Late Eocene Petrified Woods at the Lazy F Ranch,
Gonzalez County, TX
By Scott Singleton

Introduction

This report summarizes my preliminary analysis of material found at the Lazy F Ranch in Gonzalez County near Smiley. The material is found in the soil zone above as well as within the host sediments. At this point in time, no thin section work has been done on the petrified wood specimens found at this locality.

The Lazy F Ranch sits within the late Eocene Manning Fm. of the Jackson Group. This formation contains white, cross-bedded sand interlayered with flat-layered clay. It most likely is of shallow marine origin, as determined by Martesia borings in one specimen. It contains poorly silicified wood mostly comprised of Alangium, but palm and a few other genera may also be present.

Above this formation lies some lag material (pebbles to large cobbles) at the base of the soil zone. This lag material is highly silicified, much of it being translucent to transparent. Most of the translucent specimens are not identifiable because the cell structure was not preserved or was destroyed during the silicification of the wood after it was fossilized.

There are a large number of different species represented in this lag material (18 counted from material collected during one visit). Unfortunately, at this preliminary stage in my investigation many of these are unknowns (8). Thin section analysis of these specimens may provide many of the IDs, although some do not contain the necessary cell structure and thus will remain unknowns.

An attempt was made to constrain the age of this lag material given the information available to me at this point. I looked at (1) species differentiation between the translucent specimens and the opaque specimens, (2) presence or absence of growth ring development, and (3) the growth environments of the known species. This information indicates the lag material is possibly Oligocene in age, although it is still not certain whether it is all the same age or if there is some contamination from upstream formations.

Manning Formation Description

The formation is observed outcropping at several locations. One location consists of fine to medium sand, white, loosely packed, and not indurated with any significant silica cement. The material is cross-bedded and contains vertical tubes which bifurcated downward (see photo).

The tubes are filled with the same sand as was in the formation and do not differ in any way from the formation. They have been identified as a marine burrowing invertebrate called Thalassinoides. This burrow is common in Eocene shallow water sand deposits and is indicative of an environment with enough current or wave activity to move and deposit sand.

A second locality has a more clayey-type of sediment. It might be bentonite from the decomposition of volcanic ash (?). The layering from what I could observe is horizontal. The color is white to beige and possibly tan. Most of the wood in-situ specimens that could be identified (i.e. reasonable silica content) came from this locality.

The two localities are separated by about a mile in the dip direction with the sand locality being higher in the formation (younger).

Petrified Wood Within the Manning Fm.

All the samples obtained from this formation are white to beige, porous, and have a low level of silicification. Some have an outer layer of white, denser material that I believe to be common opal. The white coating would have been deposited in the fossil specimens some time after they were originally fossilized. I have seen this type of replacement before in the Manning north of Lake Livingston in East Texas.

The vast majority of the specimens are from a single species, Hunteria (Apocynaceae family). This species is common in the Texas Eocene. Small amounts of other species were also recovered. These are listed below along with the number of specimens in parentheses:

Lazy F Ranch Species List for In-Situ Manning Fm.

It is noteworthy that a single species predominates the assemblage. This is not common although it does happen in various localities. It is difficult to draw paleo-environmental conclusions from such occurrences because they could be due either to a very low-diversity forest or from preferential selection of certain species that are more prone to fossilization, either due to their growth location along banks of waterways or due to the physical characteristics of the trees (large trunks, resistant wood, etc.). Nevertheless, the above species list is a typical late Eocene Jackson Group assemblage and lays the groundwork for a little detective work concerning the lag found on top of this formation.

One large log of Hunteria was found to be shot through with borings from a marine organism (see photos).

From the characteristics of these borings, those who know marine invertebrates feel they are from a species of the wood-boring clam Martesia. This would serve to constrain the paleo-depositional environment to shallow marine. This would, in turn, imply that all wood specimens at this locality are “drift wood” and have been transported an unknown distance from their original growth location.

Petrified Wood in Lag at Base of Soil Zone

A second distinct occurrence of petrified wood occurs above the Manning Fm. outcrop. It is difficult to determine exactly where these pieces originate because most of them are merely laying as cobbles on top of the weathered Manning outcrop in areas where the soil zone has washed away. However in areas where the soil zone is present, close inspection reveals they are physically lying above the Manning Fm. and within the lower portion of the soil zone (see photos).

These specimens are all highly silicified. Most of them are translucent and contain a variety of colors, although grey, tan, beige, brown, yellow, red, and black predominate (see photo of palm).

Many contain very light colors and light can be seen through the specimens. It should be noted that not all of the specimens are translucent. Some are opaque and consist of the same colors listed above, although tan, brown, and black predominated. Some specimens grade from translucent to opaque.

The specimens are not excessively stream worn. Some show various degrees of rounding, but the majority of them are quite angular. It is true that highly silicified material is quite brittle and will fracture and break easily. However, the overall lack of extreme rounding leads one to presume that their source could not be far away. (However, as a counterpoint, Indian artifact hunters have informed me much of the angularity they saw was not natural. They believe Indians had broken many of the pieces in search of appropriate arrowhead material).

