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Speculative interpretations of Triassic climates have long played a central role in Triassic studies. Pre-1970 work focused primarily on classic arguments on the origin of red beds, as sum- marized by Krynine (1950) and Van Houten (1973). More re- cently, paleoclimatological research has focused on dynamic climatological models, such as those presented by Robinson (1973). This paradigm shift is in part due to the understanding that the red color of sedimentary rocks probably reflects mineral alteration through time of both detrital and authigenic ferric ox- ides (Van Houten, 1973). Recent research has concentrated on four interrelated climatic factors: (1) a general atmospheric circulation model, (2) northerly drift of plates across lines of latitude, (3) impact of rift topography, and (4) periodic and cyclic climate changes interpreted according to the Milankovitch astronomical theory of climate change. Large continents profoundly modify their climates through the way in which they respond to seasonal variations in solar radiation. Landmasses typically have low heat capacity and thus warm and cool rapidly, transferring these effects to air masses above. Eurasia and North America today, for example, have marked seasonal variations of temperature that cause concomi- tant changes in pressure gradient forces and in wind direction. By analogy, Pangaea probably experienced an effective monsoonal circulation. Figure 2 is an interpretive sketch of summer climatic condi- W. Manspeizer and Others tions over Laurasia and of winter conditions over Gondwanaland near the beginning of the breakup of Pangaea (Robinson, 1973). In this interpretation the rift basins were dominated by a Tethyan high-pressure cell that pumped warm, moist air into the continent. Hot dry summer winds, blowing from the west across the continent, probably had little impact on the rift basins, which lay east of the Intertropical Convergence Zone. During the Laurasian winter, however, the proto-Atlantic basins would have been dominated by a subtropical high-pressure cell, while Gondwanaland was overlain by the Intertropical Convergence Zone. Winds, blowing clockwise about the high-pressure cell, typically brought cold, dry air from aloft across the basins, but on occasion carried cool, moist maritime air from Arctic Canada to the proto- Atlantic region. Rift basins, because of their special topographic characteris- tics, create unusual climates that cannot be explained through a general atmospheric circulation model alone (Manspeizer, 198 1, 1982). The detrital basins on the American plate had substantial relief and extended for long distances across lines of latitude and prevailing wind systems (Fig. 2). As moist air (from either Tethys or Arctic Canada) was uplifted over the shoulders of the rift system, it would have cooled adiabatically, yielding rainwater for high-discharge streams and high-altitude lakes. Conversely, as the air descended into the axis of the rift, it would have warmed adiabatically, becoming hot and dry and thus increasing the rates
DETRITAL BASINS :'->: SALT BASINS 0 0 rn "moo" POTASH SALT BASINS CARBONATE BASINS @ LAND y/1 DDLERITE DIKES fl mNT FR-RE , TRANSFER FAULTS WHITE MOUNTAIN MAGMA ' SERIES NEW ENGLAND SEAMOUNT CHAIN Post-Paleozoic activity THICKNESS On nnim) MAIN SOUTH GEORGIA RIFT 20 ATLANTIS BASIN RIDDLEVILLE-DUNBARTOM-FLORENCE 21 GEORGES BANK DURHAM-SANFORO-WADESBOROCROWBUffi 3,000 22 SCOTIAN-SABLE IS. SUBBASIN FARMVILLE-ROAHOK&SCOTTSBUffi 23 SCOTIAHABENAKI SUBBASIN DAN RNER-OAtMLLE-OAVIE M. 3,200 24 AVALCTWABSON SUBWIN RCHMONC-TAYLWLLE 1,870+ 25 LDSFTANIA CUEPER 6,600 26 J E W GETTYSBURG 9.000 27 BERKANE NEWARK 7.200 28 GUERCIF WRTFORMEERFIM 5.600 29 MOU8A ON SALAUTAMDAFELT GULF OF WNE 30 KEflOUCMEN FUNDY 1,100 31 CENTRAL HIGH ATLAS SCOTlAN-OflPHEUS QRABEN 6.000 32 TBI N'TEST BLAKE PLATEAU S.OOO 38 ARSANA-ES8AOUIRA CAROLINA TROUGH 7.000 34 DOUKKALA BALTIMORE CANYON 8.000 38 BERRICHID N.Y. BIGHT BASIN 8,000 38 KIJEFfWt.KHEM1886T LONG ISLAND BASIN 3.500 37 MIUM NANTUCKET 3.000 38 SENEOU. Figure 3. Lat ikic-~arl~ Jurassic reconstruction of eastern North America and northwestern Africa, outlining the named basins and lithofacies that developed on the Variscan-Alleghany-Mauritanide orogen. Modified from Manspeizer, 1980. Line A-B marks the trend of cross section given in Figure 12. of evaporation and precipitation of gypsum, aragonite, and halite. Although information on precise paleoaltitudes is lacking, this model (orographic) suggests that Triassic-Liassic lacustrine and paludal sedimentation occurred in high-altitude basins, while evaporites formed concurrently in adjacent low-altitude troughs (see Fig. 3). According to the Milankovitch astronomical theory of climate change, periodic climate changes are caused by cycles in the seasonal distribution of sunlight reaching the earth; these cycles, in turn are controlled by the astronomical cycles of celestial me- chanics (Milankovitch, 1941). The periodicity inferred for Trias- sic lacustrine cycles is described later in this chapter by Olsen. NEWARK SUPERGROUP: LATE TRIASSIC-EARLY JURASSIC Figure 3, a pre-drift reconstruction of eastern North Amer- ica and northwest Africa, outlines the major Early Mesozoic basins and lithofacies on the broad Alleghanian, Variscan, and Mauritanide orogenic belt. Two major basin types and lithofacies are recognized: (1) Newark-type detrital rift basins, which typically are exposed onshore as half grabens containing a thick (4 to 8 km) continental clastic-lacustrine and volcanic facies; and (2) evaporite basins, which occur on rift-stage crust seaward of the string of detrital basins (e.g., Georges Bank basin) and/or on a
Page 1 and 2: The Geology of North America Vol. F
Page 3: Post-Paleozoic activity SIBERIAN Fi
Page 7 and 8: similar orientations and/or lie alo
Page 9 and 10: Post-Paleozoic activity NEWARK BASI
Page 11 and 12: Strata within these Mesozoic basins
Page 13 and 14: Post-Paleozoic activity USGS LINES
Page 15 and 16: sic marine tongue was speculated to
Page 17 and 18: contours in red are over Jurassic d
Page 19 and 20: crust. A pronounced north-northeast
Page 21 and 22: for relative contributions from iso
Page 23 and 24: Post-Paleozoic activity CULPEPER LI
Page 25 and 26: the basins and also corresponding w
Page 27 and 28: tons (1963), Thayer and others (197
Page 29 and 30: ank 0 6 I I Post-Paleozoic activity
Page 31 and 32: the north side of the basin. Accord
Page 33 and 34: - 46' - 42O Post-Paleozoic activity
Page 35 and 36: activity was initiated. The initial
Page 37 and 38: percent pyroxene (chiefly augite),
Page 39 and 40: have formed under higher conditions
Page 41 and 42: lines an elongated north-south patt
Page 43 and 44: Several studies of the intrusions w
Page 45 and 46: 0 100 200 300 Miles ^ Figure 27. Ma
Page 47 and 48: tomac Formation. They also report t
Page 49 and 50: REFERENCES CITED ' Post-Paleozoic a
Page 51 and 52: Galton, P. M., 1976, Prosauropod di
Page 53 and 54: - , 1988, Triassic-Jurassic rifting
Page 55 and 56:
Bulletin, v. 95, p. 1176-1 187. Res
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