Geology

doi: 10.1130/0091-7613(1990)018<0533:PCOSRO>2.3.CO;2 1990;18;533-536Geology

 C. Blaine Cecil Paleoclimate controls on stratigraphic repetition of chemical and siliciclastic rocks  

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Paleoclimate controls on stratigraphie repetition of chemical and siliciclastic rocks

C. Blaine Cecil U.S. Geological Survey, Reston, Virginia 22092

ABSTRACT Climate is a primary control on sediment flux from continental sources into sedimentary

systems. In warm climates, siliciclastic input is greatest under highly seasonal rainfall. Nonsea-sonal conditions favor formation of "end member" chemical rocks; perennially wet climates are conducive to coal formation, whereas dry climates produce carbonates and/or evaporites. Stratigraphic repetition of siliciclastic and chemical rocks therefore appears to be related to paleoclimate cycles as well as to transgressive-regressive events and tectonics.

INTRODUCTION Cyclic sedimentation has generally been in-

terpreted on the basis of depositional, transgres-sive-regressive, or tectonic models. However, models based on physical processes generally do not explain the stratigraphic distribution of sili-ciclastic and chemical rocks. For example, in the central Appalachian basin the Mississippian System rocks exhibit transgressive-regressive cy-cles, tectonic setting, and depositional environ-ments similar to those of the Pennsylvanian System (Donaldson et al., 1985). The Mississip-pian System is, however, almost devoid of coal beds, in marked contrast to the Pennsylvanian. This contrast has been attributed to climate change (White, 1925; Cecil et al., 1985; Donaldson et al., 1985). Because climate is an important control on sediment flux (both silici-clastic and chemical) into shelf margin, epicon-tinental shelf, and terrestrial sedimentary se-quences, the stratigraphy of such sequences must be controlled by climate as well as by base level.

CLIMATE-CHANGE MODEL The model developed here primarily encom-

passes warm climates, defined herein as gener-ally frost-free. This model evaluated paleocli-mate cycles on the basis of stratigraphic repetition of laterally extensive lithologic units, the origin of which was sensitive to climate. For example, nonmarine rocks of chemical origin such as mappable coal beds and/or leached pa-leosols represent relatively wet periods of pa-leoclimate, whereas limestone, dolomite, evapo-rites, and caliche represent periods of relative dryness.

Ideal conditions for peat and/or coal forma-tion occur under relatively wet climates where siliciclastic and dissolved-sediment load is min-imal because of leached soils, maximum vegeta-tive cover, and/or sediment-denuded upland areas (Table 1; Fig. 1). Most, if not all, minable coal beds are the result of peat that formed under such conditions. In contrast, the ideal condition for siliciclastic input into a sedimen-

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Figure 1. Sedimentological response to cli-mate change. A: Probability for clastic input in response to climate wetness (adapted from Wilson, 1973). B: Conditions for formation of chemical sediments in response to climate wetness.

TABLE 1. RESPONSES TO N0NSEAS0NAL AND SEASONAL RAINFALL UNDER TROPICAL AND SUBTROPICAL TEMPERATURES

Variable Tropical Rainy Lonq wet/short dry Wet-dry Semi arid Arid

Rainfal l High, nonseasonal Short dry season Extreme seasonality Short wet season Arid

Vegetation Rain forest Forests Grasslands Steppes Shrubs

Chemical weathering products and soi ls

Intense High-Al clays, quartz histosols, latosols

Intense to moderate; la toso ls , histosols

Moderate to res t r ic ted ; ver t i so ls , histosols?

Minimal ; vert isols

Very low; ar id iso ls

Annual erosion Bed load Suspended load Dissolved load

Highly rest r ic ted Very low Very low Very low

Restricted to moderate Low to moderate Low to moderate Low to moderate

Intense Very high Very high Moderate

Moderate to rest r ic ted Moderate Moderate High

Restricted Very low Very low Low

Continental sedimentary Response S i l i c i c l a s t i c input

restr ic ted Chemical

Highly rest r ic ted

Domed-peat deposits

Restri cted

Planar peat

Greatest

Planar peat?

