells
Fig. 2. Results of processes
active along modern
rifts
I
TABLE Minimum
and maximum
values of solutes (ppm) accumulating
in Lake Magadi
in the East African
(from Jones et al., 1977) Sink
Source Rivers TDS *
152
Na HCO,
+ CO,
Springs
Lake surface
Borehole
brines
193,000-312,000
67,OOG312,000
-267
9600-34,900
15
- 63
3900- 15,500
78,500- 124,000
27,200- 130,000
55
-200
5500-18,000
70,700-
93,000
22,500- 105,700
Cl
5
- 15
1500-
6800
36,900-
87,700
23,500-
K
7
20
49-
240
lOlO-
2210
550-
2900
SO,
5
-14
73-
250
730-
2600
130-
2880
1
-
4
50-
170
950-
1980
300-
2170
- 57
34-
104
260-
1200
70-
1500
F SiO 2
29
102,000
Br
0.14-
0.08
7-
37
150-
290
78-
360
PO,
0.0 -
0.09
1-
17
53-
97
26-
167
B
0.0 -
0.06
2-
9
20-
120
20-
100
* TDS = total dissolved
solids
rift
637
sediment eroded
into
tectonic
at increased
lakes.
rates,
and
fine-grained elastic continental waters have formed lakes.
As the bordering rift valleys deposits
highlands
are filled
are uplifted,
consequently
and by lacustrine
deposits
they
are
by coarse-
to
where impounded
Rift lakes are unique because many are very large and deep; large, deep lakes tend to have many sources of sediments and nutrients that can become trapped since lakes serve as sinks. “Sink” is a term borrowed from global ecologists who calculate mass balance relationships between the environment and the depositional basin (Dastoor et al., 1979). Nutrients such as phosphate tend to get locked into deep anoxic bottom-sediment sinks (Hutchinson, 1957). Phosphate can go back into solution when strong earthquakes, a characteristic of rift valleys, agitate bottom sediments and expel pore waters (Sims, 1975). Hot springs, including geysers, may rise along faults and carry elements dissolved from underlying rocks (Table I). Volcanic
activity,
another
characteristic
of some rifts (Ziegler,
1981), adds additional
solutes and nutrients in the form of ash and rocks that weather easily at surface temperatures and pressures. Finally, numerous high-gradient streams (Fig. 2) feed into tectonic
lakes, carrying
other sediments
and nutrients.
Hydrologic factors
The hydrologic modified because
system is rearranged by rifting (Fig. 2). Precipitation may be newly formed highlands and large volcanic cones alter weather
and drainage patterns. High-gradient streams that carry large suspended and solute loads from the faulted and weathered highland rocks can deposit the sediment as deltas in the rift-valley lakes. As rifting progresses, near-surface groundwater is driven under higher hydraulic heads from the higher valley shoulders and can leach larger amounts of the soluble elements from the fractured rocks (Bredehoeft et al., 1982). Deep-seated groundwater may be heated to form circulating convection cells (Harder et al., 1980), also effective in leaching fractured rocks. Gatenby (1980) has quantified the thermal characteristics of heated groundwater that moves back up along active faults. As a result of so many diverse sources of solutes, the surface and groundwaters of tectonic lakes differ from those of other freshwater lakes, notably in their relatively high content of total dissolved solids (TDS) (Table I) and also in their relatively high pH values. Of 31 modern
rift-valley
lakes, 29 are alkaline
(Robbins,
1982). Lakes in
warm climates, or those having an input of warm or hot water, can become thermally stratified and eventually also density- or salinity-stratified (Hutchinson, 1957). The bottoms of stratified lakes tend to be anoxic environments favorable to growth of anaerobic bacteria and an accompanying generation of both methane and H,S (Deuser et al., 1973). The salinity of tectonic lakes in desert regions increases where evaporation exceeds inflow, attaining the world’s highest levels in the Dead Sea (Hutchinson, 1957).
