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Magnetostratigraphy in the Trans-Pecos volcanic field: preliminary results from the Eocene-Oligocene Vieja group Margaret M. Testarmata and Wulf A. Gose, 1980, pp. 101-103 in: Trans Pecos Region (West Texas), Dickerson, P. W.; Hoffer, J. M.; Callender, J. F.; [eds.], New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, 308 p.
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MARGARET M. TESTARMATA and WULF A. GOSE Geophysics Laboratory Marine Science Institute University of Texas at Austin 700 The Strand Galveston, Texas 77550
INTRODUCTION establishing the reversal stratigraphy of the Vieja Group, which can The Vieja Group, which accumulated on the western edge of the then serve as a reference section for the region. Trans-Pecos volcanic field, consists of 600 m of volcaniclastic A broader objective of this study is to correlate the reversal stra- sediments subdivided by several interbedded extrusive units (fig. tigraphy of the Vieja Group with the marine magnetic anomaly se- 1). Radiometric dates (McDowell, 1978), diagnostic mammal fossils quence. Such a correlation would test the consistency between (Wilson and others, 1968; Wilson, 1978), and geologic setting sug- marine and continent dating: K-Ar dates from the Vieja Group gest that the Vieja Group represents a continuous record of depo- could be compared with ages assigned to the marine magnetic sition from 40 to 30 m.y.b.p., thereby making it ideal for a magne- time scale; and Eocene-Oligocene boundaries, defined by mam- tostratigraphic study of latest Eocene through early Oligocene. mal faunas in the Vieja Group and marine faunas in the oceans, Initial results of this study have been published in another guide- could also be compared. book (Testarmata and Gose, 1979). Since then, we have more than doubled the number of samples in order to fill in data gaps and to DATA COLLECTION extend the section for a firmer correlation. More than one thousand samples were collected from the Vieja Group in the Candelaria area (30°08 N, 104°41 W). By collecting OBJECTIVES different parts of the section at different sites and using contacts Lithologic continuity is lacking in the Trans-Pecos volcanic field, and marker beds to tie sections together, complete coverage from and biostratigraphic and radiometric dating have been able to pro- the lower part of the Colmena Formation to the Mitchell Mesa Ig- vide only a general correlation. Magnetostratigraphy applied in nimbrite was achieved. Oriented cores were collected with a por- conjunction with these other techniques may prove a useful cor- table, gasoline-powered drill. In sedimentary rocks, samples were relation tool. This study is intended to investigate this possibility by collected in stratigraphic sequence at an average spacing of 1 m. Approximately 10 samples were collected from each extrusive unit with sampling distributed throughout the thickness of the VIEJA GROUP unit. Samples were measured with a cryogenic magnetometer and be- RADIOMETRIC MAMMAL tween measurements were stored in a magnetically shielded DATES FAUNAS room in a field of a few hundred gammas. Standard techniques of PE TA N BASALT \ alternating field and thermal demagnetization were used to MITCHELL MESA IGNIM BR ITE 31.5 = 0.7 M.Y. (18) remove unstable components of magnetization. Most sedimentary samples displayed similar changes in direction during demagnetization. A strong viscous normal overprint domi- CAPOTE MTN. ASH SPRING FORMATION nating the natural remanent magnetization (NRM) was removed after demagnetization to 200°C. The stable directions revealed at 200°C were more clearly defined after heating to 400-500°C, and AIRSTRIP these data were used for determination of magnetic polarity. z Ford SS Heating of samples above 575°C, the Curie temperature of 0 0 36.5=1.2 c r 0 magnetite, usually resulted in a scatter of directions (fig. 2), indi- BRACK S RHYOL ITE 36.8 O cating that magnetite is the carrier of the stable remanence. LITTLE Upp er Marker —1 EGYPT CHAMBERS 0 Measurements of magnetization versus temperature and examina- FORMATION tion of polished sections support the dominance of magnetite as Lo wer Marker PORVENIR the carrier of remanence. As magnetite is not an authigenic BUCKSHOT IGNIMBRITE 38.6=1.2, 36.2 mineral, this implies that the sedimentary rocks carry a detrital 35.2=2.3, 36.4 remanent magnetization (DRM) acquired at the time of deposition. COLMENA FORMATION CANDELARIA Samples from extrusive rocks in most cases showed stable NRM 1— Z directions which changed very little upon demagnetization. They GILL BRECCIA 40.0=2.0 O L A display a thermal remanent magnetization (TRM) acquired at the JEFF CONGLOMERATE time of extrusion.