Moderate

Carbonates

Highly res t r ic ted

Evaporites

Note: Rainfal l patterns and responses are gradational and highly variable.

GEOLOGY, v. 18, p. 533-536, June 1990 533

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tary basin apparently occurs under seasonal rainfall (Table 1; Fig. 1), because such seasonal-ity restricts vegetative cover in upland areas (Langbein and Schumm, 1958; Schumm, 1968; Wilson, 1973; Ziegler et al., 1987). If rainfall is further diminished during a long dry and short wet seasonal phase, the sedimentary response is reduced erosion and clastic transport and in-creased formation of pedogenic and lacustrine carbonates (Table 1; Fig. 1). Further drying (semiarid and arid conditions) produces evapo-rites, and the potential for water-transported clastics is restricted to catastrophic flood events (Table 1; Fig. 1).

CLIMATE-CHANGE CLASSIFICATION The primary purpose of this paper is to inter-

pret paleoclimate change on the basis of the stratigraphic repetition of climate-sensitive rocks. A climate-change classification that takes into account both long-term climate change and shorter-term climate cycles is shown in Table 2.

Long-term change is ascribed to continents moving through latitudes (Schutter and Heckel, 1985), a process that may require several mil-lions of years and may or may not be cyclic. Orogenesis may also affect climate over millions of years (Ruddimann and Kutzbach, 1990), and may or may not be cyclic. Orogenesis may lead to either drier or wetter climates, depending upon changes in atmospheric circulation. These effects on sedimentation may be significant on a global (Ruddiman and Kutzbach, 1990) as well as a regional scale in intermontane and foreland basins.

Climate changes associated with 100 or 400 ka cycles in Earth's orbital eccentricity (Pisias and Imbrie, 1987) are classified as intermediate term. Climate fluctuations related to axial tilt (41 ka), and precession cycles (19 and 23 ka) are referred to as short term. Climate changes controlled by orbital forcing are cyclic.

It is beyond the scope of this paper to qualify climate cycles that have a duration less than the short-term cycles but greater than seasonality or instantaneous catastrophic storm events. Such climate cycles, referred to as very short term, have a duration of hundreds of years or a few

thousand years. Such cycles are well docu-mented in tropical and subtropical Africa, where the climate has changed from relatively wet to arid during the Holocene (Street and Grove, 1979; Lezine and Casanova, 1989).

CLIMATE CHANGE IN THE CARBONIFEROUS OF THE EASTERN UNITED STATES

Late Pennsylvanian cyclothems in the mid-continent have been attributed to glacio-eustatic changes in sea level; continental glaciation was controlled by climate change induced by orbital forcing (Fischer, 1986; Heckel, 1986). Trans-gressive-regressive cycles in Lower through mid-Middle Pennsylvanian strata in the Appalachian basin may have had a similar origin (Chesnut, 1989). However, the stratigraphic distribution of climate-sensitive rocks in the Carboniferous of the eastern United States indicates that climate changes were not confined to high-latitude gla-cial cycles. Major shifts in low-latitude climate (rainfall) belts accompanied high-latitude glacial cycles. The low-latitude climate cycles for the Carboniferous were probably on the order of the 100 or 400 ka eccentricity cycles corresponding to the periodicity of the transgressive-regressive cycles suggested by Heckel (1986) for the Late Pennsylvanian and by Chesnut (1989) for the Middle Pennsylvanian.

A paleoclimate curve (Fig. 2A) based on stratigraphic (groups and formations) changes in climate-sensitive lithologic units was com-piled for the Appalachian basin. The long-term climate changes (Fig. 2A) are consistent with the paleogeographic reconstructions of Scotese (1986). The eastern United States drifted northward through dry latitudes during the Mis-sissippian Period, into wet latitudes (the paleo-intertropical convergence zone) in the Early Pennsylvanian, and into dry latitudes again in the Late Pennsylvanian.

To examine the Carboniferous climates in greater detail, I applied the climate-change model to individual lithologic units and se-quences (Fig. 2B; Table 1). The lithologic re-sponses seem to be primarily the result of intermediate- and short-term climatic cycles

(Table 2) that often appear to accompany base-level change. Base level was probably controlled by glacio-eustatic transgressive-regressive cycles and, secondarily, by orogenesis.

