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Facies architecture of a Lower Cretaceous coral-rudist patch reef, Arizona
Cretaceous
Research
(1989)
10, 3 1 l-336
Facies architecture of a Lower Cretaceous coral-rudist patch reef, Arizona P. M. Hartshorne *Department Received
of Geosciences,
University
of Arizona,
3 August 1988 and accepted 28 March
Tucson, Arizona
85721, U.S.A.
1989
The Lower Cretaceous Mural Limestone marks the maximum marine incursion into southeast Arizona during Aptian-Albian time and records the middle Cretaceous transition from coral-dominated to rudist-bivalve dominated reefs. Upper Mural Limestone facies are most often dominated by corals. However, rudists form significant frameworks at some localities, one of which is described in this paper. The paleoenvironmental distribution of three potential reef-builders (corals, rudists, and ‘oysters’) were studied at this patch reef locality. Corals built the framework of the inner reef core. The rudist Petalodontiu initially gained a foothold in sheltered areas among corals and subsequently built a framework in the outer reef core. Caprinid rudists formed mounds in the outer reef to back reef areas. The rudists Toucasia and Monopleura and the oyster-like bivalve Chondrodonta formed beds or were scattered in the reef-flank and shelf lagoon sediments and did not contribute to the reef framework. Upper Mural Limestone reefs are important examples of the coexistence of corals and rudists during this middle Cretaceous fauna1 transition period. This study supports the idea that rudist-bivalves initially colonized protected back-reef areas early in the Cretaceous and only later in the Cretaceous did rudists dominate reef frameworks. KEY WORDS: Aptian-Albian;
Early
Cretaceous;
rudists;
corals;
reefs;
Arizona
1. Introduction The Cretaceous was a time of major reef-building activity throughout the tropical Tethyan Seaway, which extended from about 40” N to 20” S of the equator (Wilson, 1975). Corals were the dominant reef-builders of the Early Cretaceous, but by the Late Cretaceous reefs were dominated by the gregarious rudistid-bivalves (Kauffman & Sohl, 1974). The rudists were bizarre bivalves which first appeared in the Late Jurassic and evolved rapidly in the Late Cretaceous, only to become extinct at the end of the 1986). Rudists formed significant buildups on the Cretaceous (Nicol, carbonate platforms and offshore banks of what is now southern Europe, the Middle East, southern Asia, and currently submerged atolls of the middle Pacific Ocean (Heckel, 1974; Wilson, 1975). They also generated reefs and bioherms on the continuous band of shelf margins encircling the Cretaceous Gulf of Mexico and also in the Caribbean region (Kauffman & Sohl, 1974; Wilson, 1975; Bebout & Loucks, 1983). Outcrops of rudist buildups are found in northern Sonora and Chihuahua, Mexico, and their northern limit extends into southeast Arizona, southwest New Mexico, and central Texas in the United States (Hayes, 1970b; Bebout & Loucks, 1983). In the Lower #Present
address:
1638 E. 7th Street,
01956671/89/040311f26 $03.0010
Tucson,
Arizona
85719,
U.S.A.
;:c 1989 Academic Press Limited
312
I’. 11. liartshornr
Cretaceous upper Mural Limestone of southeast .Arizona, most reefs are dominated by corals and algae with rudists as significant but subordinate framework contributors (Scott, 1979). This study is a detailed examination of the fossils and lithologies present in a coral-rudist patch reef in southeast Arizona. The objective of the research is the elucidation of the depositional history of this particular patch reef and an assessment of the roles of the three potential reef-builders present, (1) corals, (2) rudists, and (3) the oyster-like bivalve Chondrodonta.
2. Geologic
setting
Late Jurassic to earliest Late Cretaceous sediments accumulated in SE Arizona and adjacent areas of New Mexico and Mexico in the Bisbee Basin, a NW-trending extension of the Chihuahua Trough. This intracontinental extensional depression was related to the opening of the Gulf of Mexico. Later backarc spreading extended the depression northwestward to penetrate the back side of a magmatic arc on the western continental margin (Figure 1). Further discussion of the tectonic setting of the Bisbee Basin and the surrounding region is given by many authors (Coney, 1978; Drewes, 1981; Bilodeau, 1982; Bilodeau & Lindberg, 1983; Dickinson et al., 1986, 1989).
N Estimated positlon of Early Cretoceous magmatic arc
t
Figure 1. Paleogeographic map of the southwest USA and northern Mexico showing the approximate limit of Aptian to lower Albian deposition, the position of the Early Cretaceous magmatic arc along the west side of this region, and the position of a rift shoulder bounding the north side of the Bisbee Basin. After Dickinson et al. (1989) and Bilodeau & Lindberg (1983).
Lower Cretaceous coral-rudist patch reef
2.1.
313
Bisbee Group
In southeast Arizona, sediments deposited in the Bisbee Basin during the Cretaceous are known as the Bisbee Group (Ransome, 1904). Numerous authors have described the stratigraphy and paleogeography of Bisbee Group sediments, most notably Ransome (1904), Stoyanow (1949), Gilluly (1956), Hayes (1970a, 1970b), Bilodeau i3zLindberg (1983) and Dickinson et al. (1989). The Bisbee Group can be subdivided into non-marine deposits to the NW and transgressive-regressive marine deposits to the SE (Dickinson et al., 1986). These two segments represent different depocenters and are not well correlated due to poor age control (Klute, 1987). In addition, correlation is hampered by the isolation of outcrop exposures in uplifted blocks of Basin and Range topography (Dickinson et al., 1989). My study area lies within the SE segment.
C1ntura formatIon
Morita formotlon
Glance conglomerate
? ? / Figure 2. Generalized stratigraphic section of the SE segment of the Bisbee Group in SE Arizona.
314
I’. IvI. Hartshome
The SE segment of the Bisbee Group (Figure 2) is divided into four formations (Ransome, 1904). From oldest to youngest these are (1) the Glance Conglomerate, (2) the Morita Formation, (3) the Mural Limestone, and (4) the Cintura Formation. Alluvial fan and fluvial deposits of the Glance Conglomerate were deposited during Late Jurassic to Early Cretaceous rifting (Bilodeau, 1978). The upper three formations of the Bisbee Group make up a major transgressive-regressive sequence deposited during passive thermotectonic subsidence after rifting had ceased (Bilodeau, 1982; Klute, 1987).
2.2.
Mural Limestone
The marine Mural Limestone is located above the transgressive Morita Formation and below the regressive Cintura Formation. The Mural Limestone was deposited upon a carbonate and siliciclastic shelf to shelf margin complex in the SE portion of the Bisbee Basin (Hayes, 1970a, 1970b). The formation becomes dominantly siliciclastic to the NW, pinching out west of the Huachuca Mountains and north of the Mule Mountains. This facies transition indicates the NW limit of marine conditions in SE Arizona during Aptian to early Albian time (Hayes, 1970a). To the south, the Mural Limestone thickens towards the shelf margin until it disappears beneath Tertiary and Quaternary volcanic cover about 30 km south of the Mexican border (Warzeski, 1983; Scott, 1987). The thickness of the Mural Limestone ranges from 90 to 250m in the Mule Mountains and Huachuca Mountains of southern Arizona (Hayes, 1970a). The formation is informally divided into two members: a lower unit of interbedded mudstone, limestone, siltstone, and sandstone (90-357 m, thickening westward to New Mexico), and an upper unit of massive fossiliferous limestone (So-260 m, thickening southward into Mexico) which contains coral-algal-rudist buildups (Hayes, 1970a; Scott, 1987; Warzeski, 1987). Stoyanow (1949) used ammonites to place the Aptian-Albian boundary within the lower member of the Mural Limestone. However, Scott (1987) used graphic correlation techniques to place this boundary in the basal portion of the upper Mural Limestone. The upper Mural Limestone is correlative with a portion of the Upper Tamaulipas Formation of NE Mexico, the Glen Rose Formation of central Texas, and with part of the upper U-Bar Formation of SW New Mexico (Scott, 1987; Warzeski, 1987). Numerous coral-algal-rudist patch reefs occur within the upper Mural Limestone and its correlatives in SE Arizona and SW New Mexico (Zeller, 1965, 1970; Hayes, 1970a, 1970b; Scott & Brenckle, 1977). Warzeski (1983) documents two prograding carbonate bank barrier complexes in northern Sonora, Mexico, and postulates the existence of a third hidden below volcanic overburden further to the south. Patch reefs of SE Arizona have been the focus of several studies: Grocock (1975), Scott & Brenckle (1977), Scott (1979, 1981, 1987), Roybal (1979, 1981), Warzeski (1983, 1987) and Monreal (1985).
Lower
Cretaceous
coral-rudist
patch
reef
315
study area. The patch reef described in this paper is located in the extreme SE corner of Arizona, just west of the Peloncillo Mountains in Guadalupe Canyon (N1/2, SE1/4, sec. 17, T24S, R32E, Guadalupe Canyon Quadrangle, Cochise County, Arizona; Figures 3,4). Scott (1979) defined and described the lithofacies of upper Mural Limestone bioherms throughout SE Arizona, including this locality (his Lone Butte locality #8326, Scott, 1979, figure 7A). This outcrop is a hill (locally known as Mexican Saddlehorn) of lower Mural Limestone capped by a resistant, cliff-forming, upper Mural Limestone coral-rudist bioherm (Figure 5).
3. Methods A topographic map of the top of the Mexican Saddlehorn hill was prepared using plane table and alidade. Mapping was done with a contour interval of 5 m and covered an area of approximately 900 x 1200 m. Ten stratigraphic sections spanning between 4 and 25 m in thickness were measured on the cliff faces that surround the top of the hill (Figure 4). The lithologies and the presence of macrofossils were used to correlate between sections and to define facies. Thirty-five thin-sections were made to facilitate microfacies analysis and microfossil identification.
Map area
Scale in kllometers
Figure 3.
Location
map of the study area in SE Arizona. Also shown is the location of the Paul Spur reef (PS) cited in the text.
Mexican Scddlehorn
Scale
in meters Contour
30‘--
Interval
=5 m
30
Mexico
Figure
4. Topographic
map of the study area showing the locations A-J and the cross-section Z-Z’.
of measured
4. Facies descriptions and paleoenvironmental
stratigraphic
sections
discussion
Eleven facies were identified based on field and laboratory descriptions of lithologies and fossils (Tables l-3). Most facies show little evidence of internal bedding. Gradational change between facies, both laterally and vertically, is common. The 11 facies are organized here into four groups (Figure 6), (1) the Basal Facies Group, (2) the Framework Facies Group, (3) the Framework-Flanking Facies Group, and (4) the Uppermost Blanketing Facies Group. 4.1.