TABLE
II
Minimum
and maximum
(from Livingstone, Degens
values of elements
1963; Talling
and Kulbicki,
and Talling,
1973a and b; Kilham,
(ppm) accumulating
in Lake Kivu in the East African
1965; Hecky and Degens, 1973) *
source
Sink
Murundu
hot
water
surface
River
springs
column
of anoxic sediment
_
Ag
layer
< 2-3 2 I ,200-80.560
Al As
x. 3x-- < 50
B
12-23
Ba
185-1387 200] _
_ + _ _
Pb, Zn
_ _
_
110 80- 110 80- 110
+ +
70-80
_
CU
80-95
S
Cu,
S
CU
105
_
120 80-l
_ _
Pb, Zn Cu, Pb, Zn
15
_
_
80
110
+
Cu, Pb, Zn
S
80
+
Pb, Zn
Cu, Au
+
_
_
+
_
S
_
_
80- 150
1 IO- > 200
105
80
60-75
_
Zn, Au
60-75
S
+
Cu. Au
+
P
+
CU
P
+
Pb, Zn?
+ ?
+ ? + +
CU?
P
Cu? and Richard Roberts
(1972), Large (1980), Leone et al. (1971), Neto (1970), Reinemund
(1928),
Turner-Peterson (1902).
Saxby (1977),
(1976).
G. Scott,
UNESCO
written
(1978),
commun.,
U.S. Bureau
(1955). Robbins
(1981),
Staplin
(1977),
Suszczynski
of Mines
(1976),
Weems
(1980),
(1982), (1973),
Woodworth
reporting
colors on the sheet-like
the thin-walled that
the
sheet-like
characteristic There
bisaccate
kerogen
of prokaryotes
are inherent
amorphous
kerogen
pollen where possible. has
fatty-acid
such as blue-green
problems
in all of the samples,
In Robbins profiles
with
and on
et al. ( 1979) we showed C-14
and
C-17
peaks
algae.
in using the sheet-like
kerogen:
the color of algal
tissues can be affected by the age of the colony, and can be affected by the suite of pigments expressed phenotypically depending on the species. Any unusual combination of pigments in algae will add an unsystematic error to these kinds of studies. RESERVOIRS.
SEALS.
AND
TRAPS
In addition to thermally mature source beds rich in organic matter, petroleum accumulation requires the presence of porous and permeable reservoir rocks, structural and/or stratigraphic traps, and impermeable rocks acting as seals. Reservoirs, seals, and traps are characteristic of rifted environments. Coarse-grained elastic rocks that can serve as reservoirs in rifts occur in three environments: along valley floors, along faulted slopes, and in or around lakes. Valley floor deposits are often reworked and sorted by axial and lateral rivers within graben and tilted fault-block valleys (Hay, 1967). Thick accumulations of sorted fluvial sands have been mapped (Willis, 1936; Davies, 195 1); and trough and cross-bedded micaceous sands and tuffs have been identified in river floodplains (Cooke, 1957; Hay. 1967; Crossley, 1980). Eolian sands are also typical sedimentary down deposits of rifts (Hay, 1967; McCall et al., 1967). High winds funneling narrow rift valleys produce sorted deposits Pumice gravels were seen in the Olorgesailie East African rift (McCall et al., 1967). The elastic sediments Subsidence, where
landslides,
the deposits
Landslides
of faulted and
slopes form talus and alluvial
slumping
are sorted
and sediment
of wind-blown sands (Beadle, 1974). area in the Gregory rift valley of the
can provide
by hill wash
porous
and braided
fan deposits
materials, streams
slumps have been noted in Lake Geneva
today.
particularly
(Cooke,
1957).