The influx of coarse siliciclastic sediments in the latest Devonian and earliest Mississippian (Kinderhookian and Osagean) is attributed to a relatively wet climate (K. J. Englund, 1988, per-sonal commun.). During this time the climate varied from relatively dry in the earliest Kinder-hookian to wet-dry seasonal and perhaps at times humid tropical or subtropical in the early Osagean (Fig. 2B). Such climate variations ac-count for the high clastic input and the occur-rence of coal, including minable coal beds, in Osagean-equivalent strata in the Appalachian basin. The climate then became arid in the late Osagean and early Meramecian, when siliciclas-tic input was restricted and minable evaporites were deposited along with marine carbonates in both the Appalachian and Illinois basins. The climate remained relatively dry throughout the transgressive-regressive cycles of the Meramecian and into the early Chesterian in these basins, as indicated by a low input of siliciclastic sediment and the presence of aridisols (caliche and silici-fied subareal crusts; Ettensohn et al., 1988). Later in the Chesterian, the climate became more sea-sonal, and annual rainfall may have increased slightly, as indicated by an increase in siliciclas-tic sediments. Coarse siliciclastic units may have been the result of periods of strongly seasonal rainfall. Paleosols that have characteristics sim-ilar to modern aridisols, vertisols, and lacustrine limestones indicate climates that were semiarid to seasonal during the Late Mississippian. Humid periods are indicated by a few mappable coal beds.

As eastern North America moved into the tropical rainy (nonseasonal) belt in the Early Pennsylvanian, climates varied from tropical rainy to long wet and short dry seasonal (Fig. 2B). This pattern continued through the mid-Middle Pennsylvanian. Coal developed mainly from domed ombrogenous peat deposits (Cecil et al., 1985) that formed during tropical rainy periods. Climate shifts between tropical rainy and wet-dry seasonal during the Early Pennsylvanian (Morrowan) is indicated both in the Appalachian and Illinois basins by mappable coal beds and the influx of siliciclastics, includ-ing deposits of orthoquartzites. These ortho-quartzites may have been derived from spodo-sols that developed under the effects of the tropical rainy conditions.

From the beginning of the Middle Pennsyl-vanian through the mid-Middle Pennsylvanian (Atokan), the tropical rainy part of cycles led to the formation of coal deposits. The coal beds appear to have been best developed during max-imum regression in transgressive-regressive cy-cles (Chesnut, 1989). An increasing frequency of seasonal periods is indicated by a general

TABLE 2. TROPICAL AND SUBTROPICAL CLIMATE CHANGE CLASSIFICATION

RELATIVE DURATION

CAUSE TIME (years)

Long-term Movement of continents through latitudes; Orogenesis

106-10a

105-107

Intermediate-term

100 and 400 ka cycles of orbital eccentricity

105

Short-term cycles in axial tilt and precession

104

Very short-term

not qualified 103

Instantaneous Catastrophic storms 10"2 (weeks, days, hours)

534 GEOLOGY, June 1990

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coarsening-upward sequence in the central Ap-

palachian basin and equivalent Pottsville strata

in the northern Appalachian basin. Beds of

splint coal associated with this coarsening-

upward sequence in the uppermost mid-Middle

Pennsylvanian strata also indicate periods of

seasonal drying. These coal beds are derived

from domed peat deposits that were periodically

subjected to aerobic degradation as a result of a

lowered water table (Eble and Grady, 1988).

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RAINFALL RAINFALL Figure 2. Paleoclimate interpretation for Carboniferous of eastern United States. A: Probable long-term changes except for arid interval in Mississippian, which was probably in response to intermediate- and short-term controls. Modified from Cecil et al. (1985). B: Inferred intermediate- and short-term climate cycles. Early through mid-Middle Pennsylvanian climate cycles ranged from tropical rainy (nonseasonal) to long wet season and short dry season. Ombrogenous domed-peat deposits formed during tropical rainy periods; periods of wet-dry seasonality enhanced input of siliciclastic sediment. From late Middle to end of Pennsylvanian Period, climates cycled from humid subtropical to nearly semiarid. Peat formation was limited to planar topogenous swamps during wetter periods; leached paleosols developed adjacent to these swamps. Lacustrine and pedogenic carbonates (vertisols and aridisols) formed during dry intervals.