Basal Facies Group
This lowermost group of facies consists of the fossiliferous calcareous quartz sandstone facies, the burrowed fossiliferous coated-grain packstone facies, and the fossiliferous coated-grain packstone/grainstone facies. Stratigraphically, these facies occur below all of the other facies studied.
Lower
t:igurr
Cretaceous
coral-rudist
5. View of the west side of the LIes~can
patch
Saddlehorn
317
reef
locality
(shoun
bs arrow)
‘I’he fossiliferous calcareous quartz sandstone facies locally occurs in the uppermost part of the lower Mural Limestone. About 1.5 m of this unit is exposed; erosional debris buries the basal and upper contacts. Beds crop out as thin ledges (less than 30 cm thick) that appear to be very crudely bedded into shell-rich and shell-poor layers. Fine to very fine sand-sized, angular to subangular quartz grains are cemented by sparry calcite, and exhibit corrosion on grain edges. Unbroken gastropods are common and several pectinid bivalves are present. The surface of one ledge (section I) exhibits parallel-oriented high-spired gastropods (spires pointing NW). Molluscan fragments are abundant, and smaller amounts of grapestones, reworked grains, ooids, coated grains, and other fossil fragments are present (Figure 7). ‘I’he fossiliferous calcareous quartz sandstone facies probably accumulated in a high-energy shallow subtidal to intertidal environment. This is indicated by the absence of micrite in both the matrix and fossil cavities, the abundance of quartz sand, the presence of reworked and rounded grains, and the parallel alignment of gastropods. Tidal currents removed fine-grained mud and collected and aligned the lag deposits of gastropod shells. Grapestones and reworked fossil fragments with micrite fillings and attached micrite matrix formed in muddier low-energy areas and were transported to the higher energy area. The absence of terrigenous material in any facies deposited after the sandstone indicates a cut-off of the supply of elastics. This could have been a result of deepening water or of the development of a coeval carbonate environment closer to shore. The burrowed fossiliferous coated-grain packstone facies is represented here by a single bed about 1 m thick. Mud and fossil hash occur in clumps due to
Facies
S-10 3 3& 70 60-70 30-50 5525 15560 20-55 20-55 7720
1.5 2.2556.7 4.1-6.1 5.0-8.25 0.8-11.55 1.8-13.5 0.34.0 1 .o-2.3 0.1-0.7
30-35
“” Fossils
Saddlehorn.
0.85-0.95
1.5
(m)
Thickness
1. Properties that characterize facies in the upper Mural Limestone at Mexican fragments (unidentified fossil fragments). grains, and matrix. (TR = trace amount).
Fossiliferous calcareous quartz sandstone Burrowed fossiliferous coated-grain packstone Fossiliferous coated-grain packstone/ grainstone Coral boundstone Petalodontid boundstone Caprinid floatstone/rudstone Lime mudstone/wackestone Fossiliferous wackestone/floatstone Toucasid wackestone/floatstone Chondrodontid-caprinid floatstone/ boundstone Miliolid-monopleurid lime mudstone/ wackestone
Table
Ranges
227
5-l 5
85 5-10 5520 5-15 l-10 8830 8835
5-10
10
ob Fragments
are given
TR
0
55 TR 0 TR TR TR TR
40-55
35-41
o/oGrains
for total thicknesses
of fossils,
80-Y 5
45-80
45 30-70 30-75 50-75 75595 40 80 45580
40-60
15-35
O0 Matrix
and percentages
Fossiliferous calcareous quartz sandstone Burrowed fossiliferous coated-grain packstone Fossiliferous coated-grain packstone/ grainstone Coral boundstone Petalodontid boundstone Caprinid floatstone/rudstone Lime mudstone/wackestone Fossiliferous wackestone/floatstonc Toucasid wackestoneifloatstone Chondrodontid-caprinid floatstonei boundstone Miliolid-monopleurid lime mudstonr/ wackestone
Facies
Table 2. Range of per cents of major 1,
”
02 0
o--3 TR TR 410
3 34
0
4-9 36-53 15-22 o-8 2-m19 3-20
0
G20 0~1 9-2 0 &5 O-4
0 15-35 l-.4 4-6 o-6 0 14 o- 12
TR 20-55 24 4-x 06 1 16 l-12 O-1
0
0
0
TR
0
“” Rudist
0
Branihing coral
0
Maszive coral
TR
II Coral
0
fossil types for each facies. 0
4-8
03
01 0 0 0 GlO G8
0
0
0
A’Gopleurid
0
Y’” Pet&dontid
0
“;] Caprinid
@2
” Toucasid
0
0
70 Chondrodontid
3. Percentage
F&es
of grain types for each facies.
Miliolid-monopleurid wackestone
boundstone
lime mudstone/
Fossiliferous calcareous quartz sandstone Burrowed fossiliferous coated-grain packstone Fossiliferous coated-grain packstone/ grainstone Coral boundstone Petalodontid boundstone Cap&id floatstone/rudstone Lime mudstone/wackestone Fossiliferous wackestone/floatstone Toucasid wackestone/floatstone Chondrodontid-caprinid floatstone!
Table
(A=abundant,
0
0 0
TR 0 0 0 0 0 0
55 0 0 0 0 GTR 0-TR
0
0
0
35-55
TR
0
TR-A ?-50
TR 0 ? >
2
0
“,Peloid
2-5
“oOoid
occurrence)
0
“” Coated-grain
? =possible
0
0
0 0 0 0 0 (tTR 0
0
556
0 0 Grapestone
0
0 0 0 0 0 TR 0 0
0
28-30
“,,Quartz
0
sand
0
0
0 TR 0 0 0 0 O&‘I’R 0
0 5
TR
or, Intraclast
Lower
FACIES
Cretaceous
coral-rudist
patch
FACIES
GROUPS
Uppermost
Blanketing
Facies
Miholid-monopleurid
lime
Chondrodontid-caprinid
Taucasid Framework-flanking
Facies
Fossiliferous Lime
Framework
Facies
Caprmid
Coral
Facies
mudstonelwackestone floatstonelboundstone
wackestone/fioatstone wackestone/floatstone
mudstone/wackestone
floatstonelrudstone
Petafodontid
Basal
321
reef
boundstone
boundstone
Fossiliferous
Burrowed
Fossiliferous
coated-grain
fossiliferous
calcareous
packstonelgrainstone
coated-grain
ouartz
packstone
sandstone
facies
6
t;~gure
6.
Diagram
showing
the geometry of the facies described in the text. Refer to Figure Iocations of the lettered sections.
4 for the
horizontal burrowing, giving the rock a though indistinct, pervasive, mottled appearance. The matrix is micrite with patches of sparry calcite, some of which shows evidence of neomorphism. Much of the micrite matrix may be composed of degraded peloids. Coated skeletal grains dominate and some grains are reworked (Figure 8). Whole regular and irregular echinoids are common; brachiopods and gastropods are less abundant. The single bed representing the fossiliferous coated-grain packstone; grainstone facies is about 1.5 m thick. Variable, but subequal, amounts
322
I’.
RI.
tiartshorne
Figure /. Photomlcrograph from the towlltrrous ca Ic‘arrwx the photo is a worn gastropod shell. A grapestone is below right corner of the photo. Bar = I mm.
Figure
X. Photomicrogrnph coated fossil fragments
LIuartz sandstone taucs. the wstropod and another
from the burrowed fossiliferous coated-grain packstone and patches of sparry calcite occur in 3 micritic matrix.
III
rhc cvntc~-
facirs. \Vorn Bar1 mm.
and
Lower
Cretaceous
coral-rudist
patch
reef
323
of micrite and sparry calcite form the matrix. Ghosts of grains present in the sparry calcite indicate at least some neomorphism. This facies is dominated by coated mollusc and echinoderm fragments (Figure 9). Solution boundaries between grains are common. Both the burrowed fossiliferous coated-grain packstone facies and the fossiliferous coated-grain packstone/grainstone facies are dominated by coated fossil fragments in a micrite and sparry calcite matrix. The coated grains probably formed under moderate to high energy conditions on shoals and were then moved off the shoals into lower energy areas where lime mud could not be completely winnowed away. The greater amount of mud and whole echinoids in the burrowed fossiliferous coated-grain packstone may indicate deposition in a lower energy area further from the shoal. The presence of echinoids and brachiopods show this facies probably formed under normal marine conditions. The irregular echinoids found in the burrowed facies may have been responsible for producing the burrows. Regular echinoids were also present in the burrowed facies indicating proximity to a firm substrate-perhaps even the initial coral reef colony. 4.2.
Framework
Facies Group
The framework facies group includes the coral boundstone facies, the petalodontid boundstone facies, and the caprinid floatstone/rudstone facies. This group forms a prominent mound with adjacent beds thinning outward away from the core.
Figure 9. Photomicrograph coated fossil fragments Bar= 1 mm.
from the fossiliferous coated-grain packstone/grainstone in a micrite and sparry calcite matrix. Note closer packing
facie. Worn and than in Figure 8.
The coral boundstone facies is represented here by a large elongate lenticular bed and several smaller isolated lenses. These lenticular beds exhibit thicknesses ranging from 2.256.7m. The matrix is a micritic wackestone to packstone containing whole fossils and fossil fragments of gastropods, brachiopods, rudists, and foraminifera. Massive, tabular, and laminar corals are very abundant (Actinastrea spp. and Microsolena texana are common); some branching and solitary corals are also present. Coral abundance is variable; the lens perimeters tend to have lower coral densities than the lens interiors. Corals are recrystallized and are often heavily bored by bivalves, sponges, and other organisms, and are also encrusted by algae, foraminifera, and occasionally by oysters and rudists sponges, serpulids, (Figure 10). Micrite filling and geopetal structures often occur in borings and fossil cavities. This coral boundstone, dominated by massive domal and tabular corals, formed the inner core of the reef and appears to have been the pioneer coral community recognized by Scott & Brenckle (1977) in other Mural Limestone reefs. The coral lenses formed on a variety of substrates that must have been stabilized or perhaps had become hardground surfaces. This could have permitted colonization by the corals. The climax coral community found above this pioneer community in many of the other upper Mural Limestone patch reefs, the lamellar form of the coral Microsolena encrusted by stromatolites (Scott & Brenckle, 1977; Scott, 1979; Roybal, 1981; Warzeski, 1983; Monreal, 198S), does not occur at Mexican Saddlehorn. The implications of this difference are discussed later in the Comparison Section.
Lower Cretaceous
Figure
11.
