Thursday, January 1, 2009

Allocyclicity vs Autocyclicity: An Old Debate Revisited

An old debate on sedimentary cycles and research topic choices came to mind.

Brian at Clastic Detritus has an informative post with some great images on the discovery of rhythmic sediments deposited on Mars. As the name implies rhythmic sediments are not just layered but a particular type of sediment has been deposited at a regular frequency throughout that geological section.

The origin of these rhythms is a widely researched area of stratigraphy and sedimentology. As Brian explained these rhythms or sedimentary cycles can be allocyclic in origin or autocyclic in origin. Allocyclic means that periodic changes in an external factor such as cyclic climatic changes forces the environments of deposition to change in a cyclic manner producing a cyclic sedimentary record. Autocylic means that the sedimentary processes within a sedimentary basin develop cyclic feedback loops linking sediment production, transportation and deposition. This results in similar environments of deposition appearing at and disappearing from any particular location within the basin at periodic intervals leading to a cyclic sedimentary record.

Preliminary work suggests that the Mars rhythmic sediments are likely allocylic in origin, maybe linked to cyclic climatic changes on Mars. Brian thinks they are chemical deposits formed in a large lake. Maybe; it will be fun following this research.

Coming back to planet Earth this debate on allocyclicity vs autocyclicity in carbonate strata was being played out in a big way during my graduate students days in the early-mid 90's. The focus of study was the Cambro-Ordovician passive margin platform carbonates of the central and southern Appalachians. Two big labs, one based in Virginia and the other in Tennessee developed quite opposing points of view.

At Virginia Polytechnic, J.F. Read headed the carbonate research group and championed the allocyclic model. All his students who worked on stratigraphy related projects found strong evidence of allocyclicity in the sequences they studied. Further south, Kenneth Walker of the University of Tennessee at Knoxville downplayed allocyclicity as a viable model for shallow platform carbonates and stressed internal feedback loops and autocyclicity as a better explanation for the observed facies changes. All his students found strong components of autocyclicity in the sequences they studied.

Mind you, the students from the two different labs were working on carbonate sequences from the same time period deposited in adjacent areas of the same tectono-sedimentary setting.

Even further south I worked on the Alabama and Georgia Appalachian carbonates for my thesis at Florida State Univ. I took the smart option. I avoided the debate altogether and worked on the geochemistry of cements.

But I did follow with interest the work of my colleagues up north. Both these founders of the two large research labs are influential charismatic personalities. I don't doubt that new eager graduate students imbibed a particular way of thinking about carbonate deposition by listening to their mentors. But why such a clear divide in research results?

I am in no way suggesting that students made up results to fit preconceived notions. Could it be that students were genuinely interpreting the same sequences differently because of a mental bias or simply due to an unclear understanding of the characteristics of these two different cycle modes. This may have happened in the very early days of the history of these research programs. But over the years stratigraphers have developed some pretty discriminating models of the two types of cycles. As the two research labs developed a more sophisticated understanding of the topic I suspect a more subtle bias developed.

Carbonate platforms vary in their water depth. There are really shallow areas, let's say just about knee deep water and there are areas which are few to several meters deep. It is in these slightly deeper water areas that signals of allocyclicity or external forcing are best preserved. Very shallow water areas, those in the intertidal and supratidal zone for example are more readily affected by internally produced disturbances and feedbacks in sediment deposition and production. These are areas where signals from externally forced cyclicity get swamped by locally produced cyclicity.

I didn't follow each and every thesis produced by these students, but what I am speculating is that students of Virginia Tech, started choosing as research topics slightly deeper water sequences which best made the case for allocyclicity, while students of Tennessee started choosing those environments of deposition where autocyclicity was the dominant process. The results they got may have been genuine but they were in a sense inevitable given their choice of a particular type of study material. I may be completely wrong but this is something you could test by a detailed literature review to see if there was a preference within the two labs for selecting particular environments of deposition to study.

Moving away from these carbonates to a more broader theme. How much does an influential mentor dictate the choice of your research project? I would be interested in hearing from other geobloggers. Did you have an experience of working in a large research lab with a larger than life research head, where a particular school of thought had been established and which influenced your work? Were you really critical of this established way of thinking or did you just accept it and use it as a foundation for your work?

Is it better then to choose to go to a smaller lab which might not have developed a tradition of thinking in any particular direction? And finally doesn't the influential mentor have a responsibility to develop internal differing points of view? How about assigning a couple of students on projects that might potentially disprove the established tradition?


  1. Suvrat ... great post, that's a cool review from the carbonate perspective.

    The dynamic interaction of allo and auto controls really makes me think it could be difficult to impossible to unravel the relative contributions of either in many cases. For example, the classic example of an autogenic process in clastic systems in the avulsion of a delta lobe. But, if that delta was feeding a deep-water system and now that deep-water system is abandoned (because delta lobe avulsed) wouldn't that be an "external" change from the perspective of the deep-water environment? The complex causal chain combined w/ different spatial and temporal scales make the discrimination quite difficult.

    Regarding your last paragraph ... I did my master's work in a group strongly influenced by one person, and did my Ph.D. in a larger group that wasn't dominated by anyone person or point-of-view ... for me, I like the latter better, but I wonder how different things would be for me if the two styles were in different order.

  2. Brian-

    i looked up some recent theoretical work on carbonate parasequences and people are still having problems clearly discriminating between the two cycle modes. so plenty of work to be done yet.

    regarding the example you gave on delta lobe avulsion. What if long term climatic cycles control the supply of sediment to the delta and influence avulsion frequencies? Would that be an allocyclic mode?

    Just trying to make things more difficult for you clastic types :-)
    You're right. Scale matters.

    I too worked in a very small lab and had quite a lot of freedom in expanding my thinking in many directions. so i really don't know what it is to work with 10-15 other graduate colleagues in the same lab.

  3. Suvrat says: "What if long term climatic cycles control the supply of sediment to the delta and influence avulsion frequencies?"

    Good point ... since external controls will always 'influence' autogenic processes, it comes down to deciding when they influence is minor. I guess in the case of the avulsing lobe, the sed supply surely influences the occurrence of avulsion, but can sed supply (e.g., volume of sed delivered per unit time) be used to predict the timing of the avulsion?

    It's all so complex! Since it's difficult to tease these apart, I really think it's about figuring out how they interact ... which is exactly what your example is about. In what way do the autogenic processes change when the external regime is different. Fun stuff.