Facies succession during the Cenozoic Era shows a vertical shallowing trend, ... heavier than the replacive dolomite. - 142 .03. - 133 .05. - 127 .45. - 116 .85.
The Second International Conference on Saltwater Intrusion and Coastal Aquifers — Monitoring, Modeling, and Management. Mérida, Yucatán, México, March 30 - April 2, 2003 Diagenetic processes in the Cenozoic sedimentary formations associated with the Chicxulub Impact Crater, northwestern Yucatan Peninsula, Mexico Mihai Lefticariu1, Eugene C. Perry1, Liliana Leftticariu1 1
Northern Illinois University, Department of Geology and Environmental Geosciences, DeKalb, IL 60115 1. Introduction The northwestern Yucatan Peninsula, Mexico, is a place for studying the control exerted by
the large Terminal Cretaceous Chicxulub Impact Crater on both sediment deposition and diagenesis during the Cenozoic Era. The Crater and associated impact breccia have been well preserved beneath a blanket of Cenozoic carbonate rocks with a thickness of up to 1100 m in the center and 280 m in the rim zone. Limestone, dolomitic limestone and rarely dolostone crop out in the area. The crater exerts a major influence on groundwater flow, and the minerals present in crater sediments affect groundwater quality. During most of the Tertiary Period the Crater was a distinct basin (henceforth Chicxulub Sedimentary Basin) where predominantly pelagic and outer-platform sediments were deposited until it was filled in Middle Miocene. The Basin is roughly circumscribed by the Cenote Ring (fig. 1), a geomorphic surface feature of the buried crater that has a higher permeability compared to the adjacent unfaulted zones (Perry et al., 1995a; Pope et al., 1996; Perry et al., 2002). Dolomitized shallow water facies predominate outside the Ring. Partly secondary gypsum is common within and above the impact breccia and secondary gypsum is also occasionally present in Tertiary carbonates as crack- and mold filling. The carbonate platform of the Yucatan Peninsula hosts an aquifer characterized by the presence of a freshwater lens, a saline intrusion, and a mixing-zone. Extensive petrographic and litho- and hydrogeochemical data from both the inside and outside the Ring were obtained as part of this study. They include oxygen and carbon isotope data on calcite and dolomite. 2. Sampling and analytical methodology Macroscopic observations were performed on core samples from wells located throughout the northwestern Yucatan Peninsula: UNAM, PEMEX and ICDP wells and the shallower Motul, Merida, and Cenotillo wells (fig. 1). Transmitted and reflected light, cathodoluminescence, and electron microscopy were employed to identify the main the main mineral phases and phase relationships.
1
Y-6
UNAM 1 UNAM 8
Fig. 1. Location of the study area (delimited by the outline and the northwestern coast) on the hydrogeochemical and structural map of northern Yucatan Peninsula, Mexico, from Perry et al., 2002 (the contour lines belong to the 100*SO4/Cl equivalent ratio); sampled cores are shown with filled circles
Microprobe, X-Ray diffractometry and oxygen and carbon isotope analyses were performed to better characterize the carbonate phases (calcite and dolomite).
3. Results and discussion Facies succession during the Cenozoic Era shows a vertical shallowing trend, more gradual toward the center of the Chicxulub Sedimentary Basin. Pelagic limestones, in places cherty or shaly, occur only inside this basin (fig. 2).
Planktonic foraminifera, coccolith plates, and pyrite are
common within the pelagic sequence. Dolomitized shallow water facies predominate outside it. An evaporitic facies containing bedded or chickenwire anhydrite with ptygmatic gypsum and satin spar is present above the collapse breccia in UNAM 6 well (fig. 2); this suggests subaerial exposure of
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evaporite. Secondary gypsum is common especially at the base of the Cenozoic sequences from UNAM 5, UNAM 6, and UNAM 7 cores.
B L O C K S ?? ?