Field
photograph upright
coral-rudist
32.5
patch reef
of the petalodontid boundstone facies. This vertical sectional petalodontids and abundant fragments in the matrix.
vieu
shows
Directly overlying the largest coral boundstone bed is a lenticular bed (4.1-6.1 m thick) which represents the petalodontid boundstone fakes. The facies has a micritic matrix and is dominated by the rudistid bivalve Petalodontia felixi (Douville). These rudists commonly form a framework of upright and closely packed individuals (Figure 11). Petalodontids also occur individually and in clusters, both in and out of life position, at the base of the bed and in deposits lateral to the lens. Fossil cavities are often filled with unfossiliferous micrite and some exhibit geopetal structure. Petalodontid fragments are abundant in the micritic matrix between rudist individuals, especially at the lens base. The petalodontid boundstone facies formed in the outer reef core. Corals may have provided initial shelter and support for these rudists, since scattered petalodontids occur among the corals in some areas of the upper portion of the coral boundstone. The petalodontids were then able to form a massive framework based on mutual support of adjacent individuals. Displaced individuals and displaced clusters of rudists may be evidence of
326
P. .\I. Hartshome
_~_~~
such as storm events. The abundant occasional high energy conditions, petalodontid fragments trapped in the open space between upright petalodontids were most likely a result of pervasive bioerosion. The caprinidjloatstonelrudstone facies is represented by a single lenticular bed (5.0-8.25 m thick). The matrix is a foraminiferal wackestone composed of both micrite and sparry calcite. The dominant fossils in this facies are caprinids. The two species of caprinids found in the Mural Limestone are Caprinuloidea gracilis Palmer and Coalcomana ramosa (Boehm). Other common fossils are orbitolinid and miliolid foraminifera. The caprinid rudists are not as closely packed as the petalodontids and tend to occur in recumbent rather than upright growth positions (Figure 12). The caprinids lived in the outer reef core to back-reef areas. Wilson (1975) notes that caprinids tend to occur in shallow back-reef mounds, although he comments that they also occurred in shelf margin and downslope areas. Densities of the caprinids varied, though the largest accumulation was in the area above the petalodontid boundstone. The caprinids did not appear to form a rigid framework, but instead laid loose on the substrate. In spite of this apparent lack of framework, this facies is included in the framework group because of its association with and contribution to the reef buildup.
4.3.
Framework-Flanking
Facies
Group
the lime of this group, Outcrops representing the two facies mudstone/wackestone facies and the fossiliferous wackestone/floatstone facies, occur lateral to the lenticular beds of the framework facies group.
Figure
12. Field
photograph
of the caprinid floatstonejrudst~jne facies. This shows caprinids in rwumbent positions.
vertical
sectional
Lower
Cretaceous
coral-rudist
patch
reef
327
The combined thickness of the flanking facies lessens noticeably away from the framework group of facies. The lime mudstonelwackestone facies is represented by outcrops which vary from 0.8 m to 11.55 m thick. The homogeneous micrite matrix contains the large foraminifer Orbitolina texana (Roemer) and other foraminifera, especially miliolids (Figure 13). Other fossils present are brachiopods, echinoids, and high-spired gastropods. Fossils are very sparse throughout the beds, especially in the lowermost outcrops, but increase in abundance upwards. The lime mudstone/wackestone facies represents low wave energy, shelf lagoon sedimentation in inter-reef habitats. The paucity of fossils indicates reef debris was not being transported into this area. Locally abundant accumulations of orbitolinid foraminifera indicate an environment with shallow to moderate water depths and open marine circulation (Douglass, 1960; Warzeski, 1983). The absence of laminations may suggest complete bioturbation or perhaps very slow sediment accumulation in the inter-reef areas. Outcrops of the fossiliferous wackestone/fIoatstone furies range from about 1 .X-13.5 m thick. The matrix is dominantly micrite with small amounts of sparry calcite. This facies locally grades into mudstone or packstone and is gradational between the framework facies group and the lime mudstone/wackestone facies. Fossil abundance varies locally and common fossils include corals, bivalves, rudists, echinoderms, gastropods, serpulids,
Figure 13. Photomicrograph from the lime mudstone/wackestone facies. An orbitolinid occupies most of the center of the photo. Bar= 1 mm.
foraminifer
I’. 11. Hartshorne
328
miliolids, orbitolinids, other foraminifera, calcispheres, and unidentified fragments (Figures 14, 15). No one type of fossil is dominant. The fossiliferous wackestone/floatstone facies formed the flanking beds of the reef core, containing debris washed out of the reef proper. The high diversity of fossils present indicates the sediments were derived from the adjacent framework buildup. These sediments washed out over and graded into the inter-reef lime mudstoneiwackestone. 4.4.
Uppermost
Blanketing
Facies
Group
The uppermost blanketing facies group forms a fairly uniform layer over the entire top of the Mexican Saddlehorn hill. The three facies that form this group are the toucasid wackestone/floatstone facies, the chondrodontidcaprinid floatstone/boundstone facies, and the miliolid-monopleurid lime mudstone/wackestone facies (Figure 16). Beds of the toucasid wackestone/jloatstone facies vary in thickness from 0.3 m to 4.0m. The matrix consists of micrite and sparry calcite and contains abundant miliolids, mollusc fragments, and peloidal grains (Figure 17). The rudist Toucasia hancockensis Whitney is the most abundant macrofossil. These rudists are randomly scattered and never form biostromes. Unfossiliferous micrite often fills the toucasid body cavity. The chondrodontid-caprinidjloatstone/boundstone facies occurs as a bedded unit (1 .O-2.3 m thick) with individual beds varying in thickness from 0.2 to 1.5 m. Chondrodonta, an oyster-like bivalve, and caprinids are the dominant
Figure
14.
Photomicrograph from the fossiliferous wackestone/floatstone facie. Several orbitolinids scattered throughout the photo around the valves of a brachiopod. Bar= 1 mm.
are
Lower Cretaceous
Figure
15.
Photomicrograph
from prominent
coral-rudist
329
patch reef
the fossiliferous \\,ackestone/~oatstonc facies. on the right side of the photo. Bar = 1 mm.
Coral
fragments
are
fossils, generally occurring in alternating beds. The basal bed is usually a biostrome of variably oriented chondrodontids, immediately succeeded by a bed of caprinids. The chondrodontids occur as individuals or as upright clusters (Figure 18). Remaining beds often contain both chondrodontids and caprinids, but one or the other is usually dominant. Chondrodontiddominated beds are sometimes capped by a few monopleurid rudists and may contain toucasids as a minor constituent. The miliolid-monopleurid lime mudstonelwackestone facies caps the uppermost blanketing facies group. The matrix is micrite with abundant miliolid foraminifera and lesser amounts of orbitolinids and other foraminifera (Figure 19). Varying amounts of the rudist Monopleura cf. M. marcida White are found as individuals, clusters, and thickets, often occurring in upright positions. Iron-stained and silicified Thalassinoides burrows are also present. Small patches of fossils contain gastropods and other unidentified fragments. A few oncolites were found in one small area. The uppermost blanketing facies group indicates burial of the reef buildup by more restricted lagoonal sediments. Reef growth had apparently ended due to environmental change to conditions less favourable to reefal organisms. Miliolid foraminifera are very abundant in these three facies indicating shallower and more restricted marine conditions than in the previous facies (Rose & Lidz, 1977; Warzeski, 1983). Thalassinoides indicates the presence of burrowing organisms, probably crustaceans, and may also occur in shallow water (Enos, 1983; Ekdale et al., 1984).
330
1’. 11. Hartshome
hlimketint: Figure 16. Vertical sectional \‘LWI ot the L,,>p”mosr to chcrndrodontld lwds. wackestone/floatstonz; arrows point M =monopleurid lime mudstone,‘\~a~kest(~tl~~.
t&es group (‘I’= toucasid C’= caprinid-dominated bed;
5. Comparisons The upper Mural Limestone patch reef discussed here exhibits important similarities and differences with other Lower Cretaceous reefs in Arizona and elsewhere. The organisms found at Mexican Saddlehorn are typical of those found in other upper Mural Limestone localities and the facies present in one reef can be recognized in another. The presence or absence of various facies gives clues to environmental differences between the reefs. For example, southeastward into Mexico (increasing water depth into the basin) the upper Mural Limestone thickens to such an extent that the expanded section can be further subdivided into five new members (Warzeski, 1983, 1987). This southward thickening accentuates differences and increases the complexity of the facies geometries in Mexico. The most striking difference between the Mexican Saddlehorn locality and other Mural Limestone patch reefs is the absence of the Microsolenastromatolite boundstone at Mexican Saddlehorn. Because the Mexican Saddlehorn patch reef is incompletely preserved due to faulting and erosion, it is possible that the missing portion of the reef contained this
Lower
Figure
17.
Photomicrograph
Cretaceous
coral-rudist
patch
from the toucasid ~ackestonelfloatstone abundant in the matrix. Bar = 1 mm.
331
reef
facies.
Miliolid
foraminifera
arr
boundstone. Microsolena does appear in the coral boundstone facies at Mexican Saddlehorn, but it is not associated with stromatolites and it is never a dominant community member-in contrast with the Paul Spur reefs Scott & Brenckle, 1977; Scott, 1979, 1981; (see Figure 3 for location; Koybal, 1981). Alternatively, the absence of the Microsolena-stromatolite facies may be attributed to ecologic factors. The coral growth form of this facies was originally thought to be encrusting and, along with other factors, suggested growth in a high energy environment (Scott & Brenckle, 1977; Scott, 1979; Roybal, 1981). Later study showed that the growth form was probably more ‘bush-like’ and rarely showed signs of breakage (Warzeski, 1983). Warzeski (1983) used these and other factors to conclude that the facies grew in deep water below storm wave base on the reef-slope and back-reef of the banks, and below normal wave base on the upper reef slope of the larger patch reefs. My study of the Mexican Saddlehorn locality indicates that this reef also grew in quiet water below normal wave base and may have been exposed to at least some storm waves. Scott & Brenckle (1977) noted that the pioneer community of tabular and massive corals could develop on the open lagoon substrate, but in these areas the Microsolena-stromatolite facies usually did not develop. The Mexican Saddlehorn patch reef may simply be smaller, shorter lived, and more isolated (the opposite of the expanded upper Mural Limestone of Mexico) in comparison with the other patch reefs. This would simplify the facies relationships and geometries and could result in the omission of some facies. In turn, the absence of the coral-
Lower
Cretaceous
coral-rudist
patch
reef
333
stromatolite boundstone may have allowed the petalodontids to move into the outer reef core and to build a dense boundstone. As a whole, the upper Mural Limestone coral-algal-rudist patch reefs are interpreted by Scott (1979) and Warzeski (1983) to be representative of low energy, stable marine environments. The corals and stromatolites grew in quiet conditions below wave base, and as the reef grew up into the zone of wave energy caprinids became dominant (Warzeski, 1983; Scott, 1984, 1988). Similar coral-dominated reefs are found in France (Masse & Philip, 1981), in the Glen Rose Limestone, the Stuart City Formation, and the James Limestone of Texas (Perkins, 1974; Wilson, 1975; Bebout & Loucks, 1983; Achauer, 1985), and in the Cupid0 Limestone of Mexico (Wilson & Pialli, 1977). This is in contrast to the high energy rudist-dominated reefs found in the Edwards Formation and the Glen Rose Limestone of Texas (Nelson, 1973; Perkins, 1974), in the Antilles of the Caribbean (Kauffman & Sohl, 1974), in the Middle East (Sass & Bein, 1982; Frost et al., 1983; Alsharhan, 1987), and in the El Abra Limestone of Mexico (Enos, 1974; Collins, 1988). For further discussion of the various types of Cretaceous reefs and their paleoenvironmental interpretations see Wilson (197.5), Scott (1984, 1988) and Kauffman & Johnson (1988).