Figure 1. Stratigraphy, K-Ar dates, and mammal faunas of the Vieja INTERPRETATION Group. After Walton (1972). Age of Mitchell Mesa Ignimbrite is the Magnetic polarities of most samples from the upper 400 m have average of 18 dates. been determined, and analysis of the lower portion is now under- 102 TESTARMATA and GOSE
CHAMBERS FM (TCC01) — MEXICAN SPRINGS NRM TD 2 2 0 m
40—
30—
20— N TD 560 TD 600
0 0 180 —90 +90 0 DECLINATION INCLINATION
Figure 3. Paleomagnetic data and polarity interpretation for part of the Chambers Formation. Inner columns show initial data; outer columns, data after dense sampling of selected zones. Declination and inclination values are after demagnetization to 450 to 500°C. Black indicates normal polarity; white, reverse. Same convention CHAMBERS FM FM applies to other figures. TCH 0 6 (A) Figure 2. Equal-area stereographic projections of NRM directions and directions after demagnetization at 220°C, 560°C, and 600°C alies 12 and 13, indicating a long period of reverse polarity from 33 for part of the Chambers Formation. Open symbols are in upper to 35.5 m.y. The long predominantly reverse zone in the Vieja hemisphere, crosses in lower hemisphere. Group is tentatively identified as correlating to this time period. This correlation is constrained by radiometric dates and fossil ages, as well as the fact that 400 m of Vieja sediments should represent way. When magnetic polarities are plotted in stratigraphic se- approximately 6 m.y., if 600 m were deposited in 10 m.y. If this quence, many thin polarity zones are evident. Because most of correlation is assumed correct, a comparison can be made be- these zones were based upon only one or two samples, selected tween marine and continental age dates. The radiometric dates zones were resampled at a higher density in order to insure that correspond fairly well, as do the Eocene-Oligocene boundaries. the zones were real and not due to human error. In most cases, The most noticeable disparity in correlating the Vieja Group with the existence of the thin zones was confirmed (for example, see the marine section is in the large number of short polarity intervals fig. 3). High-density sampling showed that zones were as thin as in the Vieja Group. Those of less than 40,000 years duration would 0.3 m, thus explaining why they had been represented by only one be difficult to detect in marine anomaly patterns. However, longer sample. It is likely that additional thin zones were not detected polarity intervals may be represented by small-scale amplitude because the sample spacing was greater than their thickness. For fluctuations in marine magnetic profiles. These fluctuations are regional or global correlation these zones are of little importance. often interpreted as intensity variations, but in light of the large The large-scale reversal pattern is of prime interest. However, the number of thin polarity zones found in the Vieja Group, we sug- thin zones are significant in that they imply frequent reversals of gest that they be reinterpreted as short polarity events. For exam- the geomagnetic field in early Oligocene time. ple, LaBrecque and others (1977) indicate eight "short events or Stratigraphic overlap between the sections made it possible to intensity fluctuations" in the long reverse period between attempt local correlation. The problem is that the overlap is at an anomalies 12 and 13 (fig. 5). Many of these correlate well with thin interval of predominantly reverse polarity with short normal inter- polarity zones in the Vieja Group, suggesting that they are indeed vals (fig. 4). Which normal zones correlate with each other is not polarity events. easy to determine because all zones are of approximately equal thickness, and zones recognized in one section may have been CONCLUSION missed in another if they were thinner than the sampling interval. The large number of magnetic polarities encountered in this time Prospects for regional correlation look promising. When thin interval is rather surprising. We are not aware of any other study zones are ignored, an overall reversal pattern, which should prove with a similar multitude of short magnetic events, although their useful in regional correlation, emerges. existence has been theoretically predicted (Cox, 1968). Because Because not all samples have been completely demagnetized, some of these events are represented by as little as 0.3 m of strati- we can only propose a tentative correlation with the marine polar- graphic section, they are easily missed where sampling is at 1-m in- ity time scale (fig. 5). For a unique correlation, a distinct reversal tervals, and we therefore cannot expect a one-to-one correlation pattern is necessary. This is a major problem in the Tertiary between overlapping sections. When this is taken into account, because most polarity intervals are of similar duration. However, the various sections compare reasonably well, and the magnetic marine magnetic profiles display low amplitudes between anom- correlation readily agrees with all geologic constraints.