Fig. 2. Proposed lithological columns for the sampled wells; depth is expressed in meters below present land surface
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The best chance for the sulfate minerals present at the top of the breccia layer to participate to diagenetic processes during the Cenozoic Era is related to the possibility for the freshwater lens or a mixture of saline water and freshwater to reach the breccia. Some of the secondary gypsum could have formed in this way. This might have been helped the fact that the Cenozoic deposits are much thinner outside the Cenote Ring than inside (fig. 2). Moreover, the frequency of subaerial exposure features (vugs, subaerial crusts, karst) increases upward and outward from the center of the Chicxulub Basin. Most of the original aragonite and high-magnesium calcite were replaced by low-magnesium calcite in vadose and phreatic meteoric environments prior to dolomitization; meniscus, pendular, and syntaxial calcite cements are present. Very little of the initial high-magnesium calcite or aragonite has been preserved inside the Ring. Sr in gypsum and anhydrite from the impact breccia (UNAM 5, UNAM 6, and UNAM 7 wells) varies between 900 and 3500 ppm. In addition to litho- and biostratigraphic information, Sr could prove helpful in finding the contribution of the impact breccia to carbonate diagenesis during the Cenozoic by using strontium concentrations and strontium isotope ratios in carbonates as tracers of water-rock interaction in the northwestern Yucatan Peninsula.
Benthic foraminifera tests
composed now of low-magnesium calcite have Sr concentrations up to 700 ppm; most Sr values are grouped around 500 ppm and this could be expected when subjecting to freshwater diagenesis tests initially made up of high-magnesium calcite (Tucker and Wright, 1990). Dolomite contains up to 400 ppm Sr. δ18O in whole rock, non-vug-filling, calcites ranges between –7.14‰ and +0.85‰ VPDB and δ13C between –6.77‰ and +3.31‰ VPDB. The trends present in the δ18O-δ13C plane (fig. 3) suggest that the contribution of organic matter to diagenesis became more and more significant upward in the section as the relative importance of the meteoric processes increased. They are also better defined in the wells located inside the Cenote Ring (UNAM 1, UNAM 2, and UNAM 8 wells) than in those from outside it (UNAM 5 and UNAM 6 wells).
The younger vug-filling low-
magnesium calcite precipitated from meteoric water, as suggested by the low stable isotope values. In whole rock, non-vug-filling, matrix and grain dolomites δ18O and δ13C vary between –5.54‰ and +0.87‰ VPDB, and –4.62‰ and +3.31‰ VPDB, respectively. Some of the most negative δ18O values are associated with features of subaerial exposure. Almost all the samples have negative δ18O and positive δ13C (fig. 4). Dolomite contains between 47.28 and 57.24 mole % CaCO3 with most values between 51 and 56 mole % CaCO3. The dolomites with highest Mg concentrations occur mostly in UNAM 5 well between –195 m and –105 m. We hypothesize that
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most replacive calcian dolomite formed in shallow burial conditions (no deeper than 500 m) in a fluid dominated by meteoric water. Dolomite formation was rock-buffered with respect to carbon isotopes. In general, the vug-filling dolomite is significantly heavier than the replacive dolomite.
3.5 -503 .90
3.0 -401 .20 -528 .68
-256 .40
-387 .40 -442 -412 .60 .00 -382 .80
2.5
-456 .00
-425 .80
-235 .40
-153 .13
2.0 1.5
-218 .15 -75 .90 -218 .15 -266 .46 -53 .22
1.0
-6 .00 -271 .45 -253 .48
0.5
d18O (PDB) -7.5
-7.0
-6.5
-142 .03
-6.0
-5.5
-5.0
-77 .35
-99 .55
34 .72
-4.5
-31 .90 -3.5 -4.0
-3.0
-2.5
-2.0
-1.5
-1.0 -133 .05 -0.5
-99 .45
0.5
1.0
-0.5
19 .00 -127 .45
0.0 0.0
-96 .30 -116 .85
-1.0
-9 .20 -110 .50 -6 .47
-97 .90
-48 .40
-1.5
-2 .35
-2.0
-93 .60
-2.5 7 .95
-28 .40 2 .25
-36 .25
-86 .00 -35 .00
-77 .75
5 .60
-3.0 -3.5