6. Summary and conclusions The Mexican Saddlehorn locality preserves an Early Cretaceous patch reef. Four facies groups were recognized (Figure 20). The Basal Facies Group is that set of facies found stratigraphically below the framework of the reef. These are the Fossiliferous Calcareous Quartz Sandstone, the Burrowed Fossiliferous Coated-Grain Packstone, and the Fossiliferous Coated-Grain Packstone/Grainstone. These facies record deepening marine conditions and formed the base for reef development. The Framework Facies Group formed the framework of the reef. These facies are the Coral Boundstone, the Petalodontid Boundstone, and the Caprinid Floatstone/Rudstone. The corals formed the inner reef core, the petalodontids occupied the outer reef core, and the caprinids formed mounds in the outer reef core to back-reef areas. The absence of the IVicrosolenu-stromatolite facies may be a result of the incomplete preservation of the reef due to faulting or erosion, or may be the result of environmental factors related to the relatively small size and isolation of the patch reef in the open shelf lagoon. The Framework-Flanking Facies Group formed laterally to the framework group of facies. The Fossiliferous Wackestone/Floatstone formed the flanking beds of the reef and spread out over and graded into the Lime Mudstone/Wackestone (inter-reef facies). The combined thickness of these facies decreases away from the framework. The Uppermost Blanketing Facies Group caps the entire reef buildup. ‘l’hese are the Toucasid Wackestone/Floatstone, the ChondrodontidCaprinid Floatstone/Boundstone, and the Miliolid-Monopleurid Lime Mudstone. These facies indicate the cessation of reef growth and record shallower and more restricted shelf conditions.
334
P. M.
Ilartshornr
_N IO
0
Meters
1
Shelf lagoon
Figure
20.
Schematic
cross-section 2-Z’ through the Mexican Saddlehorn locality location of line Z-Z’; see Figure 6 for key to symbols).
(see Figure
4 for the
Petalodontid rudists initially grew in sheltered areas among the corals and were then able to form a substantial framework. The other rudists did not form such a framework. These observations substantiate the view that rudists were lower energy, back-reef inhabitants early in the Cretaceous, but later rudistid groups dominated frameworks in high energy environments (Masse & Philip, 1981; Scott, 1981; Frost et al., 1983).
Acknowledgements This project represents partial fulfillment of the Master of Science Degree at the University of Arizona. Research was completed under the direction of Dr Karl W. Flessa, with further guidance from Dr Andrew S. Cohen and Dr Joseph F. Schreiber Jr. Field vehicles and assistance were enthusiastically provided by Nancy Schmidt, Karin Barovich and Matt Gray. Many thanks to John and Mary Magoffin for access to the field site and for their hospitality. Research was partially supported by a grant-in-aid of research from Sigma Xi and from the Geological Society of America, and a graduate research grant from the University of Arizona, Department of Geosciences, funded by Chevron, USA. Additional thanks to the anonymous reviewers for their extremely helpful comments.
References Achauer, C. W. 1985. Reef, lagoon, and off-reef f&es, James atoll reef (Lower Cretaceous), Fairway Field, Texas. In Lower Cretaceous depositional environments, from shoreline to slope--a Core Workshop (eds Bebout, D. & Ratcliff, D.). pp. l-12 (U nir*ersity of Texas, Bureau of Economic Geology). Alsharhan, A. S. 1987. Geology and reservoir characteristics of carbonate buildup in giant Bu Hasa oil field, Abu Dhabi, United Arab Emirates. American Association of Petroleum Geologists, Bulletin 11, 13061318. Bebout, D. G. & Loucks, R. G. 1983. Lower Cretaceous reefs, South Texas. In Carbonate depositional emironments (eds Scholle, I’. A., Bebout, D. G. & Moore, C. H.), American Association of Petroleum Geologists, Memoir 33, 441-444.
Lower
Bilodeau,
W. L. 1978. The
Glance
Cretaceous
coral-rudist
Conglomerate,
a Lower
33.5
patch reef
Cretaceous
syntectonic
deposit
in southeast-
ern Arizona. In Land of Cochise (eds Callender, J. F., Wilt, J. C. & Clemons, R. E.), pp. 209-214 (New Mexico Geological Society Guidebook, 29th Field Conference). Bilodeau, W. L. 1982. Tectonic models for Early Cretaceous rifting in southeastern Arizona. Geology 10, 466470. Bilodeau, W. L. & Lindberg, F. A. 1983. Early Cretaceous tectonics and sedimentation in southern Arizona, southwestern New Mexico, and northern Sonora, Mexico. In Mesozoic paleogeography of the west-central United States (eds Reynolds, M. W. & Dolly, E. D.), Society of Economic
Paleontologists and Mineralogists, posium 2, 173-188. Collins,
L. S. 1988.
Rocky Mountain
Section, Rocky Mountain
Paleogeography
Sym-
The
fauna1 structure of a mid-Cretaceous rudist reef core. Lethaia 21, 271-280. setting of southeastern Arizona. In Land of Cochise (eds Callender, J. C. & Clemens, R. E.). pp. 285-290. (New Mexico Geological Society Guidebook, 29th
Coney, P. J. 1978. The plate tectonic J. F., Wilt,
Field Conference). Dickinson, W. R., Fiorillo, A. R., Hall, D. L., Monreal, R., Potochnik, A. R. & Swift, P. N. 1989. Cretaceous strata of southern Arizona. In Geology of Arizona (eds Jenney, J. P. & Reynolds, S. J.). Arizona Geological Society Digest 17, in press. Dickinson, W. R., Klute, M. A. & Swift, P. N. 1986. The Bisbee Basin and its bearing on late Mesozoic paleogeographic and paleotectonic relations between the Cordilleran and Caribbean regions. In Cretaceous stratigraphy of western North America (ed. Abbott, P. L.). Society of Economic Paleontolo-
gists and Mineralogists,
Pacific Section, Book 46, 51-62. foraminiferal genus Orbitolina in North America. U. S. Geological Surve) Professional Paper 333, 52 pp. U. S. Geological Survey Professional Paper Drewes, H. D. 1981. Tectonics of southeastern Arizona. 1144,96 pp. Ekdale, A. A., Bromley, R. G., & Pemberton, S. G. 1984. Ichnology: trace fossils in sedimentology and stratigraphy. Society of Economic Paleontologists and Mineralogists, Short Course 1.5, 317 pp. Enos, P. 1974. Reefs, platforms, and basins of middle Cretaceous in northeast Mexico. American Association of Petroleum Geologists, Bulletin 58, 800-809. Enos, P. 1983. Shelf environment. In Carbonate Depositional Environments (eds Scholle, P. A., Bebout, D. G., & Moore, C. H.), American Association of Petroleum Geologists, Memoir 33, 267-295.
Douglass,
Frost,
R. C. 1960.
The
S. H., Bliefnick, Cretaceous rudist
D. M., & Harris, P. M. 1983. Deposition and porosity evolution of a Lower buildup, Shuaiba Formation of eastern Arabian Peninsula. In Carbonate Core Workshop (ed. Harris, P. M.), Society of Economic Paleontologists and Mineralo-
buildups-a gists, Core Workshop 4, 381410. Gilluly.
J.
1956.
General
geology
of
central
Cochise
County
Arizona.
U.
S.
Geological
Surzq
Professional Paper 281, 169 pp. Grocock, G. R. 1975. Stratigraphy and petrography of the upper member of the Mural Limestone in southeast Corhise County, Arizona. Unpublished M.S. thesis, University of Colorado, Boulder, 125
PP. Hayes,P.
T.
1970a.
Mesozoic
stratigraphy
of the Mule
and Huachuca
Mountains,
Arizona.
CT. S.
Geological Survey Professional Paper 658-A, 28 pp. Hayes,
P. T.
1970b.
Cretaceous
paleogeography
of southeastern
Arizona
and adjacent
areas.
1:. S.
Geological Survey Professional Paper 6.58-B, 42 pp. Heckel, P. H. 1974. Carbonate Laporte. L. F.), Society
buildups
in the geological
record:
of Economic Paleontologists
a review.
In Reefs in time and space (ed.
and Mineralogists,
Special
Publication
18,
90-l 55. Kauffman, E. G. & Johnson, C. C. 1988. The morphological and ecological evolution of middle and Upper Cretaceous reef-building rudistids. Palaios 3, 194-216. Kauffman, E. G. & Sohl, N. F. 1974. Structure and evolution of Antillean Cretaceous rudist frameworks. Verhandlungen Naturjorschende Gesellschajt, Base1 84, 399467. Klute, M. A. 1987. Tectonic significance of sandstone petrofacies within the Bisbee Basin of southeastern Arizona. In Mesozoic rocks of southern Arizona and adjacent areas (eds Dickinson, W. R. & Klute, M. A.), Arizona Geological Society Digest 18, 263-272. Masse, J.-P. & Philip, J. 1981. Cretaceous coral-rudist buildups of France. In Europeanfossil reef models (ed. Toomey. D. F.), Society of Economic Paleontologists and Mineralogists, Special Publication 30,
399-426. Monreal,
R. 1985.
Cretaceous),
Lithofacies, depositional environments, and diagenesis of the Mural Limestone (Lower Lee Siding area, Cochise County, Arizona. Unpublished M.S. thesis, University of
Arizona, Tucson, Nelson, H. F. 1973.