Prolonged periods of seasonality then developed

as a result of shifts in climate belts when North

America began to drift out of the intertropical

convergence zone into latitudes with monsoonal

circulation.

As North America continued to move north-

ward during the remainder of the Pennsylvanian

(Desmoinesian through Virgilian), the climate

became increasingly dry (Schutter and Heckel,

1985). Peat formation in the Appalachian, Illi-

nois, and mid-continent basins became restricted

to planar topogenous swamps (Cecil et al.,

1985) that developed during humid, wet

periods. Coal beds derived from such environ-

ments tend to be higher in ash and sulfur than

those derived from domed ombrogenous peat

deposits.

Dry periods within the climate cycles of the

late Middle Pennsylvanian (Desmoinesian) were

of sufficient intensity to form vertisols in positive

areas, lacustrine limestones (often containing

pedogenic features) in adjacent low-lying areas

in the Appalachian basin (Donaldson et al.,

1985), and vertisols in the mid-continent (Schut-

ter and Heckel, 1985). In contrast, high-alumina

clay beds, sometimes referred to as flint clay,

and topogenous peat developed during the wet

phase of climate cycles. The high-alumina clay

deposits formed when conditions were condu-

cive to soil leaching (Schutter and Heckel,

1985), whereas topogenous peat formed in

sediment-starved swamps. Siliciclastic units that

overlie and underlie the coal beds indicate sea-

sonal periods that were transitional between the

wet and dry phases (Fig. 3A).

Long-term drying continued through the Late

Pennsylvanian (Missourian through Virgilian),

as indicated by the appearance of aridisols (ca-

liche), and more abundant vertisols and lacus-

trine carbonates in Upper Pennsylvanian strata.

The absence of well-developed flint clay depos-

its in Upper Pennsylvanian strata is consistent

with this interpretation. Wet intervals are indi-

cated by regional development of paleosols,

which are comparable to modern inceptisols and

ultisols, that appear to have formed on slightly

positive areas contemporaneously with peat

formation. These paleosols grade laterally into

underclays and coal beds. The regional extent of

MIDDLE PENNSYLVANIAN

MAHONING SANDSTONE UFFINGTON SHALE

UPPER FREEPORT COAL BED

UNNAMED SILICICLASTIC UNIT

UPPER FREEPORT LIMESTONE

ALLEGHENY GROUP

UPPER PENNSYLVANIAN

REDSTONE LIMESTONE

PITTSBURGH SANDSTONE

UNNAMED SHALE

PITTSBURGH COAL BED

PITTSBURGH LIMESTONE

UNNAMED SILICICLASTIC UNIT

MONONGAHELA FM/ GROUP

GENERALIZED

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LEACHED SOILS (OXISOLS)

EROSION F L U V I A L S Y S T E M S

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AR ID ISOLS

LACUSTRINE LS

S A N D S T O N E SHALE

COAL

SILTS, S A N D S SEATEARTHS

LACUSTRINE LS

MODEL UPLAND BASIN

RESPONSE RESPONSE CLIMATE

CYCLE

Figure 3. A: Examples of late Middle and Late Pennsylvanian sedimentary cycle. B: General-ized sedimentary response to changes in Late Pennsylvanian paleoclimate. Coal and paleosols exhibiting characteristics of mod-ern inseptisols and oxisols indi-cate relatively wet periods; lacus-trine carbonates, vertisols, and aridosols (caliche) represent dry intervals. Siliciclastics are inter-mediate.

534 GEOLOGY, June 1990

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many of the aridisols, vertisols, and lacustrine carbonates containing pedogenic features indi-cates that climate cycles were associated with water-table (and thus perhaps base-level) change and paleotopography. The aridosols and verti-sols developed during dry intervals when the water table was low, whereas the lacustrine carbonates and interbedded shales were depos-ited during periods when the water table was high. Paleotopography is often reflected by lacustrine carbonates that grade laterally into aridisols. Figure 3B illustrates a generalized climate cycle for the Late Pennsylvanian in the Appalachian basin.