Assemblage List: The assemblage in these specimens is quite varied. No single species predominates, and there are quite a few single occurrences. The preliminary list is presented below:

Lazy F Ranch Species List for Lag Material:

Note that there are quite a few unknowns. I am not familiar with a number of the specimens based on macroscopic observation only. The main point, however, is that there are approximately 18 different species in this assemblage (which is about twice the normal number I get from a "diverse" assemblage) and that many of them are not the common species I am accustomed to seeing.

This immediately leads to suspicions about this assemblage. Remember that the principle questions we are asking about this locality are: Where are these specimens from, and what age are they? Two possibilities exist: (1) The specimens have washed in from upstream and represent debris from older formations, or (2) the specimens represent landward, terrigenous facies from newer formations (most likely Oligocene or Miocene). Note that Plio-Pleistocene alluvial terraces are locally prevalent in many areas of the Gulf Coast and serve to completely mix specimens of earlier formations.

These questions may be answered by a more thorough investigation of the different species present in this assemblage, including resolving all of the unknowns. It then will be easier to narrow down the age or ages of the specimens. (Note that this is not a foolproof method. Some specimens, such as Alangium, traverse the Eocene and Oligocene, and thus are not diagnostic of age).

However for this preliminary investigation, I will concentrate on finding characteristics of the specimens that may help in this determination. These characteristics include any possible differences in the physical appearance of the rocks (thus indicating different source formations), and the observation of growth ring intensity (a rough climatological indicator that may separate Eocene from Oligocene and later). Finally, I will assess the known species for any age indications (i.e. ones that have known origination dates or extinction dates on the Gulf Coast, such as tropical genera versus temperate genera).

Physical Characteristics: The first characteristic to investigate is physical appearance. As noted earlier, the greater percentage of these specimens were translucent from extreme silicification. I tried to avoid collecting transparent to translucent specimens that were devoid of cell structure; however some were picked up for completeness. In the course of my investigation, I noted that many of the opaque specimens (meaning a lower degree of silicification) were different species than the completely silicified specimens that had some residual cell structure. Therefore, I split these out into 3 different groups—those that were translucent from extreme silicification, those that were opaque from lesser silicification, and those that shared some translucency with some opaqueness. This breakdown is given below:

Table of fossil wood physical characteristics

I will be the first to admit that these three groups overlap somewhat. Also, the somewhat artificial distinction of “partially translucent/partly opaque” is rather arbitrary, plus the fact that if there are truly specimens that share both of these characteristics, then they don’t actually represent different populations of specimens. However, despite the arbitrariness of this separation, there do appear to be some observations that can be made: The more common specimens are mostly shared by both groups (except for #5), but starting with wood type #8 they appear to separate out nicely (of course, #13-18 only contain 1 specimen each!). If this observation proves sound, the logical conclusion is that there are actually two populations of specimens present in this collection. The translucent specimens are the most striking and are by far the greatest in abundance at this locality. These would have come from a specific geologic formation because total silicification is usually restricted in its occurrence. The opaque specimens are more common in Tertiary Gulf Coast sediments, and the ones found at this locality could have originally come from almost anywhere.

Growth Rings: The second characteristic to investigate is the degree of growth ring formation. It has been shown that the vast majority of woods grown in seasonal climates produce pronounced growth rings, but woods grown in non-seasonal tropical environments tend not to produce growth rings except where there are dry/wet seasonal variations. Thus, during the Eocene in Texas we would not expect fossil woods to exhibit pronounced growth rings, but with the progressive degradation of climate during the Oligocene and later, growth ring formation would be expected.

Observation of growth ring intensity reveals that most, if not all, of the specimens contain growth rings. Growth rings even occur in species that traverse the Eocene (e.g. Alangium), where they are normally found to be lacking in rings. This implies that, in general, we are talking about a control being placed on the age of these woods so that it is unlikely they would be from earlier in the Eocene than the host formation (Manning Fm.).

Diagnostic Species: The third characteristic is wood types that are restricted in their age ranges. Of the types listed in the lag material species list, only one is age-limited (hickory). All other species are typical Eocene species, although legumes are more prevalent from the mid-Eocene onwards. In addition, the species listed all make the transition into the Oligocene, so they are not of diagnostic value when trying to make an age determination. Hickory, however, is a temperate species that makes its appearance after the climate cools down in the Oligocene. Note that the ID is somewhat tenuous because it was made on the weathered exterior of a translucent/transparent specimen, so positive ID will not be possible. But if the ID holds up to further scrutiny, the presence of this genera would serve to constrain the age date of this material to the Oligocene. (It could not be newer than the Oligocene because further climate deterioration in the Miocene served to weed out all of the tropical species).

Further Work

This is just a preliminary investigation. To make this work stand on its own, there are several other components that need to be added:

1. Thin section analysis. There are too many unknowns for this work to be presented to anyone but an informal group of collectors or geologists.
2. Investigation of the lag. More effort needs to be put into defining the vertical location of the cobbles in the sediment layer and the horizontal extent of the lag deposits. I do know the material apparently weathered out from beneath the soil zone and was merely lying on top of exposed formation. But this is not good enough for a scientific paper.
3. Pollen analysis of lag. If it turns out that the lag can be isolated to a specific soil zone, a pollen analysis might shed some light on the time of deposition.