in the Rhine rift,
and in Lakes Tanganyika and Baikal (Forel, 1892; Hutchinson, 1957; Solonenko, 1978). Debris flows and volcanic lahars tend to have the coarsest materials (Pickford,
1978). The toes of high valley
walls accumulate
talus breccias
grading
into
Piedmont fans (Shackleton, 1978). Talus slopes tend to be stabilized today by vegetation, particularly in the wetter rifts. The deposits of slopes in ancient rifts, before the advent of land plants, undoubtedly had different characteristics. Unsorted boulders, the substrate of blue-green algae, are common along the steep western shore of Lake Tanganyika and steep eastern shore of Lake Malawi (Beadle. 1974). Coarse-grained sediments of rift lakes are deposited on beaches, deltas, reefs, bars. or as turbidites. Along the shores of rift lakes, as in other types of lakes, wave action in the shallows sorts elastic materials into beach sands and gravels (Beadle,
649
1974; McCall
et al., 1967; Crossley,
large and accumulate
as relatively
1980). Deltaic coarse-grained,
least five deltas have been formed
deposits
in large lakes can be
permeable
by rivers draining
elastic
materials;
into Lake Malawi
at
in the East
African rift (R. Crossley, Univ. Malawi, written commun., 1980). Vondra and Bowen (1978) studied pro-delta, littoral-lacustrine, barrier beach, delta-plain distributary
sands,
and
arenaceous
bioclastic
carbonate
deposits
around
and
in Lake
Rudolf (Turkana). An underwater carbonate reef exists in Lake Malawi (Eccles, 1974). Turbidites have been identified in cores from Lakes Tanganyika and Geneva (Degens et al., 1971; Reineck and Singh, The relation between the coarse-grained depends
1975). elastics and the fine-grained
on the origin of the lake. Lakes in grabens
valley walls, and coarse-grained
deposits
lake deposits
such as Lake Albert,
are mapped
have steep
on both sides (Davies,
1951).
Lakes on tilted fault blocks (half-grabens, semi-grabens), such as Lake Manyara in the East African rift, have coarse-grained deposits only on the faulted side. Lakes that are formed in combinations of grabens and tilted fault blocks, such as Lakes Baikal,
Tanganyika,
and Rudolf,
have coarse-grained
one side of the lake to another (Yuretich, 1976). While reservoir rocks are common around tectonic less common.
Thick lacustrine
deposits
that alternate
lakes, thick sealing
salts only are being deposited
from
rocks are
today in Lakes Magadi
and Natron in the East African rift, and in the Dead Sea (Eugster and Hardie, 1978; Nissenbaum, 1980). Seals can range in thickness from 50 m of trona in Lake Magadi to a lamina of clay deposited in one algal bloom. Thin clay seals are common in tectonic lakes. The spring algal bloom in temperate lakes (Russell-Hunter, 1970) or the bloom at the beginning of the rainy season in tropical lakes probably can add a relatively thick lamina. Tightly oppressed algal cells can be impervious to even bacteria (Bradley and Beard, 1969). Clays formed by flocculation where freshwater rivers enter alkaline lakes in the East African rift (Fig. 3) have been studied by Isaac (1967) and Jones et al. (1977). Structural enhanced
and stratigraphic
by rift processes.
traps are ubiquitous
The seismic sections
in rifts because
entrapment
taken across Lakes Tanganyika
is and
Kivu identified many stratigraphic and structural traps (Degens et al., 1971, 1973). Drape-folds have been noted to cross blocks thereby forming combined structuralstratigraphic traps (Cooke, 1957). facies changes are rapid. A good tectonic lake in time can be seen in are common in rifts because fault
Stratigraphic traps tend to be common because example of complex facies changes around a Roehler (1974, figs. 1, 2, and 4). Structural traps movements forming such traps can take place
annually (Solonenko, 1978). Faults form three kinds of traps: where nonporous and porous beds are brought into contact; where charged groundwaters carry particulate and dissolved elements along faults and seal them; and where a layer of impermeable clay gouge forms (Freeze and Cherry, 1979). Petroleum accumulations in ancient rift lakes are found in sealed reservoirs and in stratigraphic
and structural
traps. In the Karamay
field of China, petroleum
produc-
tion is from alluvial Angola
consist
fans (Chiyi,
1981). The seals in Angola 1974: Brice and Pardo, lacustrine
and Gabon
1981). Structural
1967: Ghignone
POSTDEPOSITIONAL
Petroleum
Wisconsin interferes
carbonate
bars, are the loci of petroleum
Brazil (Bauer.