Climate cycles and the better known trans-gressive-regressive cycles led to the stratigraphic repetition of rocks, often referred to as cyclo-thems in the Carboniferous of the eastern United States. However, a given paleoclimate is not correlated with transgressive or regressive events. The climate cycles are interpreted to be intermediate and short term, with a possible long-term orogenic effect in the Appalachian basin, and they were superposed on the long-term cycle caused by drift of North America across the paleoequator (Fig. 2).

APPLICATION OF THE CLIMATE-CHANGE MODEL TO OTHER BASINS

The paleoclimate model can also be applied to other basins. Flores (1986) suggested that the thick Paleocene-Eocene coal beds in the Powder River basin were derived from domed peat deposits. Thus, the climate of the region must have been wet and nonseasonal during periods of domed peat formation, resulting in either heavily vegetated or denuded uplands adjacent to the swamps, which reduced siliciclastic and dissolved-sediment input. Streams may have continued to flow through the swamps without aggradation or contribution of dissolved- or suspended-sediment load, except during inter-vals that were seasonal. Upon a return to sea-sonal conditions, sand transport increased and aggrading systems overwhelmed the swamps in a subsiding basin. This application of the climate-change model to the Powder River basin is consistent with the climate changes in-ferred by Nichols et al. (1989).

Intermontane basins in the Rocky Mountain region (e.g., Green River, Big Horn, and Pice-ance) have a few economic coal deposits of Pa-leocene age that suggest wet periods. Chemical sediments in the Eocene section in these basins include lacustrine limestones, evaporites, and pedogenic carbonates, which indicate a drying trend. A long-term climate change associated with orogenesis, which was accompanied by short- and intermediate-term changes, appears to have been important in these intermontane basins.

Climate cycles as well as base-level changes controlled sedimentation along the Cretaceous seaway of North America. Wetter periods re-sulted in coal formation. More seasonal periods contributed to siliciclastic deposition. Short- and long-term climate cycles accompanied long-term orogenic effects. This interpretation is con-sistent with climate cyclicity in the Late Cretaceous of trans-Pecos Texas, where strati-graphic repetition of paleosols indicates both humid and dry periods (Lehman, 1989).

CONCLUSIONS Climate is a primary allogenic control on sed-

iment flux in most sedimentary systems. Paleo-climate cycles are therefore reflected in the stratigraphic repetition of chemical and silici-clastic sedimentary rocks. Laterally extensive coal beds and/or leached paleosols are indica-tive of wet periods. Carbonate and/or evaporite units, vertisols, and aridisols reflect dry intervals. Siliciclastic flux is greatest under highly seasonal rainfall. Incorporation of paleoclimate analyses into existing allogenic models (transgressive-regressive and tectonic) should enhance our un-derstanding of many terrestrial, shelf margin, and epicontinental-shelf sedimentary sequences.

REFERENCES CITED Cecil, C.B., Stanton, R.W., Neuzil, S.G., Dulong, F.T.,

Ruppert, L.F., and Pierce, B.S., 1985, Paleocli-mate controls on late Paleozoic sedimentation and peat formation in the central Appalachian basin (U.S.A.): International Journal of Coal Geology, v. 5, p. 195-230.

Chesnut, D.R., 1989, Pennsylvanian rocks of the east-ern Kentucky coal field, in Cecil, C.B., and Eble, C.F., eds., Carboniferous geology of the eastern United States (International Geological Con-gress, 28th, field-trip guidebook T143): Wash-ington, D.C., American Geophysical Union, p. 57-60.

Donaldson, A.C., Renton, J.J., and Presley, M.W., 1985, Pennsylvanian deposystems and paleo-climates of the Appalachians: International Jour-nal of Coal Geology, v. 5, p. 167-193.

Eble, C.F., and Grady, W.C., 1988, Palynologic, petrographic and coal-quality characteristics of Middle and Upper Pennsylvanian coal beds: A comparison [abs.]: American Association of Petroleum Geologists Bulletin, v. 72, p. 906.