100 pp. The Edwards
reef
complex
and
associated
sedimentation
of Texas, Bureau of Economic Geology, Guidebook 15, 34 pp. D. 1986. Some aspects of the evolution of the rudist pelecypods. The Nautilus
in central
Texas.
University
Nicol, Perkins, B. F. Limestone
100,69-71. 1974. Paleoecology of a rudist reef complex in the Comanche Cretaceous Glen Rose of central Texas. In Aspects of Trinity Division geolog_v (ed. Perkins, B. F.), Geoscience
and Man 8, 131-173.
336
P. 5’1.Hartshorne
Ransome. F. L. 1904. The geology and ore deposits of the Bisbee Quadrangle, Arizona. i ‘. S. Geological Surwy Professional Paper 21, 16X pp. Rose, P. R. & Lidz, B. 1977. Diagnostic foraminiferal assemblages of shallow-water modern environments: South Florida and the Bahamas. Sedimenta 1 ‘I, Rosen.&1 School of Marine and _+ltmospheric Sciences, C’niwrsit_v of Miami, Florida. .iS pp. Roybal. G. H. 1979. Facies relationships in a patch reef of the upper Mural Limestone in southeastern Arizona. Unpublished M.S. thesis, University of Arizona, Tucson, 76 pp. Roybal, G. H. 1981. Facies development in a Lower Cretaceous coral-rudist patch reef (Mural Limestone, Southeast Arizona). The Mountain Geologist 18, 46&56. Sass, E. & Bein, .4. 1982. The Cretaceous carbonate platform in Israel. C’retaceous Research 3, 135-l-N Scott. R. W’. 1979. Depositional model of Early Cretaceous coral-algal-rudist reefs, Arizona. zilmerican Association of Petroleum Geologists. Bulletin 63, 1108&l 127. Scott, R. W. 1981. Biotic relations in Early (Jretaceous coral-algal-rudist reefs, Arizona. Journal of Paleontology 55, 463-478. Scott, R. W. 1984. Evolution of Early Cretaceous reefs in the Gulf of Mexico. Palaeontagraphica Americana 54, 406 -412. Scott, R. W. 1987. Stratigraphy and correlatmn of the Cretaceous Mural Limestone. Arizona and Sonora. In Mesozoic rocks of southern dgrrzona and adjacent areas (eds Dickinson, \V. R. & Klute, 11. A.), Arizona Geological Society Digest 18, 327 334. Scott, R. \V. 1988. Evolution of Late Jurassic and Early Cretaceous reef biotas. Palaios 3, 184-193. Scott, R. \V’. & Brrnckle, I’. I,. 1977. Biotic zonation of a lower Cretaceous coral-algal-rudist reef, Arizona. In Proceedings: Third International C’oral Reef Symposuun, Vol. 2 (ed. Taylor, D. I,.), pp. 183-189. i Rosensteil School of ,Varine and Atmospheric Science, University of Miami, Florida i Stoyanow, A. 1949. Lower Cretaceous stratigraphy in southeastern Arizona. Geological Society of rlmerica, Memoir 38, 169 pp. Warzeski, E. R. 1983. Facies patterns and diagenesis of a Lower Cretaceous carbonate shelf: northeastern State University of New York. Sonora and southeastern Arizona. Unpublished Ph.D. thesis, Binghamton, 401 pp. Warzeski, E. R. 1987. Revised stratigraphy of the Mural Limestone: a Lower Cretaceous carbonate shelf In Arizona and Sonora. In ,Wesozoic rocks of southern Arizona and adjacent areas (eds Dickinson, \V. R. & Klute, M. A.), Arizona Geological Soczety Digest 18, 335-363. Wilson, J. L. 1975. C’arbonatefacies ingeologir histor.v, 471 pp. (Springer-Vrrlag, New York). Wilson, J. L. & Pialli. G. 1977. A Lower Cretaceous shelf margin in northern Mexico. In C’retaceous carbonates of Texas and Mexico, applications to subsurface exploration (eds Bebout. D. G. & Loucks. R. G.), University of Texas, Bureau of Economic Geolog_v, Report of Investigations 89, 286-294. Zeller. R. A., Jr. 1965. Stratigraphy of the Big Hatchet Mountains area, New Mexico. New Mexico Bureau of Mines and Mineral Resources, Memoir 16, 128 pp. Zeller, R. A., Jr. 1970. Geology of Little Ilatchet Mountains, Hidalgo and Grant Counties, Neu Mexico. ,Veew Mexico Bureau af Mines and Mineral Resources, Bulletin 96, 26 pp.
Research
(1989)
10, 3 1 l-336
Facies architecture of a Lower Cretaceous coral-rudist patch reef, Arizona P. M. Hartshorne *Department Received
of Geosciences,
University
of Arizona,
3 August 1988 and accepted 28 March
Tucson, Arizona
85721, U.S.A.
1989
The Lower Cretaceous Mural Limestone marks the maximum marine incursion into southeast Arizona during Aptian-Albian time and records the middle Cretaceous transition from coral-dominated to rudist-bivalve dominated reefs. Upper Mural Limestone facies are most often dominated by corals. However, rudists form significant frameworks at some localities, one of which is described in this paper. The paleoenvironmental distribution of three potential reef-builders (corals, rudists, and ‘oysters’) were studied at this patch reef locality. Corals built the framework of the inner reef core. The rudist Petalodontiu initially gained a foothold in sheltered areas among corals and subsequently built a framework in the outer reef core. Caprinid rudists formed mounds in the outer reef to back reef areas. The rudists Toucasia and Monopleura and the oyster-like bivalve Chondrodonta formed beds or were scattered in the reef-flank and shelf lagoon sediments and did not contribute to the reef framework. Upper Mural Limestone reefs are important examples of the coexistence of corals and rudists during this middle Cretaceous fauna1 transition period. This study supports the idea that rudist-bivalves initially colonized protected back-reef areas early in the Cretaceous and only later in the Cretaceous did rudists dominate reef frameworks. KEY WORDS: Aptian-Albian;
Early
Cretaceous;
rudists;
corals;
reefs;
Arizona
1. Introduction The Cretaceous was a time of major reef-building activity throughout the tropical Tethyan Seaway, which extended from about 40” N to 20” S of the equator (Wilson, 1975). Corals were the dominant reef-builders of the Early Cretaceous, but by the Late Cretaceous reefs were dominated by the gregarious rudistid-bivalves (Kauffman & Sohl, 1974). The rudists were bizarre bivalves which first appeared in the Late Jurassic and evolved rapidly in the Late Cretaceous, only to become extinct at the end of the 1986). Rudists formed significant buildups on the Cretaceous (Nicol, carbonate platforms and offshore banks of what is now southern Europe, the Middle East, southern Asia, and currently submerged atolls of the middle Pacific Ocean (Heckel, 1974; Wilson, 1975). They also generated reefs and bioherms on the continuous band of shelf margins encircling the Cretaceous Gulf of Mexico and also in the Caribbean region (Kauffman & Sohl, 1974; Wilson, 1975; Bebout & Loucks, 1983). Outcrops of rudist buildups are found in northern Sonora and Chihuahua, Mexico, and their northern limit extends into southeast Arizona, southwest New Mexico, and central Texas in the United States (Hayes, 1970b; Bebout & Loucks, 1983). In the Lower #Present
address:
1638 E. 7th Street,
01956671/89/040311f26 $03.0010
Tucson,
Arizona
85719,
U.S.A.
;:c 1989 Academic Press Limited
312
I’. 11. liartshornr
Cretaceous upper Mural Limestone of southeast .Arizona, most reefs are dominated by corals and algae with rudists as significant but subordinate framework contributors (Scott, 1979). This study is a detailed examination of the fossils and lithologies present in a coral-rudist patch reef in southeast Arizona. The objective of the research is the elucidation of the depositional history of this particular patch reef and an assessment of the roles of the three potential reef-builders present, (1) corals, (2) rudists, and (3) the oyster-like bivalve Chondrodonta.
2. Geologic
setting
Late Jurassic to earliest Late Cretaceous sediments accumulated in SE Arizona and adjacent areas of New Mexico and Mexico in the Bisbee Basin, a NW-trending extension of the Chihuahua Trough. This intracontinental extensional depression was related to the opening of the Gulf of Mexico. Later backarc spreading extended the depression northwestward to penetrate the back side of a magmatic arc on the western continental margin (Figure 1). Further discussion of the tectonic setting of the Bisbee Basin and the surrounding region is given by many authors (Coney, 1978; Drewes, 1981; Bilodeau, 1982; Bilodeau & Lindberg, 1983; Dickinson et al., 1986, 1989).
N Estimated positlon of Early Cretoceous magmatic arc
t
Figure 1. Paleogeographic map of the southwest USA and northern Mexico showing the approximate limit of Aptian to lower Albian deposition, the position of the Early Cretaceous magmatic arc along the west side of this region, and the position of a rift shoulder bounding the north side of the Bisbee Basin. After Dickinson et al. (1989) and Bilodeau & Lindberg (1983).
Lower Cretaceous coral-rudist patch reef
2.1.
313
Bisbee Group
In southeast Arizona, sediments deposited in the Bisbee Basin during the Cretaceous are known as the Bisbee Group (Ransome, 1904). Numerous authors have described the stratigraphy and paleogeography of Bisbee Group sediments, most notably Ransome (1904), Stoyanow (1949), Gilluly (1956), Hayes (1970a, 1970b), Bilodeau i3zLindberg (1983) and Dickinson et al. (1989). The Bisbee Group can be subdivided into non-marine deposits to the NW and transgressive-regressive marine deposits to the SE (Dickinson et al., 1986). These two segments represent different depocenters and are not well correlated due to poor age control (Klute, 1987). In addition, correlation is hampered by the isolation of outcrop exposures in uplifted blocks of Basin and Range topography (Dickinson et al., 1989). My study area lies within the SE segment.
C1ntura formatIon
Morita formotlon
Glance conglomerate
? ? / Figure 2. Generalized stratigraphic section of the SE segment of the Bisbee Group in SE Arizona.
314
I’. IvI. Hartshome
The SE segment of the Bisbee Group (Figure 2) is divided into four formations (Ransome, 1904). From oldest to youngest these are (1) the Glance Conglomerate, (2) the Morita Formation, (3) the Mural Limestone, and (4) the Cintura Formation. Alluvial fan and fluvial deposits of the Glance Conglomerate were deposited during Late Jurassic to Early Cretaceous rifting (Bilodeau, 1978). The upper three formations of the Bisbee Group make up a major transgressive-regressive sequence deposited during passive thermotectonic subsidence after rifting had ceased (Bilodeau, 1982; Klute, 1987).
2.2.
