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USGS Open-File Report 94-023

Neogene Palynofloras And Terrestrial Paleoenvironments In Northern Latin America

Alan Graham
Kent State University, Kent, OH 44242
The database for estimating Tertiary terrestrial paleoenvironments in northern Latin America (Mexico, Central America, The Antilles), based on palynological evidence, is shown in Fig. 1 (see Note 1). The localities are scattered, and collectively provide only a low-resolution picture of environmental change. Therefore, several floras must be used to establish trends leading into the Pliocene. The two most relevant to the PRISM workshop are from the Gatun Formation, Canal region of Panama, and the Paraje Solo Formation near Coatzacoalcos, southern Veracruz state, Mexico. The chronological control on the pollen/spore-bearing part of the Gatun Formation is adequate for age assignment, but not precise. In a recent review of the marine record Coates et al. (1992) note (p. 827) that nannofossils from the middle part of the formation indicate an age range of 8.2-5.6 Ma. The 'terrestrial' phase (coastal mangrove zone) is subsurface and lacks marine palynomorphs (e.g., dinoflagellates) sufficient for tying it to the marine chronology. However, the pollen and spore-bearing lignitic material is from the upper part of the formation (top of solid rock; cores SL-49, SL-103). Coates et al. (1992, p. 827) state that "Gatunian" facies persist in Costa Rica during the Pliocene, but are absent from Panama after the earliest Pliocene. Thus, the maximum age of the plant microfossils is considered latest Gatun time (5.6 Ma) or slightly younger.

Figure 1. Map showing placement of localities discussed in text and chart illustrating the approximate ages of the formations present at these localities
This figure is available as a GIF, PICT, or TIFF (line-art) image.
Notes on Figure 1:
  1. There are no extensive, well-preserved, recently studied Tertiary macrofossil floras known from the region. The only sizable flora (the Oligocene San Sebastian flora of Puerto Rico) was last studied in 1928. All terrestrial paleoenvironmental estimates presented here are based on plant microfossil evidence.
  2. The markings indicate different stages in the completion of the studies (stipple--study complete and results published; others in progress). The Chapelton study is in press as part of GSA Memoir 182.
  3. Samples from Guatemala that have yielded palynomorphs (fair to poor) are from the Padre Miguel Group (now excluding the younger San Jacinto Formation; radiometric data indicates the Padre Miguel ignimbrites erupted between 19 and 14 Ma - middle to late Miocene), Barrios sequence (possibly Barrios Formation, Miocene), the Guastatoya Formation (age unknown, possibly Pliocene), and the Herreria (Pliocene, and possibly equivalent to the upper part of the Guastatoya). Information on the Barrios Formation fide Burkart (pers. comm.), all other from Donnelly et al. (1990). The Guatemalan lignites are not coastal brackish-water deposits containing Rhizophora (mangroves) as are the others, but were deposited in swamps that periodically became saline through evaporation, as indicated by gypsum crystals (Burkart, pers. comm.).
Figure 2. Correlation of the Paraje Solo Formation with marine faunal zones
This figure is available as a GIF, PICT, or TIFF (line-art) image.
Notes on Figure 2:
  1. Ostracodes listed for Lower and Upper Concepcion bed (N19-20) are Actinocythereis vineyardensis, Touroconcha lapidiscola, and abundant Hulingsina sp. 1, Henryhowella ex. gr. asperrima, and Puriana spp. In addition, Upper Concepcion species include abundant Cyprideis spp., Perissocytheridea spp., Basslerites? sp., Malzella conradi, and Echinocythereis margaritifera absent from the Lower Concepcion (Machain- Castillo, 1985).
  2. From the Agueguexquite Formation Akers (1979) lists the following planktic foraminifera-Globigerina bulloides apertura, G. bulloides bulloides, G. juvenilis, Globigerinoides obliquus extremus, G. obliquus obliquus, Globigerinoides quadrilobatus quadrilobatus, G. ruber, Globorotalia (Globorotalia) cultrata limbata, G. (Turborotalia) acostaensis acostaensis, G. (T.) acostaensis humerosa, Hastigerina (Hastigerina) siphonifera siphonifera, Orbulina universa, and Sphaeroideinella dehiscens dihiscens forma immatura; and the following calcareous nannofossils (in addition to those listed in Akers and Koeppel, 1973, p. 83) - Ceratolithus cristatus, Coccolithus doronicoides, Cyclococcolithina leptopora, C. macintyrei, Discoaster brouweri, D. pentaradiatus, D. surculus, D. variabilis, Gephyrocapsa caribbeanica, G. reticulata, Helicopontosphaera kamptneri, H. sp., Lithostromation perdurum, and Pseudoemiliania lacunosa.
Figure 3. Cenozoic benthic marine temperature curve with the approximate ages of formations discussed in the text
This figure is available as a GIF, PICT, or TIFF (line-art) image.
Figure 4. Position of the Paraje Solo Formation in relation to the nannofossil zones. From Machain-Castillo (1985)
This figure is available as a GIF, PICT, or TIFF (line-art) image.
The age of the Paraje Solo Formation is better constrained through marine faunal assemblages in the overlying Agueguexquite and underlying Upper Concepcion Formations within zone N20 (Fig. 2) (see Note 2). Machain- Castillo (1985, p. 123) notes that "the Paraje Solo may be in part contemporaneous to the Agueguexquite. The Agueguexquite strata contain the youngest (middle N20), most abundant, and diverse fauna, indicating an inner neritic environment of deposition and representing a local marine transgression of short duration in the northern part of the basin. By upper Agueguexquite time brackish and continental conditions returned." The Paraje Solo palynoflora contains an abundant brackish Rhizophora (mangrove) and diverse inland to upland continental component, suggesting deposition at the time and under the conditions described for the upper Agueguexquite sediments. Akers (1979) presented a similar stratigraphy and age assignment of the Lower Concepcion through Agueguexquite formations (Figs. 2 and 3) based on calcareous nannofossils and planktic foraminifera. He concluded (p. 4) that "the Agueguexquite Formation must be considered to be of middle Pliocene age, and it probably belongs just above the precise middle of zone N20." Thus, the Paraje Solo plant microfossils are Pliocene, and probably middle Pliocene from that part of the formation equivalent to the Agueguexquite, ~mid-N20, ~3-4 Ma.