MAGNETOSTRATIGRAPHY IN THE TRANS-PECOS VOLCANIC FIELD 103
C TIME POLARITY VIEJA GROUP b EPOCH MA GN MM M. Y AN OM FORD 31.5(18) SS 25 — B 7 7A — L 8 2 2 A U_ 9 30 — 10 1— 0
C 111 I------1— E 0 0 0 0_ N 35 — 13 ------0 0 E BR B R OK S BRACKS 15 RHY RHY 36 5,36.8
0 w 40 — C H LI- E _i — 18 _i BU 0
(f) N 38.6,35.2 Er Co E _J — 9 INI 36.2,36.4 0 LLI 0 .=1 .._ ---- All co 20 After LaBrecque and others, 977
0 2 Figure 5. Correlation of Vieja reversal stratigraphy (columns a and • b) with the marine magnetic time scale (left column). Column a is a simplified version of column b, in which zones less than 3 m thick to (1) are represented by short lines to the left of the column. Hachured to 11.1 zones are of undetermined polarity. Positions and ages of Mitchell 2 • 20 — LaBrecque, J. L., Kent, D. V. and Cande, S. W., 1977, Revised magnetic 0 — polarity time scale for Late Cretaceous and Cenozoic time: Geology, v. 5, p. 330-335. 1 16 km 4 k m McDowell, F. W., 1978, Potassium-argon dating in the Trans-Pecos volcanic field, in Walton, A. W. and Henry, C. D., eds., Cenozoic geology Figure 4. Magnetostratigraphic correlation of overlapping sections. of the Trans-Pecos volcanic field of Texas: Texas Bureau of Economic Geology Guidebook 19, p. 10-18. Testarmata, M. M. and Gose, W. A., 1978, Magnetostratigraphy of the Eocene-Oligocene Vieja Group, Trans-Pecos Texas, in Walton A. W., ACKNOWLEDGMENTS and Henry, C. D., eds., Cenozoic geology of the Trans-Pecos volcanic Many thanks to the Vizcaino and Chambers families, who not field of Texas: Texas Bureau of Economic Geology Guidebook 19, p. 55-66. only gave us accommodations and access to their land, but also Walton, A. W., 1972, Sedimentary petrology and zeolitic diagenesis of the helped us overcome the difficulties of field work by providing us Vieja Group (Eocene-Oligocene), Presidio County, Texas (Ph.D. disserta- with a burro for hauling water or with a homecooked meal after a tion): University of Texas, Austin, 264 p. hard day. University of Texas Marine Science Institute Contribution Wilson, J. A., 1978, Stratigraphic occurrence and correlation of early Ter- No. 396, Galveston Geophysics laboratory. tiary vertebrate faunas, Trans-Pecos Texas, part 1: Vieja area: University of Texas, Texas Memorial Museum Bulletin 25, 42 p. Wilson, J. A. Twiss, P. C., Deford, R. K. and Clabaugh, S. E. 1968, REFERENCES Stratigraphic succession, potassium-argon dates, and vertebrate faunas, Cox, A., 1968, Lengths of geomagnetic polarity intervals: Journal of Vieja Group, Rim Rock Country, Trans-Pecos Texas: American Journal of Geophysical Research, v. 73, p. 3257-3260. Science, v. 266, p. 590-604. Tobosa plant, Hilaria mutica.