Ettensohn, F.J., Dever, G.R., Jr., and Grow, G.S., 1988, A paleosol interpretation for profiles ex-hibiting subaerial exposure "crusts" from the Mississippian of the Appalachian basin, in Rein-hardt, J., and Sigleo, W.R., eds., Paleosols and weathering through geologic time: Principles and applications: Geological Society of America Spe-cial Paper 216, p. 49-79.

Fischer, A.G., 1986, Climate rhythms recorded in strata: Annual Review of Earth and Planetary Sciences, v. 14, p. 351-376.

Flores, R.M., 1986, Styles of coal deposition in Ter-tiary alluvial deposits, Powder River basin, Mon-tana and Wyoming, in Lyons, P.C., and Rice, C.L., eds., Paleoenvironments and tectonic con-trols in coal-bearing basins of the United States: Geological Society of America Special Paper 210, p. 79-102.

Heckel, P.H., 1986, Sea-level curve for Pennsylvanian eustatic marine transgressive-regressive deposi-tional cycles along midcontinent outcrop belt, North America: Geology, v. 14, p. 330-334.

Langbein, W.B., and Schumm, S.A., 1958, Yield of sediment in relation to mean annual precipita-tion: Eos (Transactions, American Geophysical Union), v. 39, p. 1076-1084.

Lehman, T.M., 1989, Upper Cretaceous (Maastrich-tian) paleosols in trans-Pecos Texas: Geological Society of America Bulletin, v. 101, p. 188-203.

Lezine, A-M., and Casanova, J., 1989, Pollen and hydrological evidence for the interpretation of past climates in tropical west Africa during the Holocene: Quaternary Science Reviews, v. 8, p. 45-55.

Nichols, D.J., Wolfe, J.A., and Pocknall, D.T., 1989, Latest Cretaceous and earliest Tertiary history of vegetation in the Powder River basin, Montana and Wyoming, in Flores, R.M., Warwick, P.D., and Moore, T., eds., Tertiary and Cretaceous coals of the Rocky Mountain region (Interna-tional Geological Congress, 28th, field-trip guidebook T132): Washington, D.C., American Geophysical Union, p. 28-33.

Pisias, N.G., and Imbrie, J., 1987, Orbital geometry, C0 2 , and Pleistocene climate: Oceanus, v. 29, p. 43-49.

Ruddimann, W.F., and Kutzbach, J.E., 1990, Late Cenozoic plateau uplift and climate change: Edinburgh, Scotland, Royal Society of Edinburgh (in press).

Schumm, S.A., 1968, Speculations concerning pa-leohydrologic controls of terrestrial sedimenta-tion: Geological Society of America, v. 59, p. 1573-1588.

Schutter, S.R., and Heckel, P.H., 1985, Missourian (early Late Pennsylvanian) climate in midconti-nent North America: International Journal of Coal Geology, v. 5, p. 111-138.

Scotese, C., 1986, Atlas of Paleozoic basemaps: Pa-leoceanographic mapping project: Austin, Uni-versity of Texas Institute for Geophysics, Technical Report 66, p. 1-23.

Street, F.A., and Grove, A.T., 1979, Global maps of lake-level fluctuations since 30,000 yrs. BP: Qua-ternary Research, v. 12, p. 83-118.

White, D., 1925, Environmental conditions of deposi-tion of coal: American Institute of Mining and Metallurgical Engineers Transactions, v. 71, p. 3-34.

Wilson, L., 1973, Variations in mean annual sediment yield as a function of mean annual precipitation: American Journal of Science, v. 273, p. 335-349.

Ziegler, A.M., Raymond, A.L., Gierlowski, T.C., Hor-rell, M.A., Rowley, D.B., and Lottes, A.L., 1987, Coal climate and terrestrial productivity: The present and Early Cretaceous compared, in Scott, A.C., ed., Coal and coal-bearing strata: Recent advances: Geological Society of London Special Publication 2, p. 25-49.

Manuscript received August 7, 1989 Revised manuscript received November 30, 1989 Manuscript accepted December 8, 1989

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