The microfossils include wind-blown and water-transported forms, and preserve an environmental record for areas delimited mostly by physiography. For Panama (Gatun Formation) the reconstructions are probably applicable to most of southern Central America where land surfaces were primarily east-west trending and where physiographic relief was moderate. For Mexico (Paraje Solo Formation) the reconstructions are probably valid for the eastern escarpment of the Sierra Madre Oriental within southern and central Veracruz state where latitudinal extent and physiographic diversity was greater.

The palynological data can provide a general quantitative estimate of climate at individual sites, but only a broad picture of regional and long- term changes. This is due more to the scattered occurrence of the material than to the imprecise nature of the technique.

The available data suggests that from the Eocene (Chapelton, Gatuncillo floras) through the lower Miocene (Uscari, Culebra, Cucaracha, and La Boca floras) current tropical conditions and relatively low relief characterized much of northern Latin America south of Mexico. In southern Central America conditions had changed by Gatun time. Pollen of the Gramineae (grasses), which was virtually absent in the lower Miocene, increased to 7.5%. Maximum estimated elevations increased from ~1200-1400 m to ~1700 m. Greater environmental and habitat diversity resulted in greater biotic diversity, with the number of taxa increasing from 44, 55, 21, and 54 types in the lower Miocene floras noted above, to 110 types in the Gatun Formation. In addition to individual elements, there was also a greater diversity of communities. All elements in the lower Miocene floras can be accommodated in four forest types- tropical wet, tropical moist (including mangrove forest), premontane wet, and possibly premontane moist. By Gatun time there were seven forest types with 11 or more taxa that can occur in each community- tropical moist (38 taxa), tropical wet (31), premontane wet (27), premontane moist (21), lower montane moist (12), premontane rainforest (11), and tropical dry forest (11). The first six of these are similar to the modern vegetation of lowland to moderate- altitude habitats on the northern (Atlantic) side of Panama where the following climates prevail (Barro Colorado Island, Gatun Lake): average annual rainfall 2750 mm/yr (range 1900-3600 mm, strongly seasonal, dry season January through April); MAT 27C (annual range 21-32C). The presence of the tropical dry forest suggests altitudes had reached a point where moist Atlantic winds were deflected to create the drier Pacific side as at present (1700 mm/yr at Balboa). The close relationship between the Mio-Pliocene Gatun vegetation and the modern lowland to mid-altitude communities suggests that the primary forcing mechanism for vegetational change at latitude 9N was physiographic relief. Coastal climates were similar to those of present during the late Miocene/earliest Pliocene, and probably since the middle to late Eocene (Gatuncillo flora).

The situation at comparable low to mid-altitudes in Veracruz (latitude 18N) by Paraje Solo time (mid-Pliocene) was different. The present- day tropical rain forest was poorly developed to absent, mid-altitude (presently 1000-2000 m) temperate communities were prominent in the lowland basin of deposition, and Picea (spruce) was present. Spruce occurs today in Mexico only in the mountains 1000 km or more to the north. The common factor in these changes is lower temperature. As an estimate, a MAT lower by ~2-3C than at present would likely produce the observed differences between modern and the Paraje Solo vegetation. The present MAT at Coatzacoalcos is 25.3C (range 22-27C), and annual rainfall is 2726 mm (range 50-525 mm, driest season January through March with some rain falling in every month). The mid-Pliocene MAT is thus estimated at ~23C.

The lack of other fossil floras in the region make it impossible to detect when the time changes from lower Miocene to Paraje Solo conditions began. However, some estimate can be gained by extrapolating from two other lines of evidence. Preliminary study of the Oligo-Miocene Simojovel flora (Chiapas, Mexico) shows a mangrove forest, with oak-pine woodland grading upwards into more temperate communities on the adjacent slopes. Since this is similar to much of the modern local vegetation, it seems reasonable to conclude that the Neogene cooling trend had not set in there by late Oligocene-early Miocene time. Also, the benthic marine temperature curve (Fig. 4) suggests the middle Miocene as a likely time, which is consistent with the limited paleobotanical evidence.

If it is assumed that temperatures at 18N latitude were ~3C cooler than at present (Paraje Solo flora), and that at 9N latitude contemporaneous to somewhat older lowland climates were comparable to the present (Gatun flora), it is tempting to reconstruct intervening terrestrial MATs by lapse rate methodologies (yielding ~0.3C per degree of latitude). The figure may ultimately prove to be approximately correct, but there is little corroborative evidence. Also, the Paraje Solo communities were growing under or adjacent to continental conditions in an area of significant physiographic relief. Thus, any exogenic change in climate read from the vegetational history would be magnified by the response of communities growing along steep altitudinal gradients. In contrast, the Gatun communities were growing on islands and peninsulas of low physiographic relief and any exogenic change in climate would be dampened. Some general estimate of temperatures in the uplands can be gained from the current worldwide mean altitudinal lapse rate of 5.0-6.0C/km.

At present it is not possible to determine more precisely the range of terrestrial paleoclimatic variability in northern Latin America during the Pliocene from the floras available. Eventually, data from the Guastatoya and Herreria Formations of Guatemala may prove useful.

References


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