faces to demonstrate that Late Pennsylvanian glacioeustasy reached amplitudes of at least. 80 m, and likely exceeded 100 m. These data rep- resent the first ...
Amplitudes of Late Pennsylvanian glacioeustasy Gerilyn S. Soreghan School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA Katherine A. Giles Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA nary ice sheets to the late Carboniferous, Crowley and Baum (1991) placed their best estimate of glacioeustatic amplitudes at 60 +/–15 m . All of these methods typically employ geologically reasonable, but imprecise assumptions, e.g., analogy to modern environments, ice sheets or tropical ocean temperature structure. Goldstein and Franseen’s (1995) quantitative approach to relative sea-level history involves locating evidence for ancient positions of sea level and subsequently determining elevational differences in those positions. This approach involves few, typically well-constrained assumptions, and provides a minimum amplitude of relative sea level change clearly demonstrable by field relations. For glacioeustatically influenced strata, this method permits quantitative measurement of eustatic fall, which constrains amplitudes of glacioeustasy. Application of this method, however, involves locating a subject system that (1) contains facies amenable to interpreting ancient sea-level positions, (2) displays significant paleotopography over a short lateral distance, and (3) is well exposed in three dimensions, allowing lateral tracing of surfaces. Shallow-water carbonate bioherms are ideal for such analysis. In Upper Pennsylvanian (early-middle Virgilian) bioherms of the eastern Orogrande basin (Sacramento Mountains, New Mexico; Fig. 1), Wilson (1967) and Goldstein (1988) traced preserved relief on exposure surfaces to demonstrate 30 to possibly 50 m of eustatic fall in this system. We targeted a
ABSTRACT The late Paleozoic is well documented as a time of significant continental glaciation, but the extents of the glaciation and attendant glacioeustasy are not well constrained because precise amplitudes of eustasy are difficult to extract from the stratigraphic record. In this paper, we use preserved relief on ancient subaerial exposure surfaces of large algal bioherms to demonstrate directly that Late Pennsylvanian glacioeustasy reached minimum amplitudes of 80 m and probably exceeded 100 m. Upper Paleozoic algal bioherms accreted predominantly during sea-level falls, but also during sea-level rises and highstands, and were capable of remarkably rapid growth rates. Eustatic amplitudes in excess of 100 m approach amounts documented for the Pleistocene, and place constraints on models for Gondwanan ice volume, climate dynamics, and potential character and magnitude of glacioclimatic fluctuations. INTRODUCTION Documentation of extensive upper Paleozoic glacial deposits in the Gondwanan continents, and coeval cyclothemic strata in the Laurasian continents, has established that the late Paleozoic was a time of large-scale continental glaciation and associated glacioeustasy (e.g., Crowell, 1978; Heckel, 1977). The extents of Gondwanan ice and corresponding eustatic excursions, however, are difficult to estimate precisely. Many have suggested that the late Paleozoic ice-house climate was similar in magnitude to that of the Pleistocene, with glacioeustatic amplitudes that possibly exceeded 100 m (e.g., Heckel, 1977). Actual measurement of paleoeustatic amplitude, however, remains elusive owing in part to the difficulty in preserving unambiguous evidence for changes in water depth, as well as the problem of distinguishing eustatic from relative sea-level change (Burton et al, 1987; Bond and Kominz, 1991; Soreghan and Dickinson, 1994). Amplitudes of late Paleozoic glacioeustasy have been estimated using a variety of techniques, but results typically hinge upon ill-constrained assumptions. The pinningpoint method of Goldstein and Franseen (1995), however, employs unambiguous field relations, e.g., preserved relief on an ancient subaerial exposure surface, to quantitatively reconstruct relative sea-level history. For glacioeustatic cycles, pin-point analysis can be used to estimate minimum amounts of sea-level fall, and thus amplitudes of glacioeustasy. In this paper, we use field measurement of preserved relief on ancient subaerial exposure surfaces to demonstrate that Late Pennsylvanian glacioeustasy reached amplitudes of at least 80 m, and likely exceeded 100 m. These data represent the first unambiguous documentation of glacioeustatic amplitudes of this magnitude, and have significance for determining: (1) extent of Late Pennsylvanian Gondwanan ice volumes and associated climate dynamics, (2) projected magnitudes of glacioclimatic change, and (3) models of late Paleozoic stratigraphic cyclicity.
METHODS OF ESTIMATING PALEOEUSTATIC AMPLITUDES, AND APPLICATIONS TO THE LATE PALEOZOIC Late Paleozoic eustatic amplitudes have been estimated using analyses of facies and/or stratal thicknesses, geochemical attributes, and ice-area estimates, and have ranged from a few tens of meters to >200 m. Heckel (1977), for example, argued for at least 100 m amplitudes on the basis of a subthermocline interpretation for the phosphatic black shale facies of Middle to Upper Pennsylvanian midcontinent cyclothems. Maynard and Leeder (1992) employed measurements of stratal thicknesses in Lower Pennsylvanian cycles of the Pennine basin to calculate a minimum of 42 m of eustatic change. Adlis et al. (1988) used oxygen isotopes as evidence for 70 m of change in two Texas cyclothems. Applying area-volume relations developed for Quater-
,
,
,
,
,
,
,
, 106˚W
U.S. Army White Sands Missile Range
0
10
20
30
km
↑
N
33˚N
Sacramento Mountains
Hembrillo Canyon
S an A n d
Alamogordo
res
NM
Pedernal Uplift
Or
Area Shown
Pedr e Bas gos in
a
Geology; March 1999; v. 27; no. 3; p. 255–258; 2 figures; 1 table.
