Thursday, February 11, 2016

Fossil Leaves of the Yumuri River Gorge in northern Cuba

Fossilized plants are among the rarest types of fossils. This is because there are many factors that prevent or allow for their preservation. One of these factors is the delicate nature and chemical composition of the plant leaves themselves. Leaves are delicate, thin, and devoid, except in rare instances, of hard or mineralized parts, and thus become fossils under very special conditions; often in exceptional conditions.

Plant remains become fossils usually as carbon or oxide films within clay or mud sediments. The chances of preservation increase if coupled with rapid burial in quiet, low energy water environments where they can absorb water, sink and settle to the bottom where they become covered with sediment and preserved. If in addition, the sediment is low in oxygen, there is less decomposition and increasing preservation. In this way, fossil leaves provide an array of indicators to the surrounding environment and their origin, and thus open a wide window to the past.

Fig. 1: Fossilized leaf and seed on the river gravels at the Yumuri river site.

This is the case of the Yumuri River gorge, near Matanzas city, in northwestern Cuba. It represents an interesting example of both fossil leaf assemblage and good preservation (Fig. 1 and 2). These fossil leaves represent an interesting case in the study of Cuban, and even the Caribbean palaeoflora. The first fossil plants in Cuba were reported by the German Johannes Felix in 1882. Ever since, fossil plants have been sporadically reported from Jurassic to the Holocene fossils (Leon, 1929) (Iturralde and Rojas, catalog available here).

The fossil leaves of the Yumuri river gorge are found within the complex arrangement of sandstone conglomerates and fossil muds of El Abra member, a variation of the Canimar formation (Iturralde, 1969). These are a mixture of small and medium pebbles, with rare fish and crab fossils within the finer sediments. This formation is very localized, existing only along the river gorge. It apparently dates between the late Miocene and the middle Pliocene. If so, this fossil leaf deposit can provide a great insight into the poorly known flora and paleoclimate of the region during that the last 6 million years. In itself a great wealth of information into the Cuban past.

Fig. 2: Sands and gravels beds of the Abra member of the Canimar fm. on the banks
of the Yumuri river gorge.

Let us start with the fossil leaves.

The shape and area of the leaves, and the type of sediment in which they are preserved, can provide an array of approximations (proxies) into the past climate and formation history of this deposit. There is a correlation between environment and the shape of plant leaves (Wilf et al. 1998 and 2000. Also, see Lindner, 2007 and citations therein). Plants with large leaves and smooth edges are most common in tropical and subtropical climates. Inversely, leaves with lobed and serrated edges, as is the case of maples, are indicative of temperate, colder climates.  The larger the area of the leaf is positively correlated with local weather such as precipitation and temperature (op. cit.). The Abra leaves, with their smooth edges, and mostly smooth elongated leaves, suggest warm, mean annual temperatures (MATs), likely greater than or about 28 degrees Celsius. The leaf margin analysis (of LMA based on the mm^2 area of the leafs) suggests warm tropical climate with mid-range precipitation (see Wilf et al. 1998 for tables).

Fig. 3: Small leaf within its fine sand tomb. Note the raised bumps on the surface, and the smooth edges.

When leaves fall they are usually spread very close to their source (Lindner, 2007). In this way, they can represent a very true local floral assemblage. The task of identifying the leaf species from El Abra was first tackled by  E. W. Berry, who reported over a dozen of species (Berry, 1936). These were mostly new species; a study that now will need revision, and is a poor indicator of age. Berry had already reported on fossil plants from the Pleistocene deposits in another of Matanzas's famous site, the San Felipe Tar Pits, near Marti (Berry, 1934). These studies of palaeofloras was followed by the notes of Roca (1922), and later Leon (1929).

Overall, the Abra fossil leaves suggests a past tropical warm and humid coastal biome. This would agree with the higher temperatures of the Pliocene, especially those of the mid-Pliocene. But once more, the better chronology is wanting here.

Fig. 4: A very small leaf attests to the fine degree of preservation of oxidized leaf film.

Fig. 5: This curved leaf may be an indication, that at least some leafs were dry
when they sank to the bottom sediments of the river.