-68 .57
-4.0 -60 .00
-38 .15
-26 .80 -87 .05
3 .45 -45 .55
UNAM wells -5.5
-17 .40
-6.0 -8 .70 4 .88
LEGEND
-5.0
-6.5
26 .38
-5 .57
1
d13C (PDB)
14 .20
-4.5
2 5 6 8
-7.0
Fig. 3. δ18O versus δ13C in whole rock non-vug-filing calcites from northwestern Yucatan Peninsula, Mexico (UNAM wells); sample depth appears next to each symbol
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3.5 -104 .50 -175 .05
-109 .80 -166 .60
3.0
-205 .40 -220 .70
-177 .30
-227 .40
-168 .40 -208 .15
-425 .30 -34 .00
2.5
-193 .50
-347 .85
-197 .80 -146 .72
-19 .68
-329 .60 -156 .30
2.0
-92 .60 -148 .40 -167 .90
-271 .00
-98 .70
-117 .70
-179 .00 -128 .00
-99 .55
-114 .15
-120 .20-20 .75
-211 .50 -86 .20
-148 .40
-141 .20
1.5
-10 .20
-135 -175 .45 .28
-114 .80
-163 .55 -211 .50
-230 .67
-99 .45
-117 .60
-75 .90 -175 .05 -53 .70
1.0
-66 .65 -47 .05
-312 .75 -130 .55.70 -338
-61 .35
-41 .60
-83 .80 -72 .50 -314 .30
-77 .50
d18O (PDB) -6.0
-5.5
-5.0
-4.0
-3.5
-3.0
0.5
-111 .13
-87 .00 9 .15 -301 .88 -133 .05
-53 .60
-4.5 -166 .80
-354 .70
-115 .32
-2.5
-2.0
-53 .45
-1.5
-30 .60
-1.0 -106 .60
-0.5
-8 .10
-60 .03 -54 .95
0.0 0.0
1.0
-0.5 -137 .55
-110 .50 -18 .70
-80 .00
0.5
-101 .90
-1.0
-127 .00
-46 .30
-97 .90
-1.5
-68 .85
-2.0
-2.5
-3.0 -43 .05
-180 .20
LEGEND
-3.5
-4.5 -87 .05
d13C (PDB)
UNAM well -4.0
1 2 5 6 7
-5.0
Fig. 4. δ18O versus δ13C in whole rock non-vug-filling dolomites from northwestern Yucatan Peninsula, Mexico (UNAM wells); sample depth appears next to each symbol
However, their stable isotope signature argues for the presence of freshwater in the dolomitizing fluid. The statistical trends of stable isotopes are less clearly defined in dolomite than in calcite. Dedolomitization is suggested by the presence of hollow dolomite crystals, which are abundant, dolomite biomolds filled with fine dolosparite, and rhombic crystals partially replaced by calcite. It is possible for the evaporites to have participated in dedolomitization once the freshwater
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lens reached the top of the impact breccia layer following a major erosional episode. In this case the process could have taken place no earlier than Middle Miocene and it could have helped karst formation, which also appears to post-date extensive dolomitization.
4. Conclusions a. The sedimentary features characteristic of subaerial exposure are more common in Cenozoic rocks from outside the Cenote Ring than inside; b. The selective exposure to meteoric conditions led to differential development of permeability, which subsequently affected diagenetic fluid flow; b. Stable isotope trends in calcite are better defined in samples from inside the Ring; c. Calcite displays more consistent carbon and oxygen isotope trends than dolomite; d. Dolomite has a greater relative abundance outside the Ring and it formed in shallow burial conditions where a fluid dominated by freshwater was present; e. Dedolomite is common in Cenozoic dolostone; f. Diagenetic processes, such as formation of low-magnesium calcite and dolomitization were more extensive outside the Ring of Cenotes than inside during most of the Cenozoic Era.
Bibliography Perry, E. C., L. E. Marin, J. McClain, G. Velazquez, 1995a. The Ring of Cenotes (sinkholes) northwest Yucatan, Mexico: its hydrogeologic characteristics and association with the Chicxulub Impact Crater. Geology, no. 23, pages 17-20. Perry, E. C., G. Velazquez-Oliman, L. Marin, 2002. The Hydrogeochemistry of the Karst Aquifer System of the Northern Yucatan Peninsula, Mexico. International Geology Review, 44, pages 191-221 Pope, K., A.C. Ocampo, G.L. Kinsland, R. Smith, 1996. Surface expression of the Chicxulub crater. Geology, vol. 24, no. 6, pages 527-530. Tucker, M. E., V. P. Wright, 1990. Carbonate sedimentology. Blackwell Scientific Publications, Oxford-London-Edinburgh-Boston-Melbourne-Berlin-Paris-Vienna, 481 pages.
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