Organ Mountains
De Ba law s
e ar in
Las Cruces
e a nd og r i n s Ba
ntains Mou
AZ
Figure 1. Location map with inset of late Paleozoic paleogeography in Orogrande basin region. Studied bioherms are exposed in Hembrillo Canyon, San Andres Mountains (AZ—Arizona, NM— New Mexico). Bioherms of Sacramento Mountains are located immediately east of Alamogordo. Modified from Algeo et al. (1991).
Diablo Platform
255
biohermal complex of the western Orogrande basin (San Andres Mountains) that has significant relief (>100 m), is superbly exposed, and is Late Pennsylvanian (middle to late Virgilian) in age, thus coincident with the time of projected maximal Gondwanan ice (Crowell, 1995). BIOHERMS OF THE WESTERN OROGRANDE BASIN The Orogrande basin (southern New Mexico) formed adjacent to the Pedernal uplift at the southwestern terminus of the Ancestral Rocky Mountains systems, and accumulated nearly 2500 m of
Figure 2. A: Photomosaic of eastern flank of studied bioherm. B: Line sketch of photo showing correlations of paleosubaerial exposure surfaces from sections 5 through 10. C: Cross section of measured sections; Rusty Bed marker unit is datum horizon. Confidence level of correlations as follows: solid line—high; long dashes—high to moderate; dotted—low. Owing to erosion and cover, surfaces 12 and 13 cannot be physically traced between sections 8 and 11, so correlation here assumes absence of cycle amalgamation or erosion. Fusulinid data from lower mound horizons indicate middle (to late?) Virgilian age (personal commun. G. Wilde, 1996, and G. Wahlman, 1998).
B
12
13
throughout the region. The strata within the immediate study area dip gently (14° WSW), and exhibit no structural disruption. The studied interval ranges to 130 m thick (datum to mound crest), and consists of carbonate “cycles” (high-frequency sequences) truncated by surfaces of ancient subaerial exposure. In mound core and flank regions, cycles consist of carbonate boundstones (algal and peloidal) and packstones, respectively. Exposure surfaces commonly are developed directly atop subtidal facies. We interpret these cycles to be of glacioeustatic origin on the basis of correlation with coeval strata studied
14
11 10
9 7
8
6
5 4 3 2 1
Top Rusty Bed
5
6
7
1
2
25 m
11
2
1
25 m
3
4
25 m
5
50 m
12
50 m
9
8
~ { z } ~ z | } ~ { y { } z , | } ~ z ~ | { y { y } z , , | } ~ z ~ | { y { } y z z , , | } ~ z ~ | { y } { y } z z , , | } ~ z ~ | } { y { y } z z , , | } } C
10 9 8 7 6 5 4 3
256
mixed carbonate-siliciclastic strata during Pennsylvanian-Permian subsidence (Kottlowski, 1963; Fig. 1). We targeted Upper Pennsylvanian algal bioherms in the Panther Seep Formation of the western Orogrande basin exposed in Hembrillo Canyon (San Andres Mountains; Fig. 2). Using standard methods (compass with Jacob staff and/or tape), we measured and described 11 closely spaced (25 to 125 m) sections spanning the bioherm periphery and extending into fully “off-mound” regions (Fig. 2). We used as our datum a marker horizon (Rusty Bed, Fig. 2) that exhibits negligible paleorelief and is traceable
6
y , | y , z | } | y y , , | } z | y y , , | } |y z y , , | } | z y y , , | } | 7
50 m
13
75 m
~{|y, } |
{ y , , ~ | { y ~ | { y | y , , ~ | { y | y , , ~ | { y | y , , 9
8
50 m
10
10
75 m
11
125 m
10 m
subtidal boundstone (core-flank) or wackestone (flank-offmound)
silty carbonate mudstone
0
pack/grainstone commonly with exposure features
covered (mud/siltstone)
14
13 12
Top Rusty Bed
~{|y, ~{|y, 13
12
10 11 9 8 7 6 5 4 3 2 1
GEOLOGY, March 1999
elsewhere in the region (Soreghan, 1994). Features that typically mark exposure surfaces include one or more of the following: (1) laminar crusts (calcrete; typically in flank and off-mound regions), (2) rhizoliths, (3) petrographic features (e.g., alveolar textures, circumgranular cracking, vadose-zone diagenesis), and (4) extensive (commonly pervasive) dolomitization associated with one or more of the above features. To precisely correlate the measured sections, we traced exposure surfaces in the field where possible, and used photomosaics where physical tracing was precluded by inaccessibility. Our facies and biostratigraphic data support our final correlations (Fig. 2). Figure 2C, plotted using the Rusty Bed datum, illustrates that preserved relief directly measurable on ancient subaerial exposure surfaces varies from