The Abra leaves show other interesting details which pertain to their physical state at the time of burial (taphonomy). Some of the leaves seem to have been preserved in a dry state. Meaning that at least some of the leaves that were preserved were dry and probably windblown by the time of deposition. This is the case of the leaf in Fig. 5, which compares with a dried example of a Mahogany Swietenia mahagoni Fig. 6 below. See how it is curved and curled?

Fig. 6: A modern example of a fallen, dry Mahogany tree leaf,
very similar to that of Fig. 5 above.

Because the sediments were so fine a mixture of mud and sands, leaf detail was preserved. Including in some examples, the original state of the leaves as they were being pressed and preserved into the thin films they are now.

The fossil plants occur only within the beds of fine sediments. These fine sediments indicate a low energy deposition environment, such as still or quiet waters, or single event deposition, such a flood that settled out quickly (Lindner, 2007). This is so because low energy waters usually do not move larger-coarser gravels, pebbles or boulders. Only larger energy events can. Especially if they are to be moved for longer distances along the flood plains of a shallow river bank. The mixture of sands and gravels of multiple sizes may support this. This is observable in the beds where the finer clasts overlie the heavier larger ones. This is called, in sedimentation stratigraphy, a graded bedding.

Fig. 7: Assemblage of fossil leaves encased in the sediment,
at various levels of deposition. Look at the bent leaf above.

These graded beds form a succession of finer beds, following larger thicker sandy beds, which indicate a cyclic variation in the formation of the deposit that can be indicative of migrating river beds, or flooding planes as stated above.These beds have marked limits in-between them, indicating abrupt changes in sedimentation regimes. Some have considered them to be variations in a river- sea level influence- regression and transgression or increasing/decreasing sea level (Iturralde, 1969). This makes sense being so close to the mouth of the bay, and if indeed this region had been a shallow, young delta in the past, much before it was lifted as it is now.

Fig. 8: Possible seed encased in fossil leafs.
Note the abrupt bed plane bellow the leafs. 

In conclusion, taking all these variations of evidence together we can reconstruct a wooded river margin, shallow, low energy, and prone to flooding, in a tropical/subtropical climate. The clastic material is most likely coming from erosion of higher inner valley lands, filtering through the incipient river gorge into the flood plains of the river deltas into the yet nonexistent bay of Matanzas.

Fig. 9: Eroded and exposed fossil leaf at El Abra, Yumuri River Gorge in Matanzas.

This is part of one of my ongoing projects and much more work needs to be done. For instance, one of the greatest problems is defining the actual age of the sediments since there are a lot mixture and reworking of younger and older material within the same beds. The other would be to revise the taxonomy of the plants involved, and other organisms found in the deposit.

If you are a plant taxonomist who can identify these fossil plants and would like to collaborate, contact me.

In the mean time stay tuned!

Cited Literature

For a catalog of Cuban plants see Estudios de Plantas Fosiles de Cuba, prepared by the National Museum in Cuba.

For other interesting fossil plant discoveries in the circum-Caribbean see Velez-Juarbe Caribbean Paleobiology blog.

Berry, E. W. 1934. Pleistocene plants from Cuba. Torrei Botanical Club 61(5): 237-240.

Berry, E. W. 1936. A Miocene flora from the Gorge of Yumuri river, Matanzas, Cuba. John Hopkins Studies in Geology 13: 1-135.

Iturralde-Vinent, M. see his literature regarding Matanzas on Biblioteca Digital Cubana de Geociencias.

Lindner Dustra, T. 2007. Paleobotany and Paleoclimatology, Part 2: Leaf Assemblages (Taphonomy, Paleoclimatology, and Paleogeography) pp. 179-202 in Part 3 of E. A. M. Koutsoukos Applied Stratigraphy, Topics in Geobiology, volume 23.

Leon, Hermano. 1929. La flora flosil de Cuba en la actualidad. La Salle: 1-6pp.

Roca, M. 1922. Nota aserca de un yacimiento de fosiles vegetales del Abra de Yumuri, Matanzas, Cuba. Memorias de la Sociedad Cubana de Historia Natural 4: 120-124.

Wilf, P. S. L. Wing, D. R. Greenwood, and C. L. Greenwood. 1998. Using fossil leaves as paleo-precipitation indicators: an Eocene example. Geology, 26: 203-206.

Wilf, P. and C. C. Labanderia. 1999. The response of plant-insect associations to Paleocene-Eocene warming. Science 284: 2153-2156.

No comments:

Post a Comment