Mural Limestone
The marine Mural Limestone is located above the transgressive Morita Formation and below the regressive Cintura Formation. The Mural Limestone was deposited upon a carbonate and siliciclastic shelf to shelf margin complex in the SE portion of the Bisbee Basin (Hayes, 1970a, 1970b). The formation becomes dominantly siliciclastic to the NW, pinching out west of the Huachuca Mountains and north of the Mule Mountains. This facies transition indicates the NW limit of marine conditions in SE Arizona during Aptian to early Albian time (Hayes, 1970a). To the south, the Mural Limestone thickens towards the shelf margin until it disappears beneath Tertiary and Quaternary volcanic cover about 30 km south of the Mexican border (Warzeski, 1983; Scott, 1987). The thickness of the Mural Limestone ranges from 90 to 250m in the Mule Mountains and Huachuca Mountains of southern Arizona (Hayes, 1970a). The formation is informally divided into two members: a lower unit of interbedded mudstone, limestone, siltstone, and sandstone (90-357 m, thickening westward to New Mexico), and an upper unit of massive fossiliferous limestone (So-260 m, thickening southward into Mexico) which contains coral-algal-rudist buildups (Hayes, 1970a; Scott, 1987; Warzeski, 1987). Stoyanow (1949) used ammonites to place the Aptian-Albian boundary within the lower member of the Mural Limestone. However, Scott (1987) used graphic correlation techniques to place this boundary in the basal portion of the upper Mural Limestone. The upper Mural Limestone is correlative with a portion of the Upper Tamaulipas Formation of NE Mexico, the Glen Rose Formation of central Texas, and with part of the upper U-Bar Formation of SW New Mexico (Scott, 1987; Warzeski, 1987). Numerous coral-algal-rudist patch reefs occur within the upper Mural Limestone and its correlatives in SE Arizona and SW New Mexico (Zeller, 1965, 1970; Hayes, 1970a, 1970b; Scott & Brenckle, 1977). Warzeski (1983) documents two prograding carbonate bank barrier complexes in northern Sonora, Mexico, and postulates the existence of a third hidden below volcanic overburden further to the south. Patch reefs of SE Arizona have been the focus of several studies: Grocock (1975), Scott & Brenckle (1977), Scott (1979, 1981, 1987), Roybal (1979, 1981), Warzeski (1983, 1987) and Monreal (1985).
Lower
Cretaceous
coral-rudist
patch
reef
315
study area. The patch reef described in this paper is located in the extreme SE corner of Arizona, just west of the Peloncillo Mountains in Guadalupe Canyon (N1/2, SE1/4, sec. 17, T24S, R32E, Guadalupe Canyon Quadrangle, Cochise County, Arizona; Figures 3,4). Scott (1979) defined and described the lithofacies of upper Mural Limestone bioherms throughout SE Arizona, including this locality (his Lone Butte locality #8326, Scott, 1979, figure 7A). This outcrop is a hill (locally known as Mexican Saddlehorn) of lower Mural Limestone capped by a resistant, cliff-forming, upper Mural Limestone coral-rudist bioherm (Figure 5).
3. Methods A topographic map of the top of the Mexican Saddlehorn hill was prepared using plane table and alidade. Mapping was done with a contour interval of 5 m and covered an area of approximately 900 x 1200 m. Ten stratigraphic sections spanning between 4 and 25 m in thickness were measured on the cliff faces that surround the top of the hill (Figure 4). The lithologies and the presence of macrofossils were used to correlate between sections and to define facies. Thirty-five thin-sections were made to facilitate microfacies analysis and microfossil identification.
Map area
Scale in kllometers
Figure 3.
Location
map of the study area in SE Arizona. Also shown is the location of the Paul Spur reef (PS) cited in the text.
Mexican Scddlehorn
Scale
in meters Contour
30‘--
Interval
=5 m
30
Mexico
Figure
4. Topographic
map of the study area showing the locations A-J and the cross-section Z-Z’.
of measured
4. Facies descriptions and paleoenvironmental
stratigraphic
sections
discussion
Eleven facies were identified based on field and laboratory descriptions of lithologies and fossils (Tables l-3). Most facies show little evidence of internal bedding. Gradational change between facies, both laterally and vertically, is common. The 11 facies are organized here into four groups (Figure 6), (1) the Basal Facies Group, (2) the Framework Facies Group, (3) the Framework-Flanking Facies Group, and (4) the Uppermost Blanketing Facies Group. 4.1.
Basal Facies Group
This lowermost group of facies consists of the fossiliferous calcareous quartz sandstone facies, the burrowed fossiliferous coated-grain packstone facies, and the fossiliferous coated-grain packstone/grainstone facies. Stratigraphically, these facies occur below all of the other facies studied.
Lower
t:igurr
Cretaceous
coral-rudist
5. View of the west side of the LIes~can
patch
Saddlehorn
317
reef
locality
(shoun
bs arrow)
‘I’he fossiliferous calcareous quartz sandstone facies locally occurs in the uppermost part of the lower Mural Limestone. About 1.5 m of this unit is exposed; erosional debris buries the basal and upper contacts. Beds crop out as thin ledges (less than 30 cm thick) that appear to be very crudely bedded into shell-rich and shell-poor layers. Fine to very fine sand-sized, angular to subangular quartz grains are cemented by sparry calcite, and exhibit corrosion on grain edges. Unbroken gastropods are common and several pectinid bivalves are present. The surface of one ledge (section I) exhibits parallel-oriented high-spired gastropods (spires pointing NW). Molluscan fragments are abundant, and smaller amounts of grapestones, reworked grains, ooids, coated grains, and other fossil fragments are present (Figure 7). ‘I’he fossiliferous calcareous quartz sandstone facies probably accumulated in a high-energy shallow subtidal to intertidal environment. This is indicated by the absence of micrite in both the matrix and fossil cavities, the abundance of quartz sand, the presence of reworked and rounded grains, and the parallel alignment of gastropods. Tidal currents removed fine-grained mud and collected and aligned the lag deposits of gastropod shells. Grapestones and reworked fossil fragments with micrite fillings and attached micrite matrix formed in muddier low-energy areas and were transported to the higher energy area. The absence of terrigenous material in any facies deposited after the sandstone indicates a cut-off of the supply of elastics. This could have been a result of deepening water or of the development of a coeval carbonate environment closer to shore. The burrowed fossiliferous coated-grain packstone facies is represented here by a single bed about 1 m thick. Mud and fossil hash occur in clumps due to
Facies
S-10 3 3& 70 60-70 30-50 5525 15560 20-55 20-55 7720
1.5 2.2556.7 4.1-6.1 5.0-8.25 0.8-11.55 1.8-13.5 0.34.0 1 .o-2.3 0.1-0.7
30-35
“” Fossils
Saddlehorn.
0.85-0.95
1.5
(m)
Thickness
1. Properties that characterize facies in the upper Mural Limestone at Mexican fragments (unidentified fossil fragments). grains, and matrix. (TR = trace amount).
Fossiliferous calcareous quartz sandstone Burrowed fossiliferous coated-grain packstone Fossiliferous coated-grain packstone/ grainstone Coral boundstone Petalodontid boundstone Caprinid floatstone/rudstone Lime mudstone/wackestone Fossiliferous wackestone/floatstone Toucasid wackestone/floatstone Chondrodontid-caprinid floatstone/ boundstone Miliolid-monopleurid lime mudstone/ wackestone
Table
Ranges
227
5-l 5
85 5-10 5520 5-15 l-10 8830 8835
5-10
10
ob Fragments
are given
TR
0
55 TR 0 TR TR TR TR
40-55
35-41
o/oGrains
for total thicknesses
of fossils,
80-Y 5
45-80
45 30-70 30-75 50-75 75595 40 80 45580
40-60
15-35
O0 Matrix
and percentages
Fossiliferous calcareous quartz sandstone Burrowed fossiliferous coated-grain packstone Fossiliferous coated-grain packstone/ grainstone Coral boundstone Petalodontid boundstone Caprinid floatstone/rudstone Lime mudstone/wackestone Fossiliferous wackestone/floatstonc Toucasid wackestoneifloatstone Chondrodontid-caprinid floatstonei boundstone Miliolid-monopleurid lime mudstonr/ wackestone
Facies
Table 2. Range of per cents of major 1,
”
02 0
o--3 TR TR 410
3 34
0
4-9 36-53 15-22 o-8 2-m19 3-20
0
G20 0~1 9-2 0 &5 O-4
0 15-35 l-.4 4-6 o-6 0 14 o- 12
TR 20-55 24 4-x 06 1 16 l-12 O-1
0
0
0
TR
0
“” Rudist
0
Branihing coral
0
Maszive coral
TR
II Coral
0
fossil types for each facies. 0
4-8
03
01 0 0 0 GlO G8
0
0
0
A’Gopleurid
0
Y’” Pet&dontid
0
“;] Caprinid
@2
” Toucasid
0
0
70 Chondrodontid
3. Percentage
F&es
of grain types for each facies.
Miliolid-monopleurid wackestone
boundstone
lime mudstone/
Fossiliferous calcareous quartz sandstone Burrowed fossiliferous coated-grain packstone Fossiliferous coated-grain packstone/ grainstone Coral boundstone Petalodontid boundstone Cap&id floatstone/rudstone Lime mudstone/wackestone Fossiliferous wackestone/floatstone Toucasid wackestone/floatstone Chondrodontid-caprinid floatstone!
Table
(A=abundant,
0
0 0
TR 0 0 0 0 0 0
55 0 0 0 0 GTR 0-TR
0
0
0
35-55
TR
0
TR-A ?-50
TR 0 ? >
2
0
“,Peloid
2-5
“oOoid
occurrence)
0
“” Coated-grain
? =possible
0
0
0 0 0 0 0 (tTR 0
0
556
0 0 Grapestone
0
0 0 0 0 0 TR 0 0
0
28-30
“,,Quartz
0
sand
0
0
0 TR 0 0 0 0 O&‘I’R 0
0 5
TR
or, Intraclast
Lower
FACIES
Cretaceous
coral-rudist
patch
FACIES
GROUPS
Uppermost
Blanketing
Facies
Miholid-monopleurid
lime
Chondrodontid-caprinid
Taucasid Framework-flanking
Facies
Fossiliferous Lime
Framework
Facies
Caprmid
Coral
Facies
mudstonelwackestone floatstonelboundstone
wackestone/fioatstone wackestone/floatstone
mudstone/wackestone
floatstonelrudstone
Petafodontid
Basal
321
reef
boundstone
boundstone
Fossiliferous
Burrowed
Fossiliferous
coated-grain
fossiliferous
calcareous
packstonelgrainstone
coated-grain
ouartz
packstone
sandstone
facies
6
t;~gure
6.