rift (Braile Mississippi
1981). The reservoir
of shallow-water
are salts from later marine and stratigraphic accumulations
and Andrade,
MOBILIZATION
invasions
(Brink,
traps, as well as offshore in the Recancavo
basin
in
deposits
ELEMENTS
of rifts. In the reactivated
Reelfoot
et al., 1982), inclusions within fluorite, sphalerite, and galena of the Valley ores of Illinois, Kentucky, Missouri, Oklahoma, Tennessee, and contain petroleum (Roedder, 1979; written commun., 1982). Petroleum with processing of copper ore mined in the Midcontinent rift at White
Pine, Michigan. The presence of petroleum in metallic petroleum reservoir rocks and ore host rocks are sometimes petroleum Metals
basin in
(Brice and Pardo,
1970).
OF METALLIC‘
has been noted in metallic
rocks in the Cabinda
and fluvial sandstones
deposits suggests that one and the same when
and metals migrated to stratigraphic and structural traps in rift valleys. appear to be mobilized in coals also. In many coal beds in the Dan
River-Danville, Deep River, and Richmond basins in the Newark rift, megascopic pyrite is found in cleats (joints). Much of this pyrite may have been deposited initially as point-particle framboids from bacterial metabolism of S-bearing organic tissues. Some of the round, octahedral, and pyritohedral holes in the organic matter forming the coal suggest loci from which pyrite was leached (Robbins, 1982). Upstream from the Mississippi Valley ore deposits are coals bearing the same mineral suite as the ore deposits. The galena and sphalerite are concentrated along cleats in the coal (Cobb,
1981).
The relationship between metal deposits and organic fuel deposits at White Pine or in the Mississippi Valley deposits could be fortuitous or could be genetic. The pathway to a fortuitous relationship could include high heat flow that cooks organic tissues in lacustrine shale and injects ore-forming hydrothermal fluids along faults. A more speculative viewpoint is that the metals are attached to and are part of the dispersed organic tissues, and that they are mobilized during petroleum generation and coal maturation. Along either pathway, the mobilization of the metals would be postdepositional. Both water (“brines”) tional
environments
and petroleum
of rifts.
Pyrolytic
are available heating
to move metals in postdeposi-
reactions,
rank
changes
in coal
(Enkohlungsprung), heated groundwater flow, and dehydration and reordering of clays produce water (Burst, 1969; Stach et al., 1975). This water ranges from 6.3 to 8.6 in pH (White et al., 1963). It may be acid due to release of H,S, CO,, carboxylic acids, and phenyl hydroxides during petroleum formation and coal maturation. Or it may be alkaline due to large amounts of solutes in the brines. Thirty elements, including Cu, Pb, and Zn, have been analyzed in oil-field brines (White et al., 1963:
651
Fletcher
and Collins,
1974). Valkovic
(1978) and Bergerioux
and Zikovsky
compiled a list of 50 elements, including Cu, Pb, and Zn, in petroleum. If there is any validity in this speculation on the relationship between and metal deposits, then one might predict that a lacustrine sequence organic tissues are still yellow will not have associated sedimentary deposits.
fossil fuel
in which the Cu-Pb-Zn
A test of the idea might be a study of the shallow Messel lakebeds
in the Rhine
rift (Ltittig,
the bisaccate
pollen
1980) where the sheet-like
(1978)
tissues are medium
(Eocene)
yellow and
grains are light yellow.
CONCLUSIONS
The interaction of physical, chemical, and biological processes has produced accumulations of precursors of fossil fuels and concentrations of metals in active and ancient rifts all over the world. Table IV shows that petroleum, coal, uranium, phosphate, Cu-Pb-Zn, and barite deposits are ubiquitous in rifted environments where lakebeds are also present. There is predictive value for locating
resources
using the understanding
of how
various processes interacted. During the active stages of rift development, sediments containing fossil-fuel precursors and metals would have accumulated at different places in the same rift system (Fig. 2). Peat might have been deposited primarily in swamps associated with river deltas and also along lake embayments and floodplains of inflowing rivers. Petroleum and phosphate precursors might have been deposited in anoxic parts of lakes. Depending on the trace elements in the rocks in the watershed
and in the rocks below the basin,
minerals
or organic
complexes
bearing
U, Cu, and Pb might have precipitated at oxygenated-anoxic interfaces along lake margins or within lakes, Cu, Pb, and Zn sulfides might have been deposited in anoxic
waters, principally
that represented
near source hot springs.
a zone of shifting
Barite might be found in a layer
anaerobiosis.
In each instance, local conditions could be expected to affect the presence of any one of the entire suite of deposits. An environment where lakes were oxygenated to the bottom would contain none. Oxygenated lakes do not leave black sediments, so this resource-poor environment would be difficult to recognize as a lake deposit. Barite probably would not be concentrated along a zone in a lake oxygenated to the bottom today such as Lake Baikal. No buildup of H,S from the putrefication of S-bearing organic tissues could be expected in environments where the anaerobic microbes are excluded. The lack of widespread oxidized-reduced interfaces means uranium-humate complexes would not have precipitated. Phosphate in organic tissues would not accumulate in oxygenated sediments because oxygen-requiring bioturbators would return it to the water column. The benthic organisms that sift the bottom muds for organic molecules also would have eliminated the petroleum-precursor molecules. Larger-scale
processes
also affect
the suite
of deposits.
Plants
may
not
have
652
invaded
the land until the Ordovician
metabolic limited
wastes until
in time. Highly
the Silurian alkaline
or Silurian, or Devonian,
and may not have put on lignified so coal deposits
lakes do not seem to preserve
plant
from plants
are
tissues, so coal
was not found associated with the saline lakebed deposits of the northernmost basins in the Newark rift. Petroleum can be eliminated in time by temperatures in excess of 2OO”C, so petroleum
obviously
would
not be expected
associated
with the meta-
morphosed Mount Isa deposit in the Leichhardt River rift. Prospecting for petroleum in rift basins is not simple. Because
heat flow is not
uniform during rifting, petroleum generation probably is not uniform in active rift basins. Heat flow in the form of regional burial metamorphism presumably would be uniform after the death of a rift system and the burial of its sediments. A test well drilled on a former hot spot would encounter evidence of cooked petroleum or black organic tissues. A few kilometers away, another well could encounter petroleum. Coal deposits in ancient rift systems might be useful for prospecting for metal deposits. The vegetation of the swamp communities can act as a filter of metals carried into the swamp environment, somewhat like activated charcoal. Even after peat is buried and changes from lignite to coal, metals can cling to coalified tissues or be concentrated along cleats. Therefore, coal might be a very good indicator of metals that were available in the watershed of tectonic lakes and swamps and might have been deposited within the lakebeds from other rivers that first did not drain through a swamp. This paper necessarily represents a cursory look at very complex issues. New ideas presented here remain to be tested rigorously. Hopefully. some light has been shed on genetic, temporal, and spatial relationships between these deposits by using modern analogs, and use can be made of these ideas in the search for fossil fuels and sedimentary metallic mineral deposits in active and ancient rifts. ACKNOWLEDGEMENTS
Many of the ideas in this paper were influenced by the thoughts of A.J. Bodenlos, W.B. Cannon, B. Cornet, U.M. Cowgill, J.P. D’Agostino. K.A. Haberyan, A.S. Iberall,
C.D. Masters,
T.J. Schmidt,
G. Scott, J.F. Windolph,
Jr., R.E. Weems, and
D.G. Ziegler. Samples for this study were provided by: G. Bain, P.B. Barton, B. Cornet, A.J. Froelich, Gulf Oil Corporation, Dieter Kock, K.Y. Lee, J.A. Sanders, G. Scott, Solite Corp. of Richmond, P.A. Thayer, A. Traverse, R.E. Weems, and D.G. Ziegler.
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