Diagram
showing
the geometry of the facies described in the text. Refer to Figure Iocations of the lettered sections.
4 for the
horizontal burrowing, giving the rock a though indistinct, pervasive, mottled appearance. The matrix is micrite with patches of sparry calcite, some of which shows evidence of neomorphism. Much of the micrite matrix may be composed of degraded peloids. Coated skeletal grains dominate and some grains are reworked (Figure 8). Whole regular and irregular echinoids are common; brachiopods and gastropods are less abundant. The single bed representing the fossiliferous coated-grain packstone; grainstone facies is about 1.5 m thick. Variable, but subequal, amounts
322
I’.
RI.
tiartshorne
Figure /. Photomlcrograph from the towlltrrous ca Ic‘arrwx the photo is a worn gastropod shell. A grapestone is below right corner of the photo. Bar = I mm.
Figure
X. Photomicrogrnph coated fossil fragments
LIuartz sandstone taucs. the wstropod and another
from the burrowed fossiliferous coated-grain packstone and patches of sparry calcite occur in 3 micritic matrix.
III
rhc cvntc~-
facirs. \Vorn Bar1 mm.
and
Lower
Cretaceous
coral-rudist
patch
reef
323
of micrite and sparry calcite form the matrix. Ghosts of grains present in the sparry calcite indicate at least some neomorphism. This facies is dominated by coated mollusc and echinoderm fragments (Figure 9). Solution boundaries between grains are common. Both the burrowed fossiliferous coated-grain packstone facies and the fossiliferous coated-grain packstone/grainstone facies are dominated by coated fossil fragments in a micrite and sparry calcite matrix. The coated grains probably formed under moderate to high energy conditions on shoals and were then moved off the shoals into lower energy areas where lime mud could not be completely winnowed away. The greater amount of mud and whole echinoids in the burrowed fossiliferous coated-grain packstone may indicate deposition in a lower energy area further from the shoal. The presence of echinoids and brachiopods show this facies probably formed under normal marine conditions. The irregular echinoids found in the burrowed facies may have been responsible for producing the burrows. Regular echinoids were also present in the burrowed facies indicating proximity to a firm substrate-perhaps even the initial coral reef colony. 4.2.
Framework
Facies Group
The framework facies group includes the coral boundstone facies, the petalodontid boundstone facies, and the caprinid floatstone/rudstone facies. This group forms a prominent mound with adjacent beds thinning outward away from the core.
Figure 9. Photomicrograph coated fossil fragments Bar= 1 mm.
from the fossiliferous coated-grain packstone/grainstone in a micrite and sparry calcite matrix. Note closer packing
facie. Worn and than in Figure 8.
The coral boundstone facies is represented here by a large elongate lenticular bed and several smaller isolated lenses. These lenticular beds exhibit thicknesses ranging from 2.256.7m. The matrix is a micritic wackestone to packstone containing whole fossils and fossil fragments of gastropods, brachiopods, rudists, and foraminifera. Massive, tabular, and laminar corals are very abundant (Actinastrea spp. and Microsolena texana are common); some branching and solitary corals are also present. Coral abundance is variable; the lens perimeters tend to have lower coral densities than the lens interiors. Corals are recrystallized and are often heavily bored by bivalves, sponges, and other organisms, and are also encrusted by algae, foraminifera, and occasionally by oysters and rudists sponges, serpulids, (Figure 10). Micrite filling and geopetal structures often occur in borings and fossil cavities. This coral boundstone, dominated by massive domal and tabular corals, formed the inner core of the reef and appears to have been the pioneer coral community recognized by Scott & Brenckle (1977) in other Mural Limestone reefs. The coral lenses formed on a variety of substrates that must have been stabilized or perhaps had become hardground surfaces. This could have permitted colonization by the corals. The climax coral community found above this pioneer community in many of the other upper Mural Limestone patch reefs, the lamellar form of the coral Microsolena encrusted by stromatolites (Scott & Brenckle, 1977; Scott, 1979; Roybal, 1981; Warzeski, 1983; Monreal, 198S), does not occur at Mexican Saddlehorn. The implications of this difference are discussed later in the Comparison Section.
Lower Cretaceous
Figure
11.
Field
photograph upright
coral-rudist
32.5
patch reef
of the petalodontid boundstone facies. This vertical sectional petalodontids and abundant fragments in the matrix.
vieu
shows
Directly overlying the largest coral boundstone bed is a lenticular bed (4.1-6.1 m thick) which represents the petalodontid boundstone fakes. The facies has a micritic matrix and is dominated by the rudistid bivalve Petalodontia felixi (Douville). These rudists commonly form a framework of upright and closely packed individuals (Figure 11). Petalodontids also occur individually and in clusters, both in and out of life position, at the base of the bed and in deposits lateral to the lens. Fossil cavities are often filled with unfossiliferous micrite and some exhibit geopetal structure. Petalodontid fragments are abundant in the micritic matrix between rudist individuals, especially at the lens base. The petalodontid boundstone facies formed in the outer reef core. Corals may have provided initial shelter and support for these rudists, since scattered petalodontids occur among the corals in some areas of the upper portion of the coral boundstone. The petalodontids were then able to form a massive framework based on mutual support of adjacent individuals. Displaced individuals and displaced clusters of rudists may be evidence of
326
P. .\I. Hartshome
_~_~~
such as storm events. The abundant occasional high energy conditions, petalodontid fragments trapped in the open space between upright petalodontids were most likely a result of pervasive bioerosion. The caprinidjloatstonelrudstone facies is represented by a single lenticular bed (5.0-8.25 m thick). The matrix is a foraminiferal wackestone composed of both micrite and sparry calcite. The dominant fossils in this facies are caprinids. The two species of caprinids found in the Mural Limestone are Caprinuloidea gracilis Palmer and Coalcomana ramosa (Boehm). Other common fossils are orbitolinid and miliolid foraminifera. The caprinid rudists are not as closely packed as the petalodontids and tend to occur in recumbent rather than upright growth positions (Figure 12). The caprinids lived in the outer reef core to back-reef areas. Wilson (1975) notes that caprinids tend to occur in shallow back-reef mounds, although he comments that they also occurred in shelf margin and downslope areas. Densities of the caprinids varied, though the largest accumulation was in the area above the petalodontid boundstone. The caprinids did not appear to form a rigid framework, but instead laid loose on the substrate. In spite of this apparent lack of framework, this facies is included in the framework group because of its association with and contribution to the reef buildup.
4.3.
Framework-Flanking
Facies
Group
the lime of this group, Outcrops representing the two facies mudstone/wackestone facies and the fossiliferous wackestone/floatstone facies, occur lateral to the lenticular beds of the framework facies group.
Figure
12. Field
photograph
of the caprinid floatstonejrudst~jne facies. This shows caprinids in rwumbent positions.
vertical
sectional
Lower
Cretaceous
coral-rudist
patch
reef
327
The combined thickness of the flanking facies lessens noticeably away from the framework group of facies. The lime mudstonelwackestone facies is represented by outcrops which vary from 0.8 m to 11.55 m thick. The homogeneous micrite matrix contains the large foraminifer Orbitolina texana (Roemer) and other foraminifera, especially miliolids (Figure 13). Other fossils present are brachiopods, echinoids, and high-spired gastropods. Fossils are very sparse throughout the beds, especially in the lowermost outcrops, but increase in abundance upwards. The lime mudstone/wackestone facies represents low wave energy, shelf lagoon sedimentation in inter-reef habitats. The paucity of fossils indicates reef debris was not being transported into this area. Locally abundant accumulations of orbitolinid foraminifera indicate an environment with shallow to moderate water depths and open marine circulation (Douglass, 1960; Warzeski, 1983). The absence of laminations may suggest complete bioturbation or perhaps very slow sediment accumulation in the inter-reef areas. Outcrops of the fossiliferous wackestone/fIoatstone furies range from about 1 .X-13.5 m thick. The matrix is dominantly micrite with small amounts of sparry calcite. This facies locally grades into mudstone or packstone and is gradational between the framework facies group and the lime mudstone/wackestone facies. Fossil abundance varies locally and common fossils include corals, bivalves, rudists, echinoderms, gastropods, serpulids,
Figure 13. Photomicrograph from the lime mudstone/wackestone facies. An orbitolinid occupies most of the center of the photo. Bar= 1 mm.
foraminifer
I’. 11. Hartshorne
328
miliolids, orbitolinids, other foraminifera, calcispheres, and unidentified fragments (Figures 14, 15). No one type of fossil is dominant. The fossiliferous wackestone/floatstone facies formed the flanking beds of the reef core, containing debris washed out of the reef proper. The high diversity of fossils present indicates the sediments were derived from the adjacent framework buildup. These sediments washed out over and graded into the inter-reef lime mudstoneiwackestone. 4.4.
Uppermost
Blanketing
Facies
Group
The uppermost blanketing facies group forms a fairly uniform layer over the entire top of the Mexican Saddlehorn hill. The three facies that form this group are the toucasid wackestone/floatstone facies, the chondrodontidcaprinid floatstone/boundstone facies, and the miliolid-monopleurid lime mudstone/wackestone facies (Figure 16). Beds of the toucasid wackestone/jloatstone facies vary in thickness from 0.3 m to 4.0m. The matrix consists of micrite and sparry calcite and contains abundant miliolids, mollusc fragments, and peloidal grains (Figure 17). The rudist Toucasia hancockensis Whitney is the most abundant macrofossil. These rudists are randomly scattered and never form biostromes. Unfossiliferous micrite often fills the toucasid body cavity. The chondrodontid-caprinidjloatstone/boundstone facies occurs as a bedded unit (1 .O-2.3 m thick) with individual beds varying in thickness from 0.2 to 1.5 m. Chondrodonta, an oyster-like bivalve, and caprinids are the dominant
Figure
14.
Photomicrograph from the fossiliferous wackestone/floatstone facie. Several orbitolinids scattered throughout the photo around the valves of a brachiopod. Bar= 1 mm.
are
Lower Cretaceous
Figure
15.
Photomicrograph
from prominent
coral-rudist
329
patch reef
the fossiliferous \\,ackestone/~oatstonc facies. on the right side of the photo. Bar = 1 mm.
Coral
fragments
are
fossils, generally occurring in alternating beds. The basal bed is usually a biostrome of variably oriented chondrodontids, immediately succeeded by a bed of caprinids. The chondrodontids occur as individuals or as upright clusters (Figure 18). Remaining beds often contain both chondrodontids and caprinids, but one or the other is usually dominant. Chondrodontiddominated beds are sometimes capped by a few monopleurid rudists and may contain toucasids as a minor constituent. The miliolid-monopleurid lime mudstonelwackestone facies caps the uppermost blanketing facies group. The matrix is micrite with abundant miliolid foraminifera and lesser amounts of orbitolinids and other foraminifera (Figure 19). Varying amounts of the rudist Monopleura cf. M. marcida White are found as individuals, clusters, and thickets, often occurring in upright positions. Iron-stained and silicified Thalassinoides burrows are also present. Small patches of fossils contain gastropods and other unidentified fragments. A few oncolites were found in one small area. The uppermost blanketing facies group indicates burial of the reef buildup by more restricted lagoonal sediments. Reef growth had apparently ended due to environmental change to conditions less favourable to reefal organisms. Miliolid foraminifera are very abundant in these three facies indicating shallower and more restricted marine conditions than in the previous facies (Rose & Lidz, 1977; Warzeski, 1983). Thalassinoides indicates the presence of burrowing organisms, probably crustaceans, and may also occur in shallow water (Enos, 1983; Ekdale et al., 1984).
330
1’. 11. Hartshome
hlimketint: Figure 16. Vertical sectional \‘LWI ot the L,,>p”mosr to chcrndrodontld lwds. wackestone/floatstonz; arrows point M =monopleurid lime mudstone,‘\~a~kest(~tl~~.
t&es group (‘I’= toucasid C’= caprinid-dominated bed;
5. Comparisons The upper Mural Limestone patch reef discussed here exhibits important similarities and differences with other Lower Cretaceous reefs in Arizona and elsewhere. The organisms found at Mexican Saddlehorn are typical of those found in other upper Mural Limestone localities and the facies present in one reef can be recognized in another. The presence or absence of various facies gives clues to environmental differences between the reefs. For example, southeastward into Mexico (increasing water depth into the basin) the upper Mural Limestone thickens to such an extent that the expanded section can be further subdivided into five new members (Warzeski, 1983, 1987). This southward thickening accentuates differences and increases the complexity of the facies geometries in Mexico. The most striking difference between the Mexican Saddlehorn locality and other Mural Limestone patch reefs is the absence of the Microsolenastromatolite boundstone at Mexican Saddlehorn. Because the Mexican Saddlehorn patch reef is incompletely preserved due to faulting and erosion, it is possible that the missing portion of the reef contained this
Lower
Figure
17.
Photomicrograph
Cretaceous
coral-rudist
patch
from the toucasid ~ackestonelfloatstone abundant in the matrix. Bar = 1 mm.
331
reef
facies.
Miliolid
foraminifera
arr
boundstone. Microsolena does appear in the coral boundstone facies at Mexican Saddlehorn, but it is not associated with stromatolites and it is never a dominant community member-in contrast with the Paul Spur reefs Scott & Brenckle, 1977; Scott, 1979, 1981; (see Figure 3 for location; Koybal, 1981). Alternatively, the absence of the Microsolena-stromatolite facies may be attributed to ecologic factors. The coral growth form of this facies was originally thought to be encrusting and, along with other factors, suggested growth in a high energy environment (Scott & Brenckle, 1977; Scott, 1979; Roybal, 1981). Later study showed that the growth form was probably more ‘bush-like’ and rarely showed signs of breakage (Warzeski, 1983). Warzeski (1983) used these and other factors to conclude that the facies grew in deep water below storm wave base on the reef-slope and back-reef of the banks, and below normal wave base on the upper reef slope of the larger patch reefs. My study of the Mexican Saddlehorn locality indicates that this reef also grew in quiet water below normal wave base and may have been exposed to at least some storm waves. Scott & Brenckle (1977) noted that the pioneer community of tabular and massive corals could develop on the open lagoon substrate, but in these areas the Microsolena-stromatolite facies usually did not develop. The Mexican Saddlehorn patch reef may simply be smaller, shorter lived, and more isolated (the opposite of the expanded upper Mural Limestone of Mexico) in comparison with the other patch reefs. This would simplify the facies relationships and geometries and could result in the omission of some facies. In turn, the absence of the coral-
Lower
Cretaceous
coral-rudist
patch
reef
333
stromatolite boundstone may have allowed the petalodontids to move into the outer reef core and to build a dense boundstone. As a whole, the upper Mural Limestone coral-algal-rudist patch reefs are interpreted by Scott (1979) and Warzeski (1983) to be representative of low energy, stable marine environments. The corals and stromatolites grew in quiet conditions below wave base, and as the reef grew up into the zone of wave energy caprinids became dominant (Warzeski, 1983; Scott, 1984, 1988). Similar coral-dominated reefs are found in France (Masse & Philip, 1981), in the Glen Rose Limestone, the Stuart City Formation, and the James Limestone of Texas (Perkins, 1974; Wilson, 1975; Bebout & Loucks, 1983; Achauer, 1985), and in the Cupid0 Limestone of Mexico (Wilson & Pialli, 1977). This is in contrast to the high energy rudist-dominated reefs found in the Edwards Formation and the Glen Rose Limestone of Texas (Nelson, 1973; Perkins, 1974), in the Antilles of the Caribbean (Kauffman & Sohl, 1974), in the Middle East (Sass & Bein, 1982; Frost et al., 1983; Alsharhan, 1987), and in the El Abra Limestone of Mexico (Enos, 1974; Collins, 1988). For further discussion of the various types of Cretaceous reefs and their paleoenvironmental interpretations see Wilson (197.5), Scott (1984, 1988) and Kauffman & Johnson (1988).
6. Summary and conclusions The Mexican Saddlehorn locality preserves an Early Cretaceous patch reef. Four facies groups were recognized (Figure 20). The Basal Facies Group is that set of facies found stratigraphically below the framework of the reef. These are the Fossiliferous Calcareous Quartz Sandstone, the Burrowed Fossiliferous Coated-Grain Packstone, and the Fossiliferous Coated-Grain Packstone/Grainstone. These facies record deepening marine conditions and formed the base for reef development. The Framework Facies Group formed the framework of the reef. These facies are the Coral Boundstone, the Petalodontid Boundstone, and the Caprinid Floatstone/Rudstone. The corals formed the inner reef core, the petalodontids occupied the outer reef core, and the caprinids formed mounds in the outer reef core to back-reef areas. The absence of the IVicrosolenu-stromatolite facies may be a result of the incomplete preservation of the reef due to faulting or erosion, or may be the result of environmental factors related to the relatively small size and isolation of the patch reef in the open shelf lagoon. The Framework-Flanking Facies Group formed laterally to the framework group of facies. The Fossiliferous Wackestone/Floatstone formed the flanking beds of the reef and spread out over and graded into the Lime Mudstone/Wackestone (inter-reef facies). The combined thickness of these facies decreases away from the framework. The Uppermost Blanketing Facies Group caps the entire reef buildup. ‘l’hese are the Toucasid Wackestone/Floatstone, the ChondrodontidCaprinid Floatstone/Boundstone, and the Miliolid-Monopleurid Lime Mudstone. These facies indicate the cessation of reef growth and record shallower and more restricted shelf conditions.
334
P. M.
Ilartshornr
_N IO
0
Meters
1
Shelf lagoon
Figure
20.
Schematic
cross-section 2-Z’ through the Mexican Saddlehorn locality location of line Z-Z’; see Figure 6 for key to symbols).
(see Figure
4 for the
Petalodontid rudists initially grew in sheltered areas among the corals and were then able to form a substantial framework. The other rudists did not form such a framework. These observations substantiate the view that rudists were lower energy, back-reef inhabitants early in the Cretaceous, but later rudistid groups dominated frameworks in high energy environments (Masse & Philip, 1981; Scott, 1981; Frost et al., 1983).
Acknowledgements This project represents partial fulfillment of the Master of Science Degree at the University of Arizona. Research was completed under the direction of Dr Karl W. Flessa, with further guidance from Dr Andrew S. Cohen and Dr Joseph F. Schreiber Jr. Field vehicles and assistance were enthusiastically provided by Nancy Schmidt, Karin Barovich and Matt Gray. Many thanks to John and Mary Magoffin for access to the field site and for their hospitality. Research was partially supported by a grant-in-aid of research from Sigma Xi and from the Geological Society of America, and a graduate research grant from the University of Arizona, Department of Geosciences, funded by Chevron, USA. Additional thanks to the anonymous reviewers for their extremely helpful comments.
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Lower
Bilodeau,
W. L. 1978. The
Glance
Cretaceous
coral-rudist
Conglomerate,
a Lower
33.5
patch reef
Cretaceous
syntectonic
deposit
in southeast-
ern Arizona. In Land of Cochise (eds Callender, J. F., Wilt, J. C. & Clemons, R. E.), pp. 209-214 (New Mexico Geological Society Guidebook, 29th Field Conference). Bilodeau, W. L. 1982. Tectonic models for Early Cretaceous rifting in southeastern Arizona. Geology 10, 466470. Bilodeau, W. L. & Lindberg, F. A. 1983. Early Cretaceous tectonics and sedimentation in southern Arizona, southwestern New Mexico, and northern Sonora, Mexico. In Mesozoic paleogeography of the west-central United States (eds Reynolds, M. W. & Dolly, E. D.), Society of Economic
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L. S. 1988.
Rocky Mountain
Section, Rocky Mountain
Paleogeography
Sym-
The
fauna1 structure of a mid-Cretaceous rudist reef core. Lethaia 21, 271-280. setting of southeastern Arizona. In Land of Cochise (eds Callender, J. C. & Clemens, R. E.). pp. 285-290. (New Mexico Geological Society Guidebook, 29th
Coney, P. J. 1978. The plate tectonic J. F., Wilt,
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gists and Mineralogists,
Pacific Section, Book 46, 51-62. foraminiferal genus Orbitolina in North America. U. S. Geological Surve) Professional Paper 333, 52 pp. U. S. Geological Survey Professional Paper Drewes, H. D. 1981. Tectonics of southeastern Arizona. 1144,96 pp. Ekdale, A. A., Bromley, R. G., & Pemberton, S. G. 1984. Ichnology: trace fossils in sedimentology and stratigraphy. Society of Economic Paleontologists and Mineralogists, Short Course 1.5, 317 pp. Enos, P. 1974. Reefs, platforms, and basins of middle Cretaceous in northeast Mexico. American Association of Petroleum Geologists, Bulletin 58, 800-809. Enos, P. 1983. Shelf environment. In Carbonate Depositional Environments (eds Scholle, P. A., Bebout, D. G., & Moore, C. H.), American Association of Petroleum Geologists, Memoir 33, 267-295.
Douglass,
Frost,
R. C. 1